<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE cref SYSTEM "cref.dtd">
<cref>
<header>
<copyright>
<year>2001</year><year>2010</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and limitations
under the License.
</legalnotice>
<title>erl_driver</title>
<prepared>Jakob Cederlund</prepared>
<responsible>Jakob Cederlund</responsible>
<docno>1</docno>
<approved></approved>
<checked></checked>
<date>2000-11-27</date>
<rev>PA1</rev>
<file>erl_driver.xml</file>
</header>
<lib>erl_driver</lib>
<libsummary>API functions for an Erlang driver</libsummary>
<description>
<p>As of erts version 5.5.3 the driver interface has been extended
(see <seealso marker="driver_entry#extended_marker">extended marker</seealso>).
The extended interface introduce
<seealso marker="erl_driver#version_management">version management</seealso>,
the possibility to pass capability flags
(see <seealso marker="driver_entry#driver_flags">driver flags</seealso>)
to the runtime system at driver initialization, and some new
driver API functions. </p>
<note>
<p>Old drivers (compiled with an <c>erl_driver.h</c> from an
earlier erts version than 5.5.3) have to be recompiled
(but does not have to use the extended interface).</p>
</note>
<p>The driver calls back to the emulator, using the API
functions declared in <c>erl_driver.h</c>. They are used for
outputting data from the driver, using timers, etc.</p>
<p>A driver is a library with a set of function that the emulator
calls, in response to Erlang functions and message
sending. There may be multiple instances of a driver, each
instance is connected to an Erlang port. Every port has a port
owner process. Communication with the port is normally done
through the port owner process.</p>
<p>Most of the functions takes the <c>port</c> handle as an
argument. This identifies the driver instance. Note that this
port handle must be stored by the driver, it is not given when
the driver is called from the emulator (see
<seealso marker="driver_entry#emulator">driver_entry</seealso>).</p>
<p>Some of the functions takes a parameter of type
<c>ErlDrvBinary</c>, a driver binary. It should be both
allocated and freed by the caller. Using a binary directly avoid
one extra copying of data.</p>
<p>Many of the output functions has a "header buffer", with
<c>hbuf</c> and <c>hlen</c> parameters. This buffer is sent as a
list before the binary (or list, depending on port mode) that is
sent. This is convenient when matching on messages received from
the port. (Although in the latest versions of Erlang, there is
the binary syntax, that enables you to match on the beginning of
a binary.)
<marker id="smp_support"></marker>
</p>
<p>In the runtime system with SMP support, drivers are locked either
on driver level or port level (driver instance level). By default
driver level locking will be used, i.e., only one emulator thread
will execute code in the driver at a time. If port level locking
is used, multiple emulator threads may execute code in the driver
at the same time. There will only be one thread at a time calling
driver call-backs corresponding to the same port, though. In order
to enable port level locking set the <c>ERL_DRV_FLAG_USE_PORT_LOCKING</c>
<seealso marker="driver_entry#driver_flags">driver flag</seealso> in
the <seealso marker="driver_entry">driver_entry</seealso>
used by the driver. When port level locking is used it is the
responsibility of the driver writer to synchronize all accesses
to data shared by the ports (driver instances).</p>
<p>Most drivers written before the runtime system with SMP
support existed will be able to run in the runtime system
with SMP support without being rewritten if driver
level locking is used.</p>
<note>
<p>It is assumed that drivers does not access other drivers. If
drivers should access each other they have to provide their own
mechanism for thread safe synchronization. Such "inter driver
communication" is strongly discouraged.</p>
</note>
<p>Previously, in the runtime system without SMP support,
specific driver call-backs were always called from the same
thread. This is <em>not</em> the case in the runtime system
with SMP support. Regardless of locking scheme used, calls
to driver call-backs may be made from different threads, e.g.,
two consecutive calls to exactly the same call-back for exactly
the same port may be made from two different threads. This
will for <em>most</em> drivers not be a problem, but it might.
Drivers that depend on all call-backs being called in the
same thread, <em>have</em> to be rewritten before being used
in the runtime system with SMP support.</p>
<note>
<p>Regardless of locking scheme used, calls to driver
call-backs may be made from different threads.</p>
</note>
<p>Most functions in this API are <em>not</em> thread-safe, i.e.,
they may <em>not</em> be called from an arbitrary thread. Function
that are not documented as thread-safe may only be called from
driver call-backs or function calls descending from a driver
call-back call. Note that driver call-backs may be called from
different threads. This, however, is not a problem for any
functions in this API, since the emulator have control over
these threads.</p>
<note>
<p>Functions not explicitly documented as thread-safe are
<em>not</em> thread-safe. Also note that some functions
are <em>only</em> thread safe when used in a runtime
system with SMP support.</p>
</note>
</description>
<section>
<title>FUNCTIONALITY</title>
<p>All functions that a driver needs to do with Erlang are
performed through driver API functions. There are functions
for the following functionality:</p>
<taglist>
<tag>Timer functions</tag>
<item>Timer functions are used to control the timer that a driver
may use. The timer will have the emulator call the
<seealso marker="driver_entry#timeout">timeout</seealso> entry
function after a specified time. Only one timer is available
for each driver instance.</item>
<tag>Queue handling</tag>
<item>
<p>Every driver instance has an associated queue. This queue is a
<c>SysIOVec</c> that works as a buffer. It's mostly used for
the driver to buffer data that should be written to a device,
it is a byte stream. If the port owner process closes the
driver, and the queue is not empty, the driver will not be
closed. This enables the driver to flush its buffers before
closing.</p>
<p>The queue can be manipulated from arbitrary threads if
a port data lock is used. See documentation of the
<seealso marker="#ErlDrvPDL">ErlDrvPDL</seealso> type for
more information.</p>
</item>
<tag>Output functions</tag>
<item>With the output functions, the driver sends data back
the emulator. They will be received as messages by the port owner
process, see <c>open_port/2</c>. The vector function and the
function taking a driver binary is faster, because that avoid
copying the data buffer. There is also a fast way of sending
terms from the driver, without going through the binary term
format.</item>
<tag>Failure</tag>
<item>The driver can exit and signal errors up to Erlang. This is
only for severe errors, when the driver can't possibly keep
open.</item>
<tag>Asynchronous calls</tag>
<item>The latest Erlang versions (R7B and later) has provision for
asynchronous function calls, using a thread pool provided by
Erlang. There is also a select call, that can be used for
asynchronous drivers.</item>
<tag><marker id="multi_threading">Multi-threading</marker></tag>
<item>
<p>A POSIX thread like API for multi-threading is provided. The
Erlang driver thread API only provide a subset of the functionality
provided by the POSIX thread API. The subset provided is
more or less the basic functionality needed for multi-threaded
programming:
</p>
<list>
<item><seealso marker="#ErlDrvTid">Threads</seealso></item>
<item><seealso marker="#ErlDrvMutex">Mutexes</seealso></item>
<item><seealso marker="#ErlDrvCond">Condition variables</seealso></item>
<item><seealso marker="#ErlDrvRWLock">Read/Write locks</seealso></item>
<item><seealso marker="#ErlDrvTSDKey">Thread specific data</seealso></item>
</list>
<p>The Erlang driver thread API can be used in conjunction with
the POSIX thread API on UN-ices and with the Windows native thread
API on Windows. The Erlang driver thread API has the advantage of
being portable, but there might exist situations where you want to
use functionality from the POSIX thread API or the Windows
native thread API.
</p>
<p>The Erlang driver thread API only return error codes when it is
reasonable to recover from an error condition. If it isn't reasonable
to recover from an error condition, the whole runtime system is
terminated. For example, if a create mutex operation fails, an error
code is returned, but if a lock operation on a mutex fails, the
whole runtime system is terminated.
</p>
<p>Note that there exist no "condition variable wait with timeout" in
the Erlang driver thread API. This is due to issues with
<c>pthread_cond_timedwait()</c>. When the system clock suddenly
is changed, it isn't always guaranteed that you will wake up from
the call as expected. An Erlang runtime system has to be able to
cope with sudden changes of the system clock. Therefore, we have
omitted it from the Erlang driver thread API. In the Erlang driver
case, timeouts can and should be handled with the timer functionality
of the Erlang driver API.
</p>
<p>In order for the Erlang driver thread API to function, thread
support has to be enabled in the runtime system. An Erlang driver
can check if thread support is enabled by use of
<seealso marker="#driver_system_info">driver_system_info()</seealso>.
Note that some functions in the Erlang driver API are thread-safe
only when the runtime system has SMP support, also this
information can be retrieved via
<seealso marker="#driver_system_info">driver_system_info()</seealso>.
Also note that a lot of functions in the Erlang driver API are
<em>not</em> thread-safe regardless of whether SMP support is
enabled or not. If a function isn't documented as thread-safe it
is <em>not</em> thread-safe.
</p>
<p><em>NOTE</em>: When executing in an emulator thread, it is
<em>very important</em> that you unlock <em>all</em> locks you
have locked before letting the thread out of your control;
otherwise, you are <em>very likely</em> to deadlock the whole
emulator. If you need to use thread specific data in an emulator
thread, only have the thread specific data set while the thread is
under your control, and clear the thread specific data before
you let the thread out of your control.
</p>
<p>In the future there will probably be debug functionality
integrated with the Erlang driver thread API. All functions
that create entities take a <c>name</c> argument. Currently
the <c>name</c> argument is unused, but it will be used when
the debug functionality has been implemented. If you name all
entities created well, the debug functionality will be able
to give you better error reports.
</p>
</item>
<tag>Adding / remove drivers</tag>
<item>A driver can add and later remove drivers.</item>
<tag>Monitoring processes</tag>
<item>A driver can monitor a process that does not own a port.</item>
<tag><marker id="version_management">Version management</marker></tag>
<item>
<p>Version management is enabled for drivers that have set the
<seealso marker="driver_entry#extended_marker">extended_marker</seealso>
field of their
<seealso marker="driver_entry">driver_entry</seealso>
to <c>ERL_DRV_EXTENDED_MARKER</c>. <c>erl_driver.h</c> defines
<c>ERL_DRV_EXTENDED_MARKER</c>,
<c>ERL_DRV_EXTENDED_MAJOR_VERSION</c>, and
<c>ERL_DRV_EXTENDED_MINOR_VERSION</c>.
<c>ERL_DRV_EXTENDED_MAJOR_VERSION</c> will be incremented when
driver incompatible changes are made to the Erlang runtime
system. Normally it will suffice to recompile drivers when the
<c>ERL_DRV_EXTENDED_MAJOR_VERSION</c> has changed, but it
could, under rare circumstances, mean that drivers have to
be slightly modified. If so, this will of course be documented.
<c>ERL_DRV_EXTENDED_MINOR_VERSION</c> will be incremented when
new features are added. The runtime system use the minor version
of the driver to determine what features to use.
The runtime system will refuse to load a driver if the major
versions differ, or if the major versions are equal and the
minor version used by the driver is greater than the one used
by the runtime system.</p>
<p>The emulator tries to check that a driver that doesn't use the
extended driver interface isn't incompatible when loading it.
It can, however, not make sure that it isn't incompatible. Therefore,
when loading a driver that doesn't use the extended driver
interface, there is a risk that it will be loaded also when
the driver is incompatible. When the driver use the extended driver
interface, the emulator can verify that it isn't of an incompatible
driver version. You are therefore advised to use the extended driver
interface.</p>
</item>
</taglist>
</section>
<section>
<title>DATA TYPES</title>
<taglist>
<tag><marker id="ErlDrvSysInfo"/>ErlDrvSysInfo</tag>
<item>
<p/>
<code type="none">
typedef struct ErlDrvSysInfo {
int driver_major_version;
int driver_minor_version;
char *erts_version;
char *otp_release;
int thread_support;
int smp_support;
int async_threads;
int scheduler_threads;
int nif_major_version;
int nif_minor_version;
} ErlDrvSysInfo;
</code>
<p>
The <c>ErlDrvSysInfo</c> structure is used for storage of
information about the Erlang runtime system.
<seealso marker="#driver_system_info">driver_system_info()</seealso>
will write the system information when passed a reference to
a <c>ErlDrvSysInfo</c> structure. A description of the
fields in the structure follow:
</p>
<taglist>
<tag><c>driver_major_version</c></tag>
<item>The value of
<seealso marker="#version_management">ERL_DRV_EXTENDED_MAJOR_VERSION</seealso>
when the runtime system was compiled. This value is the same
as the value of
<seealso marker="#version_management">ERL_DRV_EXTENDED_MAJOR_VERSION</seealso>
used when compiling the driver; otherwise, the runtime system
would have refused to load the driver.
</item>
<tag><c>driver_minor_version</c></tag>
<item>The value of
<seealso marker="#version_management">ERL_DRV_EXTENDED_MINOR_VERSION</seealso>
when the runtime system was compiled. This value might differ
from the value of
<seealso marker="#version_management">ERL_DRV_EXTENDED_MINOR_VERSION</seealso>
used when compiling the driver.
</item>
<tag><c>erts_version</c></tag>
<item>A string containing the version number of the runtime system
(the same as returned by
<seealso marker="erts:erlang#system_info_version">erlang:system_info(version)</seealso>).
</item>
<tag><c>otp_release</c></tag>
<item>A string containing the OTP release number
(the same as returned by
<seealso marker="erts:erlang#system_info_otp_release">erlang:system_info(otp_release)</seealso>).
</item>
<tag><c>thread_support</c></tag>
<item>A value <c>!= 0</c> if the runtime system has thread support;
otherwise, <c>0</c>.
</item>
<tag><c>smp_support</c></tag>
<item>A value <c>!= 0</c> if the runtime system has SMP support;
otherwise, <c>0</c>.
</item>
<tag><c>thread_support</c></tag>
<item>A value <c>!= 0</c> if the runtime system has thread support;
otherwise, <c>0</c>.
</item>
<tag><c>smp_support</c></tag>
<item>A value <c>!= 0</c> if the runtime system has SMP support;
otherwise, <c>0</c>.
</item>
<tag><c>async_threads</c></tag>
<item>The number of async threads in the async thread pool used
by <seealso marker="#driver_async">driver_async()</seealso>
(the same as returned by
<seealso marker="erts:erlang#system_info_thread_pool_size">erlang:system_info(thread_pool_size)</seealso>).
</item>
<tag><c>scheduler_threads</c></tag>
<item>The number of scheduler threads used by the runtime system
(the same as returned by
<seealso marker="erts:erlang#system_info_schedulers">erlang:system_info(schedulers)</seealso>).
</item>
<tag><c>nif_major_version</c></tag>
<item>The value of <c>ERL_NIF_MAJOR_VERSION</c> when the runtime system was compiled.
</item>
<tag><c>nif_minor_version</c></tag>
<item>The value of <c>ERL_NIF_MINOR_VERSION</c> when the runtime system was compiled.
</item>
</taglist>
</item>
<tag><marker id="ErlDrvBinary"/>ErlDrvBinary</tag>
<item>
<p/>
<code type="none">
typedef struct ErlDrvBinary {
int orig_size;
char orig_bytes[];
} ErlDrvBinary;
</code>
<p>The <c>ErlDrvBinary</c> structure is a binary, as sent
between the emulator and the driver. All binaries are
reference counted; when <c>driver_binary_free</c> is called,
the reference count is decremented, when it reaches zero,
the binary is deallocated. The <c>orig_size</c> is the size
of the binary, and <c>orig_bytes</c> is the buffer. The
<c>ErlDrvBinary</c> does not have a fixed size, its size is
<c>orig_size + 2 * sizeof(int)</c>.</p>
<note>
<p>The <c>refc</c> field has been removed. The reference count of
an <c>ErlDrvBinary</c> is now stored elsewhere. The
reference count of an <c>ErlDrvBinary</c> can be accessed via
<seealso marker="#driver_binary_get_refc">driver_binary_get_refc()</seealso>,
<seealso marker="#driver_binary_inc_refc">driver_binary_inc_refc()</seealso>,
and
<seealso marker="#driver_binary_dec_refc">driver_binary_dec_refc()</seealso>.</p>
</note>
<p>Some driver calls, such as <c>driver_enq_binary</c>,
increments the driver reference count, and others, such as
<c>driver_deq</c> decrements it.</p>
<p>Using a driver binary instead of a normal buffer, is often
faster, since the emulator doesn't need to copy the data,
only the pointer is used.</p>
<p>A driver binary allocated in the driver, with
<c>driver_alloc_binary</c>, should be freed in the driver (unless otherwise stated),
with <c>driver_free_binary</c>. (Note that this doesn't
necessarily deallocate it, if the driver is still referred
in the emulator, the ref-count will not go to zero.)</p>
<p>Driver binaries are used in the <c>driver_output2</c> and
<c>driver_outputv</c> calls, and in the queue. Also the
driver call-back <seealso marker="driver_entry#outputv">outputv</seealso> uses driver
binaries.</p>
<p>If the driver of some reason or another, wants to keep a
driver binary around, in a static variable for instance, the
reference count should be incremented,
and the binary can later be freed in the <seealso marker="driver_entry#stop">stop</seealso> call-back, with
<c>driver_free_binary</c>.</p>
<p>Note that since a driver binary is shared by the driver and
the emulator, a binary received from the emulator or sent to
the emulator, must not be changed by the driver.</p>
<p>From erts version 5.5 (OTP release R11B), orig_bytes is
guaranteed to be properly aligned for storage of an array of
doubles (usually 8-byte aligned).</p>
</item>
<tag>ErlDrvData</tag>
<item>
<p>The <c>ErlDrvData</c> is a handle to driver-specific data,
passed to the driver call-backs. It is a pointer, and is
most often casted to a specific pointer in the driver.</p>
</item>
<tag>SysIOVec</tag>
<item>
<p>This is a system I/O vector, as used by <c>writev</c> on
unix and <c>WSASend</c> on Win32. It is used in
<c>ErlIOVec</c>.</p>
</item>
<tag><marker id="ErlIOVec"/>ErlIOVec</tag>
<item>
<p/>
<code type="none">
typedef struct ErlIOVec {
int vsize;
int size;
SysIOVec* iov;
>ErlDrvBinary** binv;
} ErlIOVec;
</code>
<p>The I/O vector used by the emulator and drivers, is a list
of binaries, with a <c>SysIOVec</c> pointing to the buffers
of the binaries. It is used in <c>driver_outputv</c> and the
<seealso marker="driver_entry#outputv">outputv</seealso>
driver call-back. Also, the driver queue is an
<c>ErlIOVec</c>.</p>
</item>
<tag>ErlDrvMonitor</tag>
<item>
<p>When a driver creates a monitor for a process, a
<c>ErlDrvMonitor</c> is filled in. This is an opaque
data-type which can be assigned to but not compared without
using the supplied compare function (i.e. it behaves like a struct).</p>
<p>The driver writer should provide the memory for storing the
monitor when calling <seealso marker="#driver_monitor_process">driver_monitor_process</seealso>. The
address of the data is not stored outside of the driver, so
the <c>ErlDrvMonitor</c> can be used as any other datum, it
can be copied, moved in memory, forgotten etc.</p>
</item>
<tag><marker id="ErlDrvNowData"/>ErlDrvNowData</tag>
<item>
<p>The <c>ErlDrvNowData</c> structure holds a timestamp
consisting of three values measured from some arbitrary
point in the past. The three structure members are:</p>
<taglist>
<tag>megasecs</tag>
<item>The number of whole megaseconds elapsed since the arbitrary
point in time</item>
<tag>secs</tag>
<item>The number of whole seconds elapsed since the arbitrary
point in time</item>
<tag>microsecs</tag>
<item>The number of whole microseconds elapsed since the arbitrary
point in time</item>
</taglist>
</item>
<tag><marker id="ErlDrvPDL"/>ErlDrvPDL</tag>
<item>
<p>If certain port specific data have to be accessed from other
threads than those calling the driver call-backs, a port data lock
can be used in order to synchronize the operations on the data.
Currently, the only port specific data that the emulator
associates with the port data lock is the driver queue.</p>
<p>Normally a driver instance does not have a port data lock. If
the driver instance want to use a port data lock, it has to
create the port data lock by calling
<seealso marker="#driver_pdl_create">driver_pdl_create()</seealso>.
<em>NOTE</em>: Once the port data lock has been created, every
access to data associated with the port data lock have to be done
while having the port data lock locked. The port data lock is
locked, and unlocked, respectively, by use of
<seealso marker="#driver_pdl_lock">driver_pdl_lock()</seealso>, and
<seealso marker="#driver_pdl_unlock">driver_pdl_unlock()</seealso>.</p>
<p>A port data lock is reference counted, and when the reference
count reach zero, it will be destroyed. The emulator will at
least increment the reference count once when the lock is
created and decrement it once when the port associated with
the lock terminates. The emulator will also increment the
reference count when an async job is enqueued and decrement
it after an async job has been invoked, or canceled. Besides
this, it is the responsibility of the driver to ensure that
the reference count does not reach zero before the last use
of the lock by the driver has been made. The reference count
can be read, incremented, and decremented, respectively, by
use of
<seealso marker="#driver_pdl_get_refc">driver_pdl_get_refc()</seealso>,
<seealso marker="#driver_pdl_inc_refc">driver_pdl_inc_refc()</seealso>, and
<seealso marker="#driver_pdl_dec_refc">driver_pdl_dec_refc()</seealso>.</p>
</item>
<tag><marker id="ErlDrvTid"/>ErlDrvTid</tag>
<item>
<p>Thread identifier.</p>
<p>See also:
<seealso marker="#erl_drv_thread_create">erl_drv_thread_create()</seealso>,
<seealso marker="#erl_drv_thread_exit">erl_drv_thread_exit()</seealso>,
<seealso marker="#erl_drv_thread_join">erl_drv_thread_join()</seealso>,
<seealso marker="#erl_drv_thread_self">erl_drv_thread_self()</seealso>,
and
<seealso marker="#erl_drv_equal_tids">erl_drv_equal_tids()</seealso>.
</p>
</item>
<tag><marker id="ErlDrvThreadOpts"/>ErlDrvThreadOpts</tag>
<item>
<p/>
<code type="none">
int suggested_stack_size;
</code>
<p>Thread options structure passed to
<seealso marker="#erl_drv_thread_create">erl_drv_thread_create()</seealso>.
Currently the following fields exist:
</p>
<taglist>
<tag>suggested_stack_size</tag>
<item>A suggestion, in kilo-words, on how large stack to use. A value less
than zero means default size.
</item>
</taglist>
<p>See also:
<seealso marker="#erl_drv_thread_opts_create">erl_drv_thread_opts_create()</seealso>,
<seealso marker="#erl_drv_thread_opts_destroy">erl_drv_thread_opts_destroy()</seealso>,
and
<seealso marker="#erl_drv_thread_create">erl_drv_thread_create()</seealso>.
</p>
</item>
<tag><marker id="ErlDrvMutex"/>ErlDrvMutex</tag>
<item>
<p>Mutual exclusion lock. Used for synchronizing access to shared data.
Only one thread at a time can lock a mutex.
</p>
<p>See also:
<seealso marker="#erl_drv_mutex_create">erl_drv_mutex_create()</seealso>,
<seealso marker="#erl_drv_mutex_destroy">erl_drv_mutex_destroy()</seealso>,
<seealso marker="#erl_drv_mutex_lock">erl_drv_mutex_lock()</seealso>,
<seealso marker="#erl_drv_mutex_trylock">erl_drv_mutex_trylock()</seealso>,
and
<seealso marker="#erl_drv_mutex_unlock">erl_drv_mutex_unlock()</seealso>.
</p>
</item>
<tag><marker id="ErlDrvCond"/>ErlDrvCond</tag>
<item>
<p>Condition variable. Used when threads need to wait for a specific
condition to appear before continuing execution. Condition variables
need to be used with associated mutexes.
</p>
<p>See also:
<seealso marker="#erl_drv_cond_create">erl_drv_cond_create()</seealso>,
<seealso marker="#erl_drv_cond_destroy">erl_drv_cond_destroy()</seealso>,
<seealso marker="#erl_drv_cond_signal">erl_drv_cond_signal()</seealso>,
<seealso marker="#erl_drv_cond_broadcast">erl_drv_cond_broadcast()</seealso>,
and
<seealso marker="#erl_drv_cond_wait">erl_drv_cond_wait()</seealso>.
</p>
</item>
<tag><marker id="ErlDrvRWLock"/>ErlDrvRWLock</tag>
<item>
<p>Read/write lock. Used to allow multiple threads to read shared data
while only allowing one thread to write the same data. Multiple threads
can read lock an rwlock at the same time, while only one thread can
read/write lock an rwlock at a time.
</p>
<p>See also:
<seealso marker="#erl_drv_rwlock_create">erl_drv_rwlock_create()</seealso>,
<seealso marker="#erl_drv_rwlock_destroy">erl_drv_rwlock_destroy()</seealso>,
<seealso marker="#erl_drv_rwlock_rlock">erl_drv_rwlock_rlock()</seealso>,
<seealso marker="#erl_drv_rwlock_tryrlock">erl_drv_rwlock_tryrlock()</seealso>,
<seealso marker="#erl_drv_rwlock_runlock">erl_drv_rwlock_runlock()</seealso>,
<seealso marker="#erl_drv_rwlock_rwlock">erl_drv_rwlock_rwlock()</seealso>,
<seealso marker="#erl_drv_rwlock_tryrwlock">erl_drv_rwlock_tryrwlock()</seealso>,
and
<seealso marker="#erl_drv_rwlock_rwunlock">erl_drv_rwlock_rwunlock()</seealso>.
</p>
</item>
<tag><marker id="ErlDrvTSDKey"/>ErlDrvTSDKey</tag>
<item>
<p>Key which thread specific data can be associated with.</p>
<p>See also:
<seealso marker="#erl_drv_tsd_key_create">erl_drv_tsd_key_create()</seealso>,
<seealso marker="#erl_drv_tsd_key_destroy">erl_drv_tsd_key_destroy()</seealso>,
<seealso marker="#erl_drv_tsd_set">erl_drv_tsd_set()</seealso>,
and
<seealso marker="#erl_drv_tsd_get">erl_drv_tsd_get()</seealso>.
</p>
</item>
</taglist>
</section>
<funcs>
<func>
<name><ret>void</ret><nametext>driver_system_info(ErlDrvSysInfo *sys_info_ptr, size_t size)</nametext></name>
<fsummary>Get information about the Erlang runtime system</fsummary>
<desc>
<marker id="driver_system_info"></marker>
<p>This function will write information about the Erlang runtime
system into the
<seealso marker="#ErlDrvSysInfo">ErlDrvSysInfo</seealso>
structure referred to by the first argument. The second
argument should be the size of the
<seealso marker="#ErlDrvSysInfo">ErlDrvSysInfo</seealso>
structure, i.e., <c>sizeof(ErlDrvSysInfo)</c>.</p>
<p>See the documentation of the
<seealso marker="#ErlDrvSysInfo">ErlDrvSysInfo</seealso>
structure for information about specific fields.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_output(ErlDrvPort port, char *buf, int len)</nametext></name>
<fsummary>Send data from driver to port owner</fsummary>
<desc>
<marker id="driver_output"></marker>
<p>The <c>driver_output</c> function is used to send data from
the driver up to the emulator. The data will be received as
terms or binary data, depending on how the driver port was
opened.</p>
<p>The data is queued in the port owner process' message
queue. Note that this does not yield to the emulator. (Since
the driver and the emulator runs in the same thread.)</p>
<p>The parameter <c>buf</c> points to the data to send, and
<c>len</c> is the number of bytes.</p>
<p>The return value for all output functions is 0. (Unless the
driver is used for distribution, in which case it can fail
and return -1. For normal use, the output function always
returns 0.)</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_output2(ErlDrvPort port, char *hbuf, int hlen, char *buf, int len)</nametext></name>
<fsummary>Send data and binary data to port owner</fsummary>
<desc>
<marker id="driver_output2"></marker>
<p>The <c>driver_output2</c> function first sends <c>hbuf</c>
(length in <c>hlen</c>) data as a list, regardless of port
settings. Then <c>buf</c> is sent as a binary or list.
E.g. if <c>hlen</c> is 3 then the port owner process will
receive <c>[H1, H2, H3 | T]</c>.</p>
<p>The point of sending data as a list header, is to facilitate
matching on the data received.</p>
<p>The return value is 0 for normal use.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_output_binary(ErlDrvPort port, char *hbuf, int hlen, ErlDrvBinary* bin, int offset, int len)</nametext></name>
<fsummary>Send data from a driver binary to port owner</fsummary>
<desc>
<marker id="driver_output_binary"></marker>
<p>This function sends data to port owner process from a
driver binary, it has a header buffer (<c>hbuf</c>
and <c>hlen</c>) just like <c>driver_output2</c>. The
<c>hbuf</c> parameter can be <c>NULL</c>.</p>
<p>The parameter <c>offset</c> is an offset into the binary and
<c>len</c> is the number of bytes to send.</p>
<p>Driver binaries are created with <c>driver_alloc_binary</c>.</p>
<p>The data in the header is sent as a list and the binary as
an Erlang binary in the tail of the list.</p>
<p>E.g. if <c>hlen</c> is 2, then the port owner process will
receive <c><![CDATA[[H1, H2 | <<T>>]]]></c>.</p>
<p>The return value is 0 for normal use.</p>
<p>Note that, using the binary syntax in Erlang, the driver
application can match the header directly from the binary,
so the header can be put in the binary, and hlen can be set
to 0.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_outputv(ErlDrvPort port, char* hbuf, int hlen, ErlIOVec *ev, int skip)</nametext></name>
<fsummary>Send vectorized data to port owner</fsummary>
<desc>
<marker id="driver_outputv"></marker>
<p>This function sends data from an IO vector, <c>ev</c>, to
the port owner process. It has a header buffer (<c>hbuf</c>
and <c>hlen</c>), just like <c>driver_output2</c>.</p>
<p>The <c>skip</c> parameter is a number of bytes to skip of
the <c>ev</c> vector from the head.</p>
<p>You get vectors of <c>ErlIOVec</c> type from the driver
queue (see below), and the <seealso marker="driver_entry#outputv">outputv</seealso> driver entry
function. You can also make them yourself, if you want to
send several <c>ErlDrvBinary</c> buffers at once. Often
it is faster to use <c>driver_output</c> or
<c>driver_output_binary</c>.</p>
<p>E.g. if <c>hlen</c> is 2 and <c>ev</c> points to an array of
three binaries, the port owner process will receive <c><![CDATA[[H1, H2, <<B1>>, <<B2>> | <<B3>>]]]></c>.</p>
<p>The return value is 0 for normal use.</p>
<p>The comment for <c>driver_output_binary</c> applies for
<c>driver_outputv</c> too.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_vec_to_buf(ErlIOVec *ev, char *buf, int len)</nametext></name>
<fsummary>Collect data segments into a buffer</fsummary>
<desc>
<marker id="driver_vec_to_buf"></marker>
<p>This function collects several segments of data, referenced
by <c>ev</c>, by copying them in order to the buffer
<c>buf</c>, of the size <c>len</c>.</p>
<p>If the data is to be sent from the driver to the port owner
process, it is faster to use <c>driver_outputv</c>.</p>
<p>The return value is the space left in the buffer, i.e. if
the <c>ev</c> contains less than <c>len</c> bytes it's the
difference, and if <c>ev</c> contains <c>len</c> bytes or
more, it's 0. This is faster if there is more than one header byte,
since the binary syntax can construct integers directly from
the binary.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_set_timer(ErlDrvPort port, unsigned long time)</nametext></name>
<fsummary>Set a timer to call the driver</fsummary>
<desc>
<marker id="driver_set_timer"></marker>
<p>This function sets a timer on the driver, which will count
down and call the driver when it is timed out. The
<c>time</c> parameter is the time in milliseconds before the
timer expires.</p>
<p>When the timer reaches 0 and expires, the driver entry
function <seealso marker="driver_entry#emulator">timeout</seealso> is called.</p>
<p>Note that there is only one timer on each driver instance;
setting a new timer will replace an older one.</p>
<p>Return value i 0 (-1 only when the <c>timeout</c> driver
function is <c>NULL</c>).</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_cancel_timer(ErlDrvPort port)</nametext></name>
<fsummary>Cancel a previously set timer</fsummary>
<desc>
<marker id="driver_cancel_timer"></marker>
<p>This function cancels a timer set with
<c>driver_set_timer</c>.</p>
<p>The return value is 0.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_read_timer(ErlDrvPort port, unsigned long *time_left)</nametext></name>
<fsummary>Read the time left before timeout</fsummary>
<desc>
<marker id="driver_read_timer"></marker>
<p>This function reads the current time of a timer, and places
the result in <c>time_left</c>. This is the time in
milliseconds, before the timeout will occur.</p>
<p>The return value is 0.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_get_now(ErlDrvNowData *now)</nametext></name>
<fsummary>Read a system timestamp</fsummary>
<desc>
<marker id="driver_get_now"></marker>
<p>This function reads a timestamp into the memory pointed to by
the parameter <c>now</c>. See the description of <seealso marker="#ErlDrvNowData">ErlDrvNowData</seealso> for
specification of it's fields. </p>
<p>The return value is 0 unless the <c>now</c> pointer is not
valid, in which case it is < 0. </p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_select(ErlDrvPort port, ErlDrvEvent event, int mode, int on)</nametext></name>
<fsummary>Provide an event for having the emulator call the driver</fsummary>
<desc>
<marker id="driver_select"></marker>
<p>This function is used by drivers to provide the emulator with
events to check for. This enables the emulator to call the driver
when something has happened asynchronously.</p>
<p>The <c>event</c> argument identifies an OS-specific event object.
On Unix systems, the functions <c>select</c>/<c>poll</c> are used. The
event object must be a socket or pipe (or other object that
<c>select</c>/<c>poll</c> can use).
On windows, the Win32 API function <c>WaitForMultipleObjects</c>
is used. This places other restriction on the event object.
Refer to the Win32 SDK documentation.</p>
<p>The <c>on</c> parameter should be <c>1</c> for setting events
and <c>0</c> for clearing them.</p>
<p>The <c>mode</c> argument is bitwise-or combination of
<c>ERL_DRV_READ</c>, <c>ERL_DRV_WRITE</c> and <c>ERL_DRV_USE</c>.
The first two specifies whether to wait for read events and/or write
events. A fired read event will call
<seealso marker="driver_entry#ready_input">ready_input</seealso>
while a fired write event will call
<seealso marker="driver_entry#ready_output">ready_output</seealso>.
</p>
<note>
<p>Some OS (Windows) does not differ between read and write events.
The call-back for a fired event then only depends on the value of <c>mode</c>.</p>
</note>
<p><c>ERL_DRV_USE</c> specifies if we are using the event object or if we want to close it.
On an emulator with SMP support, it is not safe to clear all events
and then close the event object after <c>driver_select</c> has
returned. Another thread may still be using the event object
internally. To safely close an event object call
<c>driver_select</c> with <c>ERL_DRV_USE</c> and <c>on==0</c>. That
will clear all events and then call
<seealso marker="driver_entry#stop_select">stop_select</seealso>
when it is safe to close the event object.
<c>ERL_DRV_USE</c> should be set together with the first event
for an event object. It is harmless to set <c>ERL_DRV_USE</c>
even though it already has been done. Clearing all events but keeping
<c>ERL_DRV_USE</c> set will indicate that we are using the event
object and probably will set events for it again.</p>
<note>
<p>ERL_DRV_USE was added in OTP release R13. Old drivers will still work
as before. But it is recommended to update them to use <c>ERL_DRV_USE</c> and
<c>stop_select</c> to make sure that event objects are closed in a safe way.</p>
</note>
<p>The return value is 0 (Failure, -1, only if the
<c>ready_input</c>/<c>ready_output</c> is
<c>NULL</c>.</p>
</desc>
</func>
<func>
<name><ret>void *</ret><nametext>driver_alloc(size_t size)</nametext></name>
<fsummary>Allocate memory</fsummary>
<desc>
<marker id="driver_alloc"></marker>
<p>This function allocates a memory block of the size specified
in <c>size</c>, and returns it. This only fails on out of
memory, in that case <c>NULL</c> is returned. (This is most
often a wrapper for <c>malloc</c>).</p>
<p>Memory allocated must be explicitly freed with a corresponding
call to <c>driver_free</c> (unless otherwise stated).</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void *</ret><nametext>driver_realloc(void *ptr, size_t size)</nametext></name>
<fsummary>Resize an allocated memory block</fsummary>
<desc>
<marker id="driver_realloc"></marker>
<p>This function resizes a memory block, either in place, or by
allocating a new block, copying the data and freeing the old
block. A pointer is returned to the reallocated memory. On
failure (out of memory), <c>NULL</c> is returned. (This is
most often a wrapper for <c>realloc</c>.)</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>driver_free(void *ptr)</nametext></name>
<fsummary>Free an allocated memory block</fsummary>
<desc>
<marker id="driver_free"></marker>
<p>This function frees the memory pointed to by <c>ptr</c>. The
memory should have been allocated with
<c>driver_alloc</c>. All allocated memory should be
deallocated, just once. There is no garbage collection in
drivers.</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvBinary*</ret><nametext>driver_alloc_binary(int size)</nametext></name>
<fsummary>Allocate a driver binary</fsummary>
<desc>
<marker id="driver_alloc_binary"></marker>
<p>This function allocates a driver binary with a memory block
of at least <c>size</c> bytes, and returns a pointer to it,
or NULL on failure (out of memory). When a driver binary has
been sent to the emulator, it must not be altered. Every
allocated binary should be freed by a corresponding call to
<c>driver_free_binary</c> (unless otherwise stated).</p>
<p>Note that a driver binary has an internal reference counter,
this means that calling <c>driver_free_binary</c> it may not
actually dispose of it. If it's sent to the emulator, it may
be referenced there.</p>
<p>The driver binary has a field, <c>orig_bytes</c>, which
marks the start of the data in the binary.</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvBinary*</ret><nametext>driver_realloc_binary(ErlDrvBinary *bin, int size)</nametext></name>
<fsummary>Resize a driver binary</fsummary>
<desc>
<marker id="driver_realloc_binary"></marker>
<p>This function resizes a driver binary, while keeping the
data. The resized driver binary is returned. On failure (out
of memory), <c>NULL</c> is returned.</p>
<p>This function is only thread-safe when the emulator with SMP
support is used.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>driver_free_binary(ErlDrvBinary *bin)</nametext></name>
<fsummary>Free a driver binary</fsummary>
<desc>
<marker id="driver_free_binary"></marker>
<p>This function frees a driver binary <c>bin</c>, allocated
previously with <c>driver_alloc_binary</c>. Since binaries
in Erlang are reference counted, the binary may still be
around.</p>
<p>This function is only thread-safe when the emulator with SMP
support is used.</p>
</desc>
</func>
<func>
<name><ret>long</ret><nametext>driver_binary_get_refc(ErlDrvBinary *bin)</nametext></name>
<fsummary>Get the reference count of a driver binary</fsummary>
<desc>
<marker id="driver_binary_get_refc"></marker>
<p>Returns current reference count on <c>bin</c>.</p>
<p>This function is only thread-safe when the emulator with SMP
support is used.</p>
</desc>
</func>
<func>
<name><ret>long</ret><nametext>driver_binary_inc_refc(ErlDrvBinary *bin)</nametext></name>
<fsummary>Increment the reference count of a driver binary</fsummary>
<desc>
<marker id="driver_binary_inc_refc"></marker>
<p>Increments the reference count on <c>bin</c> and returns
the reference count reached after the increment.</p>
<p>This function is only thread-safe when the emulator with SMP
support is used.</p>
</desc>
</func>
<func>
<name><ret>long</ret><nametext>driver_binary_dec_refc(ErlDrvBinary *bin)</nametext></name>
<fsummary>Decrement the reference count of a driver binary</fsummary>
<desc>
<marker id="driver_binary_dec_refc"></marker>
<p>Decrements the reference count on <c>bin</c> and returns
the reference count reached after the decrement.</p>
<p>This function is only thread-safe when the emulator with SMP
support is used.</p>
<note>
<p>You should normally decrement the reference count of a
driver binary by calling
<seealso marker="#driver_free_binary">driver_free_binary()</seealso>.
<c>driver_binary_dec_refc()</c> does <em>not</em> free
the binary if the reference count reaches zero. <em>Only</em>
use <c>driver_binary_dec_refc()</c> when you are sure
<em>not</em> to reach a reference count of zero.</p>
</note>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_enq(ErlDrvPort port, char* buf, int len)</nametext></name>
<fsummary>Enqueue data in the driver queue</fsummary>
<desc>
<marker id="driver_enq"></marker>
<p>This function enqueues data in the driver queue. The data in
<c>buf</c> is copied (<c>len</c> bytes) and placed at the
end of the driver queue. The driver queue is normally used
in a FIFO way.</p>
<p>The driver queue is available to queue output from the
emulator to the driver (data from the driver to the emulator
is queued by the emulator in normal erlang message
queues). This can be useful if the driver has to wait for
slow devices etc, and wants to yield back to the
emulator. The driver queue is implemented as an ErlIOVec.</p>
<p>When the queue contains data, the driver won't close, until
the queue is empty.</p>
<p>The return value is 0.</p>
<p>This function can be called from an arbitrary thread if a
<seealso marker="#ErlDrvPDL">port data lock</seealso>
associated with the <c>port</c> is locked by the calling
thread during the call.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_pushq(ErlDrvPort port, char* buf, int len)</nametext></name>
<fsummary>Push data at the head of the driver queue</fsummary>
<desc>
<marker id="driver_pushq"></marker>
<p>This function puts data at the head of the driver queue. The
data in <c>buf</c> is copied (<c>len</c> bytes) and placed
at the beginning of the queue.</p>
<p>The return value is 0.</p>
<p>This function can be called from an arbitrary thread if a
<seealso marker="#ErlDrvPDL">port data lock</seealso>
associated with the <c>port</c> is locked by the calling
thread during the call.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_deq(ErlDrvPort port, int size)</nametext></name>
<fsummary>Dequeue data from the head of the driver queue</fsummary>
<desc>
<marker id="driver_deq"></marker>
<p>This function dequeues data by moving the head pointer
forward in the driver queue by <c>size</c> bytes. The data
in the queue will be deallocated.</p>
<p>The return value is the number of bytes remaining in the queue
or -1 on failure.</p>
<p>This function can be called from an arbitrary thread if a
<seealso marker="#ErlDrvPDL">port data lock</seealso>
associated with the <c>port</c> is locked by the calling
thread during the call.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_sizeq(ErlDrvPort port)</nametext></name>
<fsummary>Return the size of the driver queue</fsummary>
<desc>
<marker id="driver_sizeq"></marker>
<p>This function returns the number of bytes currently in the
driver queue.</p>
<p>This function can be called from an arbitrary thread if a
<seealso marker="#ErlDrvPDL">port data lock</seealso>
associated with the <c>port</c> is locked by the calling
thread during the call.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_enq_bin(ErlDrvPort port, ErlDrvBinary *bin, int offset, int len)</nametext></name>
<fsummary>Enqueue binary in the driver queue</fsummary>
<desc>
<marker id="driver_enq_bin"></marker>
<p>This function enqueues a driver binary in the driver
queue. The data in <c>bin</c> at <c>offset</c> with length
<c>len</c> is placed at the end of the queue. This function
is most often faster than <c>driver_enq</c>, because the
data doesn't have to be copied.</p>
<p>This function can be called from an arbitrary thread if a
<seealso marker="#ErlDrvPDL">port data lock</seealso>
associated with the <c>port</c> is locked by the calling
thread during the call.</p>
<p>The return value is 0.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_pushq_bin(ErlDrvPort port, ErlDrvBinary *bin, int offset, int len)</nametext></name>
<fsummary>Push binary at the head of the driver queue</fsummary>
<desc>
<marker id="driver_pushq_bin"></marker>
<p>This function puts data in the binary <c>bin</c>, at
<c>offset</c> with length <c>len</c> at the head of the
driver queue. It is most often faster than
<c>driver_pushq</c>, because the data doesn't have to be
copied.</p>
<p>This function can be called from an arbitrary thread if a
<seealso marker="#ErlDrvPDL">port data lock</seealso>
associated with the <c>port</c> is locked by the calling
thread during the call.</p>
<p>The return value is 0.</p>
</desc>
</func>
<func>
<name><ret>SysIOVec*</ret><nametext>driver_peekq(ErlDrvPort port, int *vlen)</nametext></name>
<fsummary>Get the driver queue as a vector</fsummary>
<desc>
<marker id="driver_peekq"></marker>
<p>This function retrieves the driver queue as a pointer to an
array of <c>SysIOVec</c>s. It also returns the number of
elements in <c>vlen</c>. This is the only way to get data
out of the queue.</p>
<p>Nothing is remove from the queue by this function, that must be done
with <c>driver_deq</c>.</p>
<p>The returned array is suitable to use with the Unix system
call <c>writev</c>.</p>
<p>This function can be called from an arbitrary thread if a
<seealso marker="#ErlDrvPDL">port data lock</seealso>
associated with the <c>port</c> is locked by the calling
thread during the call.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_enqv(ErlDrvPort port, ErlIOVec *ev, int skip)</nametext></name>
<fsummary>Enqueue vector in the driver queue</fsummary>
<desc>
<marker id="driver_enqv"></marker>
<p>This function enqueues the data in <c>ev</c>, skipping the
first <c>skip</c> bytes of it, at the end of the driver
queue. It is faster than <c>driver_enq</c>, because the data
doesn't have to be copied.</p>
<p>The return value is 0.</p>
<p>This function can be called from an arbitrary thread if a
<seealso marker="#ErlDrvPDL">port data lock</seealso>
associated with the <c>port</c> is locked by the calling
thread during the call.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_pushqv(ErlDrvPort port, ErlIOVec *ev, int skip)</nametext></name>
<fsummary>Push vector at the head of the driver queue</fsummary>
<desc>
<marker id="driver_pushqv"></marker>
<p>This function puts the data in <c>ev</c>, skipping the first
<c>skip</c> bytes of it, at the head of the driver queue.
It is faster than <c>driver_pushq</c>, because the data
doesn't have to be copied.</p>
<p>The return value is 0.</p>
<p>This function can be called from an arbitrary thread if a
<seealso marker="#ErlDrvPDL">port data lock</seealso>
associated with the <c>port</c> is locked by the calling
thread during the call.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvPDL</ret><nametext>driver_pdl_create(ErlDrvPort port)</nametext></name>
<fsummary>Create a port data lock</fsummary>
<desc>
<marker id="driver_pdl_create"></marker>
<p>This function creates a port data lock associated with
the <c>port</c>. <em>NOTE</em>: Once a port data lock has
been created, it has to be locked during all operations
on the driver queue of the <c>port</c>.</p>
<p>On success a newly created port data lock is returned. On
failure <c>NULL</c> is returned. <c>driver_pdl_create()</c> will
fail if <c>port</c> is invalid or if a port data lock already has
been associated with the <c>port</c>.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>driver_pdl_lock(ErlDrvPDL pdl)</nametext></name>
<fsummary>Lock port data lock</fsummary>
<desc>
<marker id="driver_pdl_lock"></marker>
<p>This function locks the port data lock passed as argument
(<c>pdl</c>).</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>driver_pdl_unlock(ErlDrvPDL pdl)</nametext></name>
<fsummary>Unlock port data lock</fsummary>
<desc>
<marker id="driver_pdl_unlock"></marker>
<p>This function unlocks the port data lock passed as argument
(<c>pdl</c>).</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>long</ret><nametext>driver_pdl_get_refc(ErlDrvPDL pdl)</nametext></name>
<fsummary></fsummary>
<desc>
<marker id="driver_pdl_get_refc"></marker>
<p>This function returns the current reference count of
the port data lock passed as argument (<c>pdl</c>).</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>long</ret><nametext>driver_pdl_inc_refc(ErlDrvPDL pdl)</nametext></name>
<fsummary></fsummary>
<desc>
<marker id="driver_pdl_inc_refc"></marker>
<p>This function increments the reference count of
the port data lock passed as argument (<c>pdl</c>).</p>
<p>The current reference count after the increment has
been performed is returned.</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>long</ret><nametext>driver_pdl_dec_refc(ErlDrvPDL pdl)</nametext></name>
<fsummary></fsummary>
<desc>
<marker id="driver_pdl_dec_refc"></marker>
<p>This function decrements the reference count of
the port data lock passed as argument (<c>pdl</c>).</p>
<p>The current reference count after the decrement has
been performed is returned.</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_monitor_process(ErlDrvPort port, ErlDrvTermData process, ErlDrvMonitor *monitor)</nametext></name>
<fsummary>Monitor a process from a driver</fsummary>
<desc>
<marker id="driver_monitor_process"></marker>
<p>Start monitoring a process from a driver. When a process is
monitored, a process exit will result in a call to the
provided <seealso marker="driver_entry#process_exit">process_exit</seealso> call-back
in the <seealso marker="driver_entry">ErlDrvEntry</seealso>
structure. The <c>ErlDrvMonitor</c> structure is filled in, for later
removal or compare.</p>
<p>The <c>process</c> parameter should be the return value of an
earlier call to <seealso marker="#driver_caller">driver_caller</seealso> or <seealso marker="#driver_connected">driver_connected</seealso> call.</p>
<p>The function returns 0 on success, < 0 if no call-back is
provided and > 0 if the process is no longer alive.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_demonitor_process(ErlDrvPort port, const ErlDrvMonitor *monitor)</nametext></name>
<fsummary>Stop monitoring a process from a driver</fsummary>
<desc>
<marker id="driver_demonitor_process"></marker>
<p>This function cancels an monitor created earlier. </p>
<p>The function returns 0 if a monitor was removed and > 0
if the monitor did no longer exist.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvTermData</ret><nametext>driver_get_monitored_process(ErlDrvPort port, const ErlDrvMonitor *monitor)</nametext></name>
<fsummary>Retrieve the process id from a monitor</fsummary>
<desc>
<marker id="driver_get_monitored_process"></marker>
<p>The function returns the process id associated with a living
monitor. It can be used in the <c>process_exit</c> call-back to
get the process identification for the exiting process.</p>
<p>The function returns <c>driver_term_nil</c> if the monitor
no longer exists.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_compare_monitors(const ErlDrvMonitor *monitor1, const ErlDrvMonitor *monitor2)</nametext></name>
<fsummary>Compare two monitors</fsummary>
<desc>
<marker id="driver_compare_monitors"></marker>
<p>This function is used to compare two <c>ErlDrvMonitor</c>s. It
can also be used to imply some artificial order on monitors,
for whatever reason.</p>
<p>The function returns 0 if <c>monitor1</c> and
<c>monitor2</c> are equal, < 0 if <c>monitor1</c> is less
than <c>monitor2</c> and > 0 if <c>monitor1</c> is greater
than <c>monitor2</c>.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>add_driver_entry(ErlDrvEntry *de)</nametext></name>
<fsummary>Add a driver entry</fsummary>
<desc>
<marker id="add_driver_entry"></marker>
<p>This function adds a driver entry to the list of drivers
known by Erlang. The <seealso marker="driver_entry#init">init</seealso> function of the <c>de</c>
parameter is called.</p>
<note>
<p>To use this function for adding drivers residing in
dynamically loaded code is dangerous. If the driver code
for the added driver resides in the same dynamically
loaded module (i.e. <c>.so</c> file) as a normal
dynamically loaded driver (loaded with the <c>erl_ddll</c>
interface), the caller should call <seealso marker="#driver_lock_driver">driver_lock_driver</seealso> before
adding driver entries.</p>
<p>Use of this function is generally deprecated.</p>
</note>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>remove_driver_entry(ErlDrvEntry *de)</nametext></name>
<fsummary>Remove a driver entry</fsummary>
<desc>
<marker id="remove_driver_entry"></marker>
<p>This function removes a driver entry <c>de</c> previously
added with <c>add_driver_entry</c>.</p>
<p>Driver entries added by the <c>erl_ddll</c> erlang interface can
not be removed by using this interface.</p>
</desc>
</func>
<func>
<name><ret>char*</ret><nametext>erl_errno_id(int error)</nametext></name>
<fsummary>Get erlang error atom name from error number</fsummary>
<desc>
<marker id="erl_errno_id"></marker>
<p>This function returns the atom name of the erlang error,
given the error number in <c>error</c>. Error atoms are:
<c>einval</c>, <c>enoent</c>, etc. It can be used to make
error terms from the driver.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>set_busy_port(ErlDrvPort port, int on)</nametext></name>
<fsummary>Signal or unsignal port as busy</fsummary>
<desc>
<marker id="set_busy_port"></marker>
<p>This function set and resets the busy status of the port. If
<c>on</c> is 1, the port is set to busy, if it's 0 the port
is set to not busy.</p>
<p>When the port is busy, sending to it with <c>Port ! Data</c>
or <c>port_command/2</c>, will block the port owner process,
until the port is signaled as not busy.</p>
<p>If the
<seealso marker="driver_entry#driver_flags"><![CDATA[ERL_DRV_FLAG_SOFT_BUSY]]></seealso>
has been set in the
<seealso marker="driver_entry">driver_entry</seealso>,
data can be forced into the driver via
<seealso marker="erlang#port_command/3">port_command(Port, Data, [force])</seealso>
even though the driver has signaled that it is busy.
</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>set_port_control_flags(ErlDrvPort port, int flags)</nametext></name>
<fsummary>Set flags on how to handle control entry function</fsummary>
<desc>
<marker id="set_port_control_flags"></marker>
<p>This function sets flags for how the <seealso marker="driver_entry#control">control</seealso> driver entry
function will return data to the port owner process. (The
<c>control</c> function is called from <c>port_control/3</c>
in erlang.)</p>
<p>Currently there are only two meaningful values for
<c>flags</c>: 0 means that data is returned in a list, and
<c>PORT_CONTROL_FLAG_BINARY</c> means data is returned as
a binary from <c>control</c>.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_failure_eof(ErlDrvPort port)</nametext></name>
<fsummary>Fail with EOF</fsummary>
<desc>
<marker id="driver_failure_eof"></marker>
<p>This function signals to erlang that the driver has
encountered an EOF and should be closed, unless the port was
opened with the <c>eof</c> option, in that case eof is sent
to the port. Otherwise, the port is close and an
<c>'EXIT'</c> message is sent to the port owner process.</p>
<p>The return value is 0.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_failure_atom(ErlDrvPort port, char *string)</nametext></name>
<name><ret>int</ret><nametext>driver_failure_posix(ErlDrvPort port, int error)</nametext></name>
<name><ret>int</ret><nametext>driver_failure(ErlDrvPort port, int error)</nametext></name>
<fsummary>Fail with error</fsummary>
<desc>
<marker id="driver_failure_atom"></marker>
<marker id="driver_failure_posix"></marker>
<marker id="driver_failure"></marker>
<p>These functions signal to Erlang that the driver has
encountered an error and should be closed. The port is
closed and the tuple <c>{'EXIT', error, Err}</c>, is sent to
the port owner process, where error is an error atom
(<c>driver_failure_atom</c> and
<c>driver_failure_posix</c>), or an integer
(<c>driver_failure</c>).</p>
<p>The driver should fail only when in severe error situations,
when the driver cannot possibly keep open, for instance
buffer allocation gets out of memory. Normal errors is more
appropriate to handle with sending error codes with
<c>driver_output</c>.</p>
<p>The return value is 0.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvTermData</ret><nametext>driver_connected(ErlDrvPort port)</nametext></name>
<fsummary>Return the port owner process</fsummary>
<desc>
<marker id="driver_connected"></marker>
<p>This function returns the port owner process.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvTermData</ret><nametext>driver_caller(ErlDrvPort port)</nametext></name>
<fsummary>Return the process making the driver call</fsummary>
<desc>
<marker id="driver_caller"></marker>
<p>This function returns the process id of the process that
made the current call to the driver. The process id can be
used with <c>driver_send_term</c> to send back data to the
caller. <c>driver_caller()</c> only return valid data
when currently executing in one of the following driver
callbacks:</p>
<taglist>
<tag><seealso marker="driver_entry#start">start</seealso></tag>
<item>Called from <c>open_port/2</c>.</item>
<tag><seealso marker="driver_entry#output">output</seealso></tag>
<item>Called from <c>erlang:send/2</c>, and
<c>erlang:port_command/2</c></item>
<tag><seealso marker="driver_entry#outputv">outputv</seealso></tag>
<item>Called from <c>erlang:send/2</c>, and
<c>erlang:port_command/2</c></item>
<tag><seealso marker="driver_entry#control">control</seealso></tag>
<item>Called from <c>erlang:port_control/3</c></item>
<tag><seealso marker="driver_entry#call">call</seealso></tag>
<item>Called from <c>erlang:port_call/3</c></item>
</taglist>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_output_term(ErlDrvPort port, ErlDrvTermData* term, int n)</nametext></name>
<fsummary>Send term data from driver to port owner</fsummary>
<desc>
<marker id="driver_output_term"></marker>
<p>This functions sends data in the special driver term
format. This is a fast way to deliver term data from a
driver. It also needs no binary conversion, so the port
owner process receives data as normal Erlang terms.</p>
<p>The <c>term</c> parameter points to an array of
<c>ErlDrvTermData</c>, with <c>n</c> elements. This array
contains terms described in the driver term format. Every
term consists of one to four elements in the array. The
term first has a term type, and then arguments.</p>
<p>Tuple and lists (with the exception of strings, see below),
are built in reverse polish notation, so that to build a
tuple, the elements are given first, and then the tuple
term, with a count. Likewise for lists.</p>
<p>A tuple must be specified with the number of elements. (The
elements precedes the <c>ERL_DRV_TUPLE</c> term.)</p>
<p>A list must be specified with the number of elements,
including the tail, which is the last term preceding
<c>ERL_DRV_LIST</c>.</p>
<p>The special term <c>ERL_DRV_STRING_CONS</c> is used to
"splice" in a string in a list, a string given this way is
not a list per se, but the elements are elements of the
surrounding list.</p>
<pre>
Term type Argument(s)
===========================================
ERL_DRV_NIL
ERL_DRV_ATOM ErlDrvTermData atom (from driver_mk_atom(char *string))
ERL_DRV_INT ErlDrvSInt integer
ERL_DRV_UINT ErlDrvUInt integer
ERL_DRV_INT64 ErlDrvSInt64 *integer_ptr
ERL_DRV_UINT64 ErlDrvUInt64 *integer_ptr
ERL_DRV_PORT ErlDrvTermData port (from driver_mk_port(ErlDrvPort port))
ERL_DRV_BINARY ErlDrvBinary *bin, ErlDrvUInt len, ErlDrvUInt offset
ERL_DRV_BUF2BINARY char *buf, ErlDrvUInt len
ERL_DRV_STRING char *str, int len
ERL_DRV_TUPLE int sz
ERL_DRV_LIST int sz
ERL_DRV_PID ErlDrvTermData pid (from driver_connected(ErlDrvPort port) or driver_caller(ErlDrvPort port))
ERL_DRV_STRING_CONS char *str, int len
ERL_DRV_FLOAT double *dbl
ERL_DRV_EXT2TERM char *buf, ErlDrvUInt len
</pre>
<p>The unsigned integer data type <c>ErlDrvUInt</c> and the
signed integer data type <c>ErlDrvSInt</c> are 64 bits wide
on a 64 bit runtime system and 32 bits wide on a 32 bit
runtime system. They were introduced in erts version 5.6,
and replaced some of the <c>int</c> arguments in the list above.
</p>
<p>The unsigned integer data type <c>ErlDrvUInt64</c> and the
signed integer data type <c>ErlDrvSInt64</c> are always 64 bits
wide. They were introduced in erts version 5.7.4.
</p>
<p>To build the tuple <c>{tcp, Port, [100 | Binary]}</c>, the
following call could be made.</p>
<code type="none"><![CDATA[
ErlDrvBinary* bin = ...
ErlDrvPort port = ...
ErlDrvTermData spec[] = {
ERL_DRV_ATOM, driver_mk_atom("tcp"),
ERL_DRV_PORT, driver_mk_port(port),
ERL_DRV_INT, 100,
ERL_DRV_BINARY, bin, 50, 0,
ERL_DRV_LIST, 2,
ERL_DRV_TUPLE, 3,
};
driver_output_term(port, spec, sizeof(spec) / sizeof(spec[0]));
]]>
</code>
<p>Where <c>bin</c> is a driver binary of length at least 50
and <c>port</c> is a port handle. Note that the <c>ERL_DRV_LIST</c>
comes after the elements of the list, likewise the
<c>ERL_DRV_TUPLE</c>.</p>
<p>The term <c>ERL_DRV_STRING_CONS</c> is a way to construct
strings. It works differently from how <c>ERL_DRV_STRING</c>
works. <c>ERL_DRV_STRING_CONS</c> builds a string list in
reverse order, (as opposed to how <c>ERL_DRV_LIST</c>
works), concatenating the strings added to a list. The tail
must be given before <c>ERL_DRV_STRING_CONS</c>.</p>
<p>The <c>ERL_DRV_STRING</c> constructs a string, and ends
it. (So it's the same as <c>ERL_DRV_NIL</c> followed by
<c>ERL_DRV_STRING_CONS</c>.)</p>
<code type="none"><![CDATA[
/* to send [x, "abc", y] to the port: */
ErlDrvTermData spec[] = {
ERL_DRV_ATOM, driver_mk_atom("x"),
ERL_DRV_STRING, (ErlDrvTermData)"abc", 3,
ERL_DRV_ATOM, driver_mk_atom("y"),
ERL_DRV_NIL,
ERL_DRV_LIST, 4
};
driver_output_term(port, spec, sizeof(spec) / sizeof(spec[0]));
]]></code>
<p></p>
<code type="none"><![CDATA[
/* to send "abc123" to the port: */
ErlDrvTermData spec[] = {
ERL_DRV_NIL, /* with STRING_CONS, the tail comes first */
ERL_DRV_STRING_CONS, (ErlDrvTermData)"123", 3,
ERL_DRV_STRING_CONS, (ErlDrvTermData)"abc", 3,
};
driver_output_term(port, spec, sizeof(spec) / sizeof(spec[0]));
]]></code>
<p>The <c>ERL_DRV_EXT2TERM</c> term type is used for passing a
term encoded with the
<seealso marker="erl_ext_dist">external format</seealso>,
i.e., a term that has been encoded by
<seealso marker="erts:erlang#term_to_binary/2">erlang:term_to_binary</seealso>,
<seealso marker="erl_interface:ei">erl_interface</seealso>, etc.
For example, if <c>binp</c> is a pointer to an <c>ErlDrvBinary</c>
that contains the term <c>{17, 4711}</c> encoded with the
<seealso marker="erl_ext_dist">external format</seealso>
and you want to wrap it in a two tuple with the tag <c>my_tag</c>,
i.e., <c>{my_tag, {17, 4711}}</c>, you can do as follows:
</p>
<code type="none"><![CDATA[
ErlDrvTermData spec[] = {
ERL_DRV_ATOM, driver_mk_atom("my_tag"),
ERL_DRV_EXT2TERM, (ErlDrvTermData) binp->orig_bytes, binp->orig_size
ERL_DRV_TUPLE, 2,
};
driver_output_term(port, spec, sizeof(spec) / sizeof(spec[0]));
]]></code>
<p>If you want to pass a binary and doesn't already have the content
of the binary in an <c>ErlDrvBinary</c>, you can benefit from using
<c>ERL_DRV_BUF2BINARY</c> instead of creating an <c>ErlDrvBinary</c>
via <c>driver_alloc_binary()</c> and then pass the binary via
<c>ERL_DRV_BINARY</c>. The runtime system will often allocate
binaries smarter if <c>ERL_DRV_BUF2BINARY</c> is used.
However, if the content of the binary to pass already resides in
an <c>ErlDrvBinary</c>, it is normally better to pass the binary
using <c>ERL_DRV_BINARY</c> and the <c>ErlDrvBinary</c> in question.
</p>
<p>The <c>ERL_DRV_UINT</c>, <c>ERL_DRV_BUF2BINARY</c>, and
<c>ERL_DRV_EXT2TERM</c> term types were introduced in the 5.6
version of erts.
</p>
<p>Note that this function is <em>not</em> thread-safe, not
even when the emulator with SMP support is used.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvTermData</ret><nametext>driver_mk_atom(char* string)</nametext></name>
<fsummary>Make an atom from a name</fsummary>
<desc>
<marker id="driver_mk_atom"></marker>
<p>This function returns an atom given a name
<c>string</c>. The atom is created and won't change, so the
return value may be saved and reused, which is faster than
looking up the atom several times.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvTermData</ret><nametext>driver_mk_port(ErlDrvPort port)</nametext></name>
<fsummary>Make a erlang term port from a port</fsummary>
<desc>
<marker id="driver_mk_port"></marker>
<p>This function converts a port handle to the erlang term
format, usable in the <c>driver_output_send</c> function.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_send_term(ErlDrvPort port, ErlDrvTermData receiver, ErlDrvTermData* term, int n)</nametext></name>
<fsummary>Send term data to other process than port owner process</fsummary>
<desc>
<marker id="driver_send_term"></marker>
<p>This function is the only way for a driver to send data to
<em>other</em> processes than the port owner process. The
<c>receiver</c> parameter specifies the process to receive
the data.</p>
<p>The parameters <c>term</c> and <c>n</c> does the same thing
as in <seealso marker="#driver_output_term">driver_output_term</seealso>.</p>
<p>This function is only thread-safe when the emulator with SMP
support is used.</p>
</desc>
</func>
<func>
<name><ret>long</ret><nametext>driver_async (ErlDrvPort port, unsigned int* key, void (*async_invoke)(void*), void* async_data, void (*async_free)(void*))</nametext></name>
<fsummary>Perform an asynchronous call within a driver</fsummary>
<desc>
<marker id="driver_async"></marker>
<p>This function performs an asynchronous call. The function
<c>async_invoke</c> is invoked in a thread separate from the
emulator thread. This enables the driver to perform
time-consuming, blocking operations without blocking the
emulator.</p>
<p>Erlang is by default started without an async thread pool. The
number of async threads that the runtime system should use
is specified by the
<seealso marker="erl#async_thread_pool_size">+A</seealso>
command line argument of <seealso marker="erl">erl(1)</seealso>.
If no async thread pool is available, the call is made
synchronously in the thread calling <c>driver_async()</c>. The
current number of async threads in the async thread pool can be
retrieved via
<seealso marker="#driver_system_info">driver_system_info()</seealso>.</p>
<p>If there is a thread pool available, a thread will be
used. If the <c>key</c> argument is null, the threads from the
pool are used in a round-robin way, each call to
<c>driver_async</c> uses the next thread in the pool. With the
<c>key</c> argument set, this behaviour is changed. The two
same values of <c>*key</c> always get the same thread.</p>
<p>To make sure that a driver instance always uses the same
thread, the following call can be used:</p>
<p></p>
<code type="none"><![CDATA[
unsigned int myKey = (unsigned int) myPort;
r = driver_async(myPort, &myKey, myData, myFunc);
]]></code>
<p>It is enough to initialize <c>myKey</c> once for each
driver instance.</p>
<p>If a thread is already working, the calls will be
queued up and executed in order. Using the same thread for
each driver instance ensures that the calls will be made in
sequence.</p>
<p>The <c>async_data</c> is the argument to the functions
<c>async_invoke</c> and <c>async_free</c>. It's typically a
pointer to a structure that contains a pipe or event that
can be used to signal that the async operation completed.
The data should be freed in <c>async_free</c>, because it's
called if <c>driver_async_cancel</c> is called.</p>
<p>When the async operation is done, <seealso marker="driver_entry#ready_async">ready_async</seealso> driver
entry function is called. If <c>async_ready</c> is null in
the driver entry, the <c>async_free</c> function is called
instead.</p>
<p>The return value is a handle to the asynchronous task, which
can be used as argument to <c>driver_async_cancel</c>.</p>
<note>
<p>As of erts version 5.5.4.3 the default stack size for
threads in the async-thread pool is 16 kilowords,
i.e., 64 kilobyte on 32-bit architectures.
This small default size has been chosen since the
amount of async-threads might be quite large. The
default stack size is enough for drivers delivered
with Erlang/OTP, but might not be sufficiently large
for other dynamically linked in drivers that use the
driver_async() functionality. A suggested stack size
for threads in the async-thread pool can be configured
via the
<seealso marker="erl#async_thread_stack_size">+a</seealso>
command line argument of
<seealso marker="erl">erl(1)</seealso>.</p>
</note>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_async_cancel(long id)</nametext></name>
<fsummary>Cancel an asynchronous call</fsummary>
<desc>
<marker id="driver_async_cancel"></marker>
<p>This function cancels an asynchronous operation, by removing
it from the queue. Only functions in the queue can be
cancelled; if a function is executing, it's too late to
cancel it. The <c>async_free</c> function is also called.</p>
<p>The return value is 1 if the operation was removed from the
queue, otherwise 0.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>driver_lock_driver(ErlDrvPort port)</nametext></name>
<fsummary>Make sure the driver is never unloaded</fsummary>
<desc>
<marker id="driver_lock_driver"></marker>
<p>This function locks the driver used by the port <c>port</c>
in memory for the rest of the emulator process
lifetime. After this call, the driver behaves as one of Erlang's
statically linked in drivers.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvPort</ret><nametext>driver_create_port(ErlDrvPort port, ErlDrvTermData owner_pid, char* name, ErlDrvData drv_data)</nametext></name>
<fsummary>Create a new port (driver instance)</fsummary>
<desc>
<p>This function creates a new port executing the same driver
code as the port creating the new port.
A short description of the arguments:</p>
<taglist>
<tag><c>port</c></tag>
<item>The port handle of the port (driver instance) creating
the new port.</item>
<tag><c>owner_pid</c></tag>
<item>The process id of the Erlang process which will be
owner of the new port. This process will be linked
to the new port. You usually want to use
<c>driver_caller(port)</c> as <c>owner_pid</c>.</item>
<tag><c>name</c></tag>
<item>The port name of the new port. You usually want to
use the same port name as the driver name
(<seealso marker="driver_entry#driver_name">driver_name</seealso>
field of the
<seealso marker="driver_entry">driver_entry</seealso>).</item>
<tag><c>drv_data</c></tag>
<item>The driver defined handle that will be passed in subsequent
calls to driver call-backs. Note, that the
<seealso marker="driver_entry#start">driver start call-back</seealso>
will not be called for this new driver instance.
The driver defined handle is normally created in the
<seealso marker="driver_entry#start">driver start call-back</seealso>
when a port is created via
<seealso marker="erts:erlang#open_port/2">erlang:open_port/2</seealso>. </item>
</taglist>
<p>The caller of <c>driver_create_port()</c> is allowed to
manipulate the newly created port when <c>driver_create_port()</c>
has returned. When
<seealso marker="#smp_support">port level locking</seealso>
is used, the creating port is, however, only allowed to
manipulate the newly created port until the current driver
call-back that was called by the emulator returns.</p>
<note>
<p>When
<seealso marker="#smp_support">port level locking</seealso>
is used, the creating port is only allowed to manipulate
the newly created port until the current driver call-back
returns.</p>
</note>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>erl_drv_thread_create(char *name,
ErlDrvTid *tid,
void * (*func)(void *),
void *arg,
ErlDrvThreadOpts *opts)</nametext></name>
<fsummary>Create a thread</fsummary>
<desc>
<marker id="erl_drv_thread_create"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>name</c></tag>
<item>A string identifying the created thread. It will be used
to identify the thread in planned future debug
functionality.
</item>
<tag><c>tid</c></tag>
<item>A pointer to a thread identifier variable.</item>
<tag><c>func</c></tag>
<item>A pointer to a function to execute in the created thread.</item>
<tag><c>arg</c></tag>
<item>A pointer to argument to the <c>func</c> function.</item>
<tag><c>opts</c></tag>
<item>A pointer to thread options to use or <c>NULL</c>.</item>
</taglist>
<p>This function creates a new thread. On success <c>0</c> is returned;
otherwise, an <c>errno</c> value is returned to indicate the error.
The newly created thread will begin executing in the function pointed
to by <c>func</c>, and <c>func</c> will be passed <c>arg</c> as
argument. When <c>erl_drv_thread_create()</c> returns the thread
identifier of the newly created thread will be available in
<c>*tid</c>. <c>opts</c> can be either a <c>NULL</c> pointer, or a
pointer to an
<seealso marker="#ErlDrvThreadOpts">ErlDrvThreadOpts</seealso>
structure. If <c>opts</c> is a <c>NULL</c> pointer, default options
will be used; otherwise, the passed options will be used.
</p>
<warning><p>You are not allowed to allocate the
<seealso marker="#ErlDrvThreadOpts">ErlDrvThreadOpts</seealso>
structure by yourself. It has to be allocated and
initialized by
<seealso marker="#erl_drv_thread_opts_create">erl_drv_thread_opts_create()</seealso>.
</p></warning>
<p>The created thread will terminate either when <c>func</c> returns
or if
<seealso marker="#erl_drv_thread_exit">erl_drv_thread_exit()</seealso>
is called by the thread. The exit value of the thread is either
returned from <c>func</c> or passed as argument to
<seealso marker="#erl_drv_thread_exit">erl_drv_thread_exit()</seealso>.
The driver creating the thread has the responsibility of joining the
thread, via
<seealso marker="#erl_drv_thread_join">erl_drv_thread_join()</seealso>,
before the driver is unloaded. It is not possible to create
"detached" threads, i.e., threads that don't need to be joined.
</p>
<warning><p>All created threads need to be joined by the driver before
it is unloaded. If the driver fails to join all threads
created before it is unloaded, the runtime system will
most likely crash when the code of the driver is unloaded.
</p></warning>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvThreadOpts *</ret><nametext>erl_drv_thread_opts_create(char *name)</nametext></name>
<fsummary>Create thread options</fsummary>
<desc>
<marker id="erl_drv_thread_opts_create"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>name</c></tag>
<item>A string identifying the created thread options. It will be used
to identify the thread options in planned future debug
functionality.
</item>
</taglist>
<p>This function allocates and initialize a thread option
structure. On failure <c>NULL</c> is returned. A thread option
structure is used for passing options to
<seealso marker="#erl_drv_thread_create">erl_drv_thread_create()</seealso>.
If the structure isn't modified before it is passed to
<seealso marker="#erl_drv_thread_create">erl_drv_thread_create()</seealso>,
the default values will be used.
</p>
<warning><p>You are not allowed to allocate the
<seealso marker="#ErlDrvThreadOpts">ErlDrvThreadOpts</seealso>
structure by yourself. It has to be allocated and
initialized by <c>erl_drv_thread_opts_create()</c>.
</p></warning>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_thread_opts_destroy(ErlDrvThreadOpts *opts)</nametext></name>
<fsummary>Destroy thread options</fsummary>
<desc>
<marker id="erl_drv_thread_opts_destroy"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>opts</c></tag>
<item>A pointer to thread options to destroy.</item>
</taglist>
<p>This function destroys thread options previously created by
<seealso marker="#erl_drv_thread_opts_create">erl_drv_thread_opts_create()</seealso>.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_thread_exit(void *exit_value)</nametext></name>
<fsummary>Terminate calling thread</fsummary>
<desc>
<marker id="erl_drv_thread_exit"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>exit_value</c></tag>
<item>A pointer to an exit value or <c>NULL</c>.</item>
</taglist>
<p>This function terminates the calling thread with the exit
value passed as argument. You are only allowed to terminate
threads created with
<seealso marker="#erl_drv_thread_create">erl_drv_thread_create()</seealso>.
The exit value can later be retrieved by another thread via
<seealso marker="#erl_drv_thread_join">erl_drv_thread_join()</seealso>.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>erl_drv_thread_join(ErlDrvTid tid, void **exit_value)</nametext></name>
<fsummary>Join with another thread</fsummary>
<desc>
<marker id="erl_drv_thread_join"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>tid</c></tag>
<item>The thread identifier of the thread to join.</item>
<tag><c>exit_value</c></tag>
<item>A pointer to a pointer to an exit value, or <c>NULL</c>.</item>
</taglist>
<p>This function joins the calling thread with another thread, i.e.,
the calling thread is blocked until the thread identified by
<c>tid</c> has terminated. On success <c>0</c> is returned;
otherwise, an <c>errno</c> value is returned to indicate the error.
A thread can only be joined once. The behavior of joining
more than once is undefined, an emulator crash is likely. If
<c>exit_value == NULL</c>, the exit value of the terminated thread
will be ignored; otherwise, the exit value of the terminated thread
will be stored at <c>*exit_value</c>.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvTid</ret><nametext>erl_drv_thread_self(void)</nametext></name>
<fsummary>Get the thread identifier of the current thread</fsummary>
<desc>
<marker id="erl_drv_thread_self"></marker>
<p>This function returns the thread identifier of the
calling thread.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>erl_drv_equal_tids(ErlDrvTid tid1, ErlDrvTid tid2)</nametext></name>
<fsummary>Compare thread identifiers for equality</fsummary>
<desc>
<marker id="erl_drv_equal_tids"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>tid1</c></tag>
<item>A thread identifier.</item>
<tag><c>tid2</c></tag>
<item>A thread identifier.</item>
</taglist>
<p>This function compares two thread identifiers for equality,
and returns <c>0</c> it they aren't equal, and
a value not equal to <c>0</c> if they are equal.</p>
<note><p>A Thread identifier may be reused very quickly after
a thread has terminated. Therefore, if a thread
corresponding to one of the involved thread identifiers
has terminated since the thread identifier was saved,
the result of <c>erl_drv_equal_tids()</c> might not give
expected result.
</p></note>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvMutex *</ret><nametext>erl_drv_mutex_create(char *name)</nametext></name>
<fsummary>Create a mutex</fsummary>
<desc>
<marker id="erl_drv_mutex_create"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>name</c></tag>
<item>A string identifying the created mutex. It will be used
to identify the mutex in planned future debug functionality.
</item>
</taglist>
<p>This function creates a mutex and returns a pointer to it. On
failure <c>NULL</c> is returned. The driver creating the mutex
has the responsibility of destroying it before the driver is
unloaded.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_mutex_destroy(ErlDrvMutex *mtx)</nametext></name>
<fsummary>Destroy a mutex</fsummary>
<desc>
<marker id="erl_drv_mutex_destroy"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>mtx</c></tag>
<item>A pointer to a mutex to destroy.</item>
</taglist>
<p>This function destroys a mutex previously created by
<seealso marker="#erl_drv_mutex_create">erl_drv_mutex_create()</seealso>.
The mutex has to be in an unlocked state before being
destroyed.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_mutex_lock(ErlDrvMutex *mtx)</nametext></name>
<fsummary>Lock a mutex</fsummary>
<desc>
<marker id="erl_drv_mutex_lock"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>mtx</c></tag>
<item>A pointer to a mutex to lock.</item>
</taglist>
<p>This function locks a mutex. The calling thread will be
blocked until the mutex has been locked. A thread
which currently has locked the mutex may <em>not</em> lock
the same mutex again.
</p>
<warning><p>If you leave a mutex locked in an emulator thread
when you let the thread out of your control, you will
<em>very likely</em> deadlock the whole emulator.
</p></warning>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>erl_drv_mutex_trylock(ErlDrvMutex *mtx)</nametext></name>
<fsummary>Try lock a mutex</fsummary>
<desc>
<marker id="erl_drv_mutex_trylock"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>mtx</c></tag>
<item>A pointer to a mutex to try to lock.</item>
</taglist>
<p>This function tries to lock a mutex. If successful <c>0</c>,
is returned; otherwise, <c>EBUSY</c> is returned. A thread
which currently has locked the mutex may <em>not</em> try to
lock the same mutex again.
</p>
<warning><p>If you leave a mutex locked in an emulator thread
when you let the thread out of your control, you will
<em>very likely</em> deadlock the whole emulator.
</p></warning>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_mutex_unlock(ErlDrvMutex *mtx)</nametext></name>
<fsummary>Unlock a mutex</fsummary>
<desc>
<marker id="erl_drv_mutex_unlock"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>mtx</c></tag>
<item>A pointer to a mutex to unlock.</item>
</taglist>
<p>This function unlocks a mutex. The mutex currently has to be
locked by the calling thread.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvCond *</ret><nametext>erl_drv_cond_create(char *name)</nametext></name>
<fsummary>Create a condition variable</fsummary>
<desc>
<marker id="erl_drv_cond_create"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>name</c></tag>
<item>A string identifying the created condition variable. It
will be used to identify the condition variable in planned
future debug functionality.
</item>
</taglist>
<p>This function creates a condition variable and returns a
pointer to it. On failure <c>NULL</c> is returned. The driver
creating the condition variable has the responsibility of
destroying it before the driver is unloaded.</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_cond_destroy(ErlDrvCond *cnd)</nametext></name>
<fsummary>Destroy a condition variable</fsummary>
<desc>
<marker id="erl_drv_cond_destroy"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>cnd</c></tag>
<item>A pointer to a condition variable to destroy.</item>
</taglist>
<p>This function destroys a condition variable previously
created by
<seealso marker="#erl_drv_cond_create">erl_drv_cond_create()</seealso>.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_cond_signal(ErlDrvCond *cnd)</nametext></name>
<fsummary>Signal on a condition variable</fsummary>
<desc>
<marker id="erl_drv_cond_signal"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>cnd</c></tag>
<item>A pointer to a condition variable to signal on.</item>
</taglist>
<p>This function signals on a condition variable. That is, if
other threads are waiting on the condition variable being
signaled, <em>one</em> of them will be woken.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_cond_broadcast(ErlDrvCond *cnd)</nametext></name>
<fsummary>Broadcast on a condition variable</fsummary>
<desc>
<marker id="erl_drv_cond_broadcast"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>cnd</c></tag>
<item>A pointer to a condition variable to broadcast on.</item>
</taglist>
<p>This function broadcasts on a condition variable. That is, if
other threads are waiting on the condition variable being
broadcasted on, <em>all</em> of them will be woken.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_cond_wait(ErlDrvCond *cnd, ErlDrvMutex *mtx)</nametext></name>
<fsummary>Wait on a condition variable</fsummary>
<desc>
<marker id="erl_drv_cond_wait"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>cnd</c></tag>
<item>A pointer to a condition variable to wait on.</item>
<tag><c>mtx</c></tag>
<item>A pointer to a mutex to unlock while waiting.</item>
<tag><c></c></tag>
<item></item>
</taglist>
<p>This function waits on a condition variable. The calling
thread is blocked until another thread wakes it by signaling
or broadcasting on the condition variable. Before the calling
thread is blocked it unlocks the mutex passed as argument, and
when the calling thread is woken it locks the same mutex before
returning. That is, the mutex currently has to be locked by
the calling thread when calling this function.
</p>
<note><p><c>erl_drv_cond_wait()</c> might return even though
no-one has signaled or broadcasted on the condition
variable. Code calling <c>erl_drv_cond_wait()</c> should
always be prepared for <c>erl_drv_cond_wait()</c>
returning even though the condition that the thread was
waiting for hasn't occurred. That is, when returning from
<c>erl_drv_cond_wait()</c> always check if the condition
has occurred, and if not call <c>erl_drv_cond_wait()</c>
again.
</p></note>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>ErlDrvRWLock *</ret><nametext>erl_drv_rwlock_create(char *name)</nametext></name>
<fsummary>Create an rwlock</fsummary>
<desc>
<marker id="erl_drv_rwlock_create"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>name</c></tag>
<item>A string identifying the created rwlock. It will be used to
identify the rwlock in planned future debug functionality.
</item>
</taglist>
<p>This function creates an rwlock and returns a pointer to it. On
failure <c>NULL</c> is returned. The driver creating the rwlock
has the responsibility of destroying it before the driver is
unloaded.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_rwlock_destroy(ErlDrvRWLock *rwlck)</nametext></name>
<fsummary>Destroy an rwlock</fsummary>
<desc>
<marker id="erl_drv_rwlock_destroy"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>rwlck</c></tag>
<item>A pointer to an rwlock to destroy.</item>
</taglist>
<p>This function destroys an rwlock previously created by
<seealso marker="#erl_drv_rwlock_create">erl_drv_rwlock_create()</seealso>.
The rwlock has to be in an unlocked state before being destroyed.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_rwlock_rlock(ErlDrvRWLock *rwlck)</nametext></name>
<fsummary>Read lock an rwlock</fsummary>
<desc>
<marker id="erl_drv_rwlock_rlock"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>rwlck</c></tag>
<item>A pointer to an rwlock to read lock.</item>
</taglist>
<p>This function read locks an rwlock. The calling thread will be
blocked until the rwlock has been read locked. A thread
which currently has read or read/write locked the rwlock may
<em>not</em> lock the same rwlock again.
</p>
<warning><p>If you leave an rwlock locked in an emulator thread
when you let the thread out of your control, you will
<em>very likely</em> deadlock the whole emulator.
</p></warning>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>erl_drv_rwlock_tryrlock(ErlDrvRWLock *rwlck)</nametext></name>
<fsummary>Try to read lock an rwlock</fsummary>
<desc>
<marker id="erl_drv_rwlock_tryrlock"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>rwlck</c></tag>
<item>A pointer to an rwlock to try to read lock.</item>
</taglist>
<p>This function tries to read lock an rwlock. If successful
<c>0</c>, is returned; otherwise, <c>EBUSY</c> is returned.
A thread which currently has read or read/write locked the
rwlock may <em>not</em> try to lock the same rwlock again.
</p>
<warning><p>If you leave an rwlock locked in an emulator thread
when you let the thread out of your control, you will
<em>very likely</em> deadlock the whole emulator.
</p></warning>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_rwlock_runlock(ErlDrvRWLock *rwlck)</nametext></name>
<fsummary>Read unlock an rwlock</fsummary>
<desc>
<marker id="erl_drv_rwlock_runlock"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>rwlck</c></tag>
<item>A pointer to an rwlock to read unlock.</item>
</taglist>
<p>This function read unlocks an rwlock. The rwlock currently
has to be read locked by the calling thread.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_rwlock_rwlock(ErlDrvRWLock *rwlck)</nametext></name>
<fsummary>Read/Write lock an rwlock</fsummary>
<desc>
<marker id="erl_drv_rwlock_rwlock"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>rwlck</c></tag>
<item>A pointer to an rwlock to read/write lock.</item>
</taglist>
<p>This function read/write locks an rwlock. The calling thread
will be blocked until the rwlock has been read/write locked.
A thread which currently has read or read/write locked the
rwlock may <em>not</em> lock the same rwlock again.
</p>
<warning><p>If you leave an rwlock locked in an emulator thread
when you let the thread out of your control, you will
<em>very likely</em> deadlock the whole emulator.
</p></warning>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>erl_drv_rwlock_tryrwlock(ErlDrvRWLock *rwlck)</nametext></name>
<fsummary>Try to read/write lock an rwlock</fsummary>
<desc>
<marker id="erl_drv_rwlock_tryrwlock"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>rwlck</c></tag>
<item>A pointer to an rwlock to try to read/write lock.</item>
</taglist>
<p>This function tries to read/write lock an rwlock. If successful
<c>0</c>, is returned; otherwise, <c>EBUSY</c> is returned.
A thread which currently has read or read/write locked the
rwlock may <em>not</em> try to lock the same rwlock again.
</p>
<warning><p>If you leave an rwlock locked in an emulator thread
when you let the thread out of your control, you will
<em>very likely</em> deadlock the whole emulator.
</p></warning>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_rwlock_rwunlock(ErlDrvRWLock *rwlck)</nametext></name>
<fsummary>Read/Write unlock an rwlock</fsummary>
<desc>
<marker id="erl_drv_rwlock_rwunlock"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>rwlck</c></tag>
<item>A pointer to an rwlock to read/write unlock.</item>
</taglist>
<p>This function read/write unlocks an rwlock. The rwlock
currently has to be read/write locked by the calling thread.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>erl_drv_tsd_key_create(char *name, ErlDrvTSDKey *key)</nametext></name>
<fsummary>Create a thread specific data key</fsummary>
<desc>
<marker id="erl_drv_tsd_key_create"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>name</c></tag>
<item>A string identifying the created key. It will be used
to identify the key in planned future debug
functionality.
</item>
<tag><c>key</c></tag>
<item>A pointer to a thread specific data key variable.</item>
</taglist>
<p>This function creates a thread specific data key. On success
<c>0</c> is returned; otherwise, an <c>errno</c> value is returned
to indicate the error. The driver creating the key has the
responsibility of destroying it before the driver is unloaded.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_tsd_key_destroy(ErlDrvTSDKey key)</nametext></name>
<fsummary>Destroy a thread specific data key</fsummary>
<desc>
<marker id="erl_drv_tsd_key_destroy"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>key</c></tag>
<item>A thread specific data key to destroy.</item>
</taglist>
<p>This function destroys a thread specific data key
previously created by
<seealso marker="#erl_drv_tsd_key_create">erl_drv_tsd_key_create()</seealso>.
All thread specific data using this key in all threads
have to be cleared (see
<seealso marker="#erl_drv_tsd_set">erl_drv_tsd_set()</seealso>)
prior to the call to <c>erl_drv_tsd_key_destroy()</c>.
</p>
<warning><p>A destroyed key is very likely to be reused soon.
Therefore, if you fail to clear the thread specific
data using this key in a thread prior to destroying
the key, you will <em>very likely</em> get unexpected
errors in other parts of the system.
</p></warning>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void</ret><nametext>erl_drv_tsd_set(ErlDrvTSDKey key, void *data)</nametext></name>
<fsummary>Set thread specific data</fsummary>
<desc>
<marker id="erl_drv_tsd_set"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>key</c></tag>
<item>A thread specific data key.</item>
<tag><c>data</c></tag>
<item>A pointer to data to associate with <c>key</c>
in calling thread.
</item>
</taglist>
<p>This function sets thread specific data associated with
<c>key</c> for the calling thread. You are only allowed to set
thread specific data for threads while they are fully under your
control. For example, if you set thread specific data in a thread
calling a driver call-back function, it has to be cleared, i.e.
set to <c>NULL</c>, before returning from the driver call-back
function.
</p>
<warning><p>If you fail to clear thread specific data in an
emulator thread before letting it out of your control,
you might not ever be able to clear this data with
later unexpected errors in other parts of the system as
a result.
</p></warning>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>void *</ret><nametext>erl_drv_tsd_get(ErlDrvTSDKey key)</nametext></name>
<fsummary>Get thread specific data</fsummary>
<desc>
<marker id="erl_drv_tsd_get"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>key</c></tag>
<item>A thread specific data key.</item>
</taglist>
<p>This function returns the thread specific data
associated with <c>key</c> for the calling thread.
If no data has been associated with <c>key</c> for
the calling thread, <c>NULL</c> is returned.
</p>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>erl_drv_putenv(char *key, char *value)</nametext></name>
<fsummary>Set the value of an environment variable</fsummary>
<desc>
<marker id="erl_drv_putenv"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>key</c></tag>
<item>A null terminated string containing the
name of the environment variable.</item>
<tag><c>value</c></tag>
<item>A null terminated string containing the
new value of the environment variable.</item>
</taglist>
<p>This function sets the value of an environment variable.
It returns <c>0</c> on success, and a value <c>!= 0</c> on
failure.
</p>
<note><p>The result of passing the empty string ("") as a value
is platform dependent. On some platforms the value of the
variable is set to the empty string, on others, the
environment variable is removed.</p>
</note>
<warning><p>Do <em>not</em> use libc's <c>putenv</c> or similar
C library interfaces from a driver.
</p></warning>
<p>This function is thread-safe.</p>
</desc>
</func>
<func>
<name><ret>int</ret><nametext>erl_drv_getenv(char *key, char *value, size_t *value_size)</nametext></name>
<fsummary>Get the value of an environment variable</fsummary>
<desc>
<marker id="erl_drv_getenv"></marker>
<p>Arguments:</p>
<taglist>
<tag><c>key</c></tag>
<item>A null terminated string containing the
name of the environment variable.</item>
<tag><c>value</c></tag>
<item>A pointer to an output buffer.</item>
<tag><c>value_size</c></tag>
<item>A pointer to an integer. The integer is both used for
passing input and output sizes (see below).
</item>
</taglist>
<p>This function retrieves the value of an environment variable.
When called, <c>*value_size</c> should contain the size of
the <c>value</c> buffer. On success <c>0</c> is returned,
the value of the environment variable has been written to
the <c>value</c> buffer, and <c>*value_size</c> contains the
string length (excluding the terminating null character) of
the value written to the <c>value</c> buffer. On failure,
i.e., no such environment variable was found, a value less than
<c>0</c> is returned. When the size of the <c>value</c>
buffer is too small, a value greater than <c>0</c> is returned
and <c>*value_size</c> has been set to the buffer size needed.
</p>
<warning><p>Do <em>not</em> use libc's <c>getenv</c> or similar
C library interfaces from a driver.
</p></warning>
<p>This function is thread-safe.</p>
</desc>
</func>
</funcs>
<section>
<title>SEE ALSO</title>
<p><seealso marker="driver_entry">driver_entry(3)</seealso>,
<seealso marker="kernel:erl_ddll">erl_ddll(3)</seealso>,
<seealso marker="erts:erlang">erlang(3)</seealso></p>
<p>An Alternative Distribution Driver (ERTS User's
Guide Ch. 3)</p>
</section>
</cref>
