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 caller. Each NIF must also have an implementation in Erlang that will be invoked if the function is called before the NIF library has been successfully loaded. A typical such stub implementation 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.
Use this functionality with extreme care!
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 not 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.
A native function that crash will crash the whole VM.
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.
A native function that do
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:
A minimal example of a NIF library can look like this:
/* niftest.c */
#include "erl_nif.h"
static ERL_NIF_TERM hello(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
{
return enif_make_string(env, "Hello world!", ERL_NIF_LATIN1);
}
static ErlNifFunc nif_funcs[] =
{
{"hello", 0, hello}
};
ERL_NIF_INIT(niftest,nif_funcs,NULL,NULL,NULL,NULL)
and the Erlang module would have to look something like this:
-module(niftest).
-export([init/0, hello/0]).
init() ->
erlang:load_nif("./niftest", 0).
hello() ->
"NIF library not loaded".
and compile and test something like this (on Linux):
$> gcc -fPIC -shared -o niftest.so niftest.c -I $ERL_ROOT/usr/include/
$> erl
1> c(niftest).
{ok,niftest}
2> niftest:hello().
"NIF library not loaded"
3> niftest:init().
ok
4> niftest:hello().
"Hello world!"
A better solution for a real module is to take advantage of
the new directive
A NIF does not have to be exported, it can be local to the module. Note however that unused local stub functions will be optimized away by the compiler causing loading of the NIF library to fail.
A loaded NIF library is tied to the Erlang module code version
that loaded it. If the module is upgraded with a new version, the
new Erlang code will have to load its own NIF library (or maybe choose not
to). The new code version can however choose to load the exact
same NIF library as the old code if it wants to. Sharing the same
dynamic library will mean that static data defined by the library
will be shared as well. To avoid unintentionally shared static
data, each Erlang module code can keep its own private data. This
private data can be set when the NIF library is loaded and
then retrieved by calling
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.
All functions that a NIF library needs to do with Erlang are performed through the NIF API functions. There are functions for the following functionality:
Any Erlang terms can be passed to a NIF as function arguments and
be returned as function return values. The terms are of C-type
All terms of type
Terms of type binary are accessed with the help of the struct type
The raw data pointed to by
Binaries are sequences of whole bytes. Bitstrings with an arbitrary bit length have no support yet.
The use of resource objects is a safe way to return pointers to
native data structures from a NIF. A resource object is
just a block of memory allocated with
All resource objects are created as instances of some resource type.
This makes resources from different modules to be distinguishable.
A resource type is created by calling
Here is a template example of how to create and return a resource object.
ERL_NIF_TERM term;
MyStruct* obj = enif_alloc_resource(my_resource_type, sizeof(MyStruct));
/* initialize struct ... */
term = enif_make_resource(env, obj);
if (keep_a_reference_of_our_own) {
/* store 'obj' in static variable, private data or other resource object */
}
else {
enif_release_resource(obj);
/* resource now only owned by "Erlang" */
}
return term;
Note that once
Another usage of resource objects is to create binary terms with
user defined memory management.
Resource types support upgrade in runtime by allowing a loaded NIF library to takeover an already existing resource type and thereby "inherit" all existing objects of that type. The destructor of the new library will thereafter be called for the inherited objects and the library with the old destructor function can be safely unloaded. Existing resource objects, of a module that is upgraded, must either be deleted or taken over by the new NIF library. The unloading of a library will be postponed as long as there exist resource objects with a destructor function in the library.
A NIF is thread-safe without any explicit synchronization as
long as it acts as a pure function and only reads the supplied
arguments. As soon as you write towards a shared state either through
static variables or
The library initialization callbacks
A NIF that cannot execute in a millisecond or less is called a "dirty NIF" since
it performs work that the Erlang runtime cannot handle cleanly. Applications
that make use of such functions must indicate to the runtime that the functions are
dirty so they can be handled specially. To schedule a dirty NIF for execution, the
application calls
All dirty NIFs must ultimately invoke the
Dirty NIF support is available only when the emulator is configured with dirty
schedulers enabled. This feature is currently disabled by default. To determine whether
the dirty NIF API is available, native code can check to see if the C preprocessor macro
This is the magic macro to initialize a NIF library. It should be evaluated in global file scope.
If compiling a nif for static inclusion via --enable-static-nifs you have to define STATIC_ERLANG_NIF before the ERL_NIF_INIT declaration.
The library will fail to load if
Works the same as
The library will fail to load if
The reload mechanism is deprecated. It was only intended
as a development feature. Do not use it as an upgrade method for
live production systems. It might be removed in future releases. Be sure
to pass
Works the same as
The library will fail to load if
Variables of type
A process bound environment is passed as the first argument to all NIFs. All function arguments passed to a NIF will belong to that environment. The return value from a NIF must also be a term belonging to the same environment. In addition a process bound environment contains transient information about the calling Erlang process. The environment is only valid in the thread where it was supplied as argument until the NIF returns. It is thus useless and dangerous to store pointers to process bound environments between NIF calls.
A process independent environment is created by calling
All elements of a list/tuple must belong to the same environment as the
list/tuple itself. Terms can be copied between environments with
typedef struct {
const char* name;
unsigned arity;
ERL_NIF_TERM (*fptr)(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[]);
} ErlNifFunc;
Describes a NIF by its name, arity and implementation.
typedef struct {
unsigned size;
unsigned char* data;
} ErlNifBinary;
Note that
Each instance of
typedef void ErlNifResourceDtor(ErlNifEnv* env, void* obj);
The function prototype of a resource destructor function. A destructor function is not allowed to call any term-making functions.
typedef enum {
ERL_NIF_LATIN1
}ErlNifCharEncoding;
The character encoding used in strings and atoms. The only
supported encoding is currently
Used by
A native signed 64-bit integer type.
A native unsigned 64-bit integer type.
Allocate memory of
Allocate a new binary of size
Return true on success or false if allocation failed.
Allocate a new process independent environment. The environment can
be used to hold terms that is not bound to any process. Such terms can
later be copied to a process environment with
Return pointer to the new environment.
Allocate a memory managed resource object of type
Free all terms in an environment and clear it for reuse. The environment must
have been allocated with
Return an integer less than, equal to, or greater than
zero if
Same as
Same as
Same as
Same as
Same as
Give the runtime system a hint about how much CPU time the current NIF call has consumed
since last hint, or since the start of the NIF if no previous hint has been given.
The time is given as a
Note that it is up to the runtime system to determine if and how to use this information.
Implementations on some platforms may use other means in order to determine consumed
CPU time. Lengthy NIFs should regardless of this frequently call
Returns 1 if the timeslice is exhausted, or 0 otherwise. If 1 is returned the NIF should return as soon as possible in order for the process to yield.
Argument
This function is provided to better support co-operative scheduling, improve system responsiveness,
and make it easier to prevent misbehaviors of the VM due to a NIF monopolizing a scheduler thread.
It can be used to divide
A convenience function that a dirty NIF can use as a finalizer that simply
return its
This function is available only when the emulator is configured with dirty
schedulers enabled. This feature is currently disabled by default. To determine whether
the dirty NIF API is available, native code can check to see if the C preprocessor macro
Same as
Free memory allocated by
Free an environment allocated with
Write a null-terminated string, in the buffer pointed to by
Set
Set
Set
Set
If
Set
Set
Set
Set
Return true on success or false if
Write a null-terminated string, in the buffer pointed to by
If
Return true on success or false if
Set
Set
Set
Check at runtime for the presence of dirty scheduler threads. If the emulator is
built with threading support, dirty scheduler threads are available and
If dirty scheduler threads are not available in the emulator, calls to
This function is available only when the emulator is configured with dirty
schedulers enabled. This feature is currently disabled by default. To determine whether
the dirty NIF API is available, native code can check to see if the C preprocessor macro
Initialize the structure pointed to by
Initialize the structure pointed to by
Return true if
Return true if
Return true if
Return true if
Return true if
Return true if
Return true if the two terms are identical. Corresponds to the
Erlang operators
Check to see if the current NIF is executing on a dirty scheduler thread. If the
emulator is built with threading support, calling
This function is available only when the emulator is configured with dirty
schedulers enabled. This feature is currently disabled by default. To determine whether
the dirty NIF API is available, native code can check to see if the C preprocessor macro
Return true if
Return true if
Return true if
Return true if
Return true if
Add a reference to resource object
Create an atom term from the null-terminated C-string
Create an atom term from the string
Make a badarg exception to be returned from a NIF, and set
an associated exception reason in
Make a binary term from
Make a copy of term
Create a floating-point term from a
Try to create the term of an already existing atom from
the null-terminated C-string
Try to create the term of an already existing atom from the
string
Create an integer term.
Create an integer term from a signed 64-bit integer.
Create an ordinary list term of length
Create an ordinary list term with length indicated by the
function name. Prefer these functions (macros) over the variadic
Create a list cell
Create an ordinary list containing the elements of array
Set
Create an integer term from a
Allocate a binary of size
Return a pointer to the raw binary data and set
Make a pid term from
Create a reference like
Create an opaque handle to a memory managed resource object
obtained by
Note that the only defined behaviour of using a resource term in
an Erlang program is to store it and send it between processes on the
same node. Other operations such as matching or
Create a binary term that is memory managed by a resource object
Several binary terms may be managed by the same resource object. The destructor will not be called until the last binary is garbage collected. This can be useful as a way to return different parts of a larger binary buffer.
As with
Create a list containing the characters of the
null-terminated string
Create a list containing the characters of the string
Make a subbinary of binary
Create a tuple term of arity
Create a tuple term with length indicated by the
function name. Prefer these functions (macros) over the variadic
Create a tuple containing the elements of array
Create an integer term from an
Create an integer term from an unsigned 64-bit integer.
Create an integer term from an
Same as
Same as
Same as
Same as
Same as
Create or takeover a resource type identified by the string
The two flag values can be combined with bitwise-or. The name of the
resource type is local to the calling module. Argument
On success, return a pointer to the resource type and
Note that
Return the pointer to the private data that was set by
Was previously named
Change the size of a binary
Release a binary obtained from
Remove a reference to resource object
Same as
Same as
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Same as
Same as
Same as
Schedule dirty NIF
Be aware that
A dirty NIF may not invoke the
This function is available only when the emulator is configured with dirty schedulers
enabled. This feature is currently disabled by default. To determine whether the dirty NIF API
is available, native code can check to see if the C preprocessor macro
When a dirty NIF finishes executing, it must schedule a finalizer function to return
its result to the original NIF caller. The dirty NIF passes
Without exception, all dirty NIFs must invoke
This function is available only when the emulator is configured with dirty
schedulers enabled. This feature is currently disabled by default. To determine whether
the dirty NIF API is available, native code can check to see if the C preprocessor macro
Initialize the pid variable
Send a message to a process.
Return true on success, or false if
The message environment
This function is only thread-safe when the emulator with SMP support is used. It can only be used in a non-SMP emulator from a NIF-calling thread.
Get the byte size of a resource object
Same as
Same as
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