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-rw-r--r--erts/doc/src/erl_nif.xml97
1 files changed, 68 insertions, 29 deletions
diff --git a/erts/doc/src/erl_nif.xml b/erts/doc/src/erl_nif.xml
index f484e9eaf7..f00f7b9f46 100644
--- a/erts/doc/src/erl_nif.xml
+++ b/erts/doc/src/erl_nif.xml
@@ -34,30 +34,6 @@
<lib>erl_nif</lib>
<libsummary>API functions for an Erlang NIF library</libsummary>
<description>
- <note><p>The NIF concept is officially supported from R14B. NIF source code
- written for earlier experimental versions might need adaption to run on R14B.</p>
- <p>No incompatible changes between <em>R14B</em> and R14A.</p>
- <p>Incompatible changes between <em>R14A</em> and R13B04:</p>
- <list>
- <item>Environment argument removed for <c>enif_alloc</c>,
- <c>enif_realloc</c>, <c>enif_free</c>, <c>enif_alloc_binary</c>,
- <c>enif_realloc_binary</c>, <c>enif_release_binary</c>,
- <c>enif_alloc_resource</c>, <c>enif_release_resource</c>,
- <c>enif_is_identical</c> and <c>enif_compare</c>.</item>
- <item>Character encoding argument added to <c>enif_get_atom</c>
- and <c>enif_make_existing_atom</c>.</item>
- <item>Module argument added to <c>enif_open_resource_type</c>
- while changing name spaces of resource types from global to module local.</item>
- </list>
- <p>Incompatible changes between <em>R13B04</em> and R13B03:</p>
- <list>
- <item>The function prototypes of the NIFs have changed to expect <c>argc</c> and <c>argv</c>
- arguments. The arity of a NIF is by that no longer limited to 3.</item>
- <item><c>enif_get_data</c> renamed as <c>enif_priv_data</c>.</item>
- <item><c>enif_make_string</c> got a third argument for character encoding.</item>
- </list>
- </note>
-
<p>A NIF library contains native implementation of some functions
of an Erlang module. The native implemented functions (NIFs) are
called like any other functions without any difference to the
@@ -67,6 +43,57 @@
is to throw an exception. But it can also be used as a fallback
implementation if the NIF library is not implemented for some
architecture.</p>
+ <marker id="WARNING"/>
+ <warning><p><em>Use this functionality with extreme care!</em></p>
+ <p>A native function is executed as a direct extension of the
+ native code of the VM. Execution is not made in a safe environment.
+ The VM can <em>not</em> provide the same services as provided when
+ executing Erlang code, such as preemptive scheduling or memory
+ protection. If the native function doesn't behave well, the whole
+ VM will misbehave.</p>
+ <list>
+ <item><p>A native function that crash will crash the whole VM.</p></item>
+ <item><p>An erroneously implemented native function might cause
+ a VM internal state inconsistency which may cause a crash of the VM,
+ or miscellaneous misbehaviors of the VM at any point after the call
+ to the native function.</p></item>
+ <item><p>A native function that do <seealso marker="#lengthy_work">lengthy
+ work</seealso> before returning will degrade responsiveness of the VM,
+ and may cause miscellaneous strange behaviors. Such strange behaviors
+ include, but are not limited to, extreme memory usage, and bad load
+ balancing between schedulers. Strange behaviors that might occur due
+ to lengthy work may also vary between OTP releases.</p></item>
+ </list>
+ </warning>
+
+ <p>The NIF concept is officially supported from R14B. NIF source code
+ written for earlier experimental versions might need adaption to run on R14B
+ or later versions:</p>
+ <list>
+ <item>No incompatible changes between <em>R14B</em> and R14A.</item>
+ <item>Incompatible changes between <em>R14A</em> and R13B04:
+ <list>
+ <item>Environment argument removed for <c>enif_alloc</c>,
+ <c>enif_realloc</c>, <c>enif_free</c>, <c>enif_alloc_binary</c>,
+ <c>enif_realloc_binary</c>, <c>enif_release_binary</c>,
+ <c>enif_alloc_resource</c>, <c>enif_release_resource</c>,
+ <c>enif_is_identical</c> and <c>enif_compare</c>.</item>
+ <item>Character encoding argument added to <c>enif_get_atom</c>
+ and <c>enif_make_existing_atom</c>.</item>
+ <item>Module argument added to <c>enif_open_resource_type</c>
+ while changing name spaces of resource types from global to module local.</item>
+ </list>
+ </item>
+ <item>Incompatible changes between <em>R13B04</em> and R13B03:
+ <list>
+ <item>The function prototypes of the NIFs have changed to expect <c>argc</c> and <c>argv</c>
+ arguments. The arity of a NIF is by that no longer limited to 3.</item>
+ <item><c>enif_get_data</c> renamed as <c>enif_priv_data</c>.</item>
+ <item><c>enif_make_string</c> got a third argument for character encoding.</item>
+ </list>
+ </item>
+ </list>
+
<p>A minimal example of a NIF library can look like this:</p>
<p/>
<code type="none">
@@ -136,7 +163,23 @@ ok
then retrieved by calling <seealso marker="#enif_priv_data">enif_priv_data</seealso>.</p>
<p>There is no way to explicitly unload a NIF library. A library will be
automatically unloaded when the module code that it belongs to is purged
- by the code server.</p>
+ by the code server.</p>
+
+ <p><marker id="lengthy_work"/>
+ As mentioned in the <seealso marker="#WARNING">warning</seealso> text at
+ the beginning of this document it is of vital importance that a native function
+ does return relatively fast. It is hard to give an exact maximum amount
+ of time that a native function is allowed to work, but as a rule of thumb
+ a well behaving native function should return to its caller before a
+ millisecond has passed. This can be achieved using different approaches.
+ If you have full control over the code that are to execute in the native
+ function, the best approach is to divide the work into multiple chunks of
+ work and call the native function multiple times. This might, however,
+ not always be possible, e.g. when calling third party libraries. In this
+ case you typically want to dispatch the work to another thread, return
+ from the native function, and wait for the result. The thread can send
+ the result back to the calling thread using message passing. Information
+ about thread primitives can be found below.</p>
</description>
<section>
<title>FUNCTIONALITY</title>
@@ -266,10 +309,6 @@ ok
mutable.</p>
<p>The library initialization callbacks <c>load</c>, <c>reload</c> and
<c>upgrade</c> are all thread-safe even for shared state data.</p>
- <p>Avoid doing lengthy work in NIF calls as that may degrade the
- responsiveness of the VM. NIFs are called directly by the same scheduler
- thread that executed the calling Erlang code. The calling scheduler will thus
- be blocked from doing any other work until the NIF returns.</p>
</item>
</taglist>
</section>