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erl_nif(3erl) | C Library Functions | erl_nif(3erl) |
NAME¶
erl_nif - API functions for an Erlang NIF libraryDESCRIPTION¶
Note:
The NIF concept is officially supported from R14B. NIF source code written for
earlier experimental versions might need adaption to run on R14B.
No incompatible changes between R14B and R14A.
Incompatible changes between R14A and R13B04:
- *
- Environment argument removed for enif_alloc, enif_realloc, enif_free, enif_alloc_binary, enif_realloc_binary, enif_release_binary, enif_alloc_resource, enif_release_resource, enif_is_identical and enif_compare.
- *
- Character encoding argument added to enif_get_atom and enif_make_existing_atom.
- *
- Module argument added to enif_open_resource_type while changing name spaces of resource types from global to module local.
- *
- The function prototypes of the NIFs have changed to expect argc and argv arguments. The arity of a NIF is by that no longer limited to 3.
- *
- enif_get_data renamed as enif_priv_data.
- *
- enif_make_string got a third argument for character encoding.
/* 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 on_load to automatically load the NIF library when the module is loaded.
Note:
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.
FUNCTIONALITY¶
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:- Read and write Erlang terms:
- 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 ERL_NIF_TERM and can only be read or written using API functions. Most functions to read the content of a term are prefixed enif_get_ and usually return true (or false) if the term was of the expected type (or not). The functions to write terms are all prefixed enif_make_ and usually return the created ERL_NIF_TERM. There are also some functions to query terms, like enif_is_atom, enif_is_identical and enif_compare.
All terms of type ERL_NIF_TERM belong to an environment of type
ErlNifEnv. The lifetime of a term is controlled by the lifetime of its
environment object. All API functions that read or write terms has the
environment, that the term belongs to, as the first function argument.
- Binaries:
- Terms of type binary are accessed with the help of the struct type ErlNifBinary that contains a pointer (data) to the raw binary data and the length ( size) of the data in bytes. Both data and size are read-only and should only be written using calls to API functions. Instances of ErlNifBinary are however always allocated by the user (usually as local variables).
The raw data pointed to by data is only mutable after a call to
enif_alloc_binary or enif_realloc_binary. All other functions
that operates on a binary will leave the data as read-only. A mutable binary
must in the end either be freed with enif_release_binary or made
read-only by transferring it to an Erlang term with enif_make_binary.
But it does not have to happen in the same NIF call. Read-only binaries do not
have to be released.
enif_make_new_binary can be used as a shortcut to allocate and return a
binary in the same NIF call.
Binaries are sequences of whole bytes. Bitstrings with an arbitrary bit length
have no support yet.
- Resource objects:
- The use of resource objects is a way to return pointers to native data structures from a NIF in a safe way. A resource object is just a block of memory allocated with enif_alloc_resource. A handle ("safe pointer") to this memory block can then be returned to Erlang by the use of enif_make_resource. The term returned by enif_make_resource is totally opaque in nature. It can be stored and passed between processes on the same node, but the only real end usage is to pass it back as an argument to a NIF. The NIF can then call enif_get_resource and get back a pointer to the memory block that is guaranteed to still be valid. A resource object will not be deallocated until the last handle term has been garbage collected by the VM and the resource has been released with enif_release_resource (not necessarily in that order).
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 enif_open_resource_type when a library is loaded.
Objects of that resource type can then later be allocated and
enif_get_resource verifies that the resource is of the expected type. A
resource type can have a user supplied destructor function that is
automatically called when resources of that type are released (by either the
garbage collector or enif_release_resource). Resource types are
uniquely identified by a supplied name string and the name of the implementing
module.
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 enif_make_resource creates the term to return to Erlang,
the code can choose to either keep its own native pointer to the allocated
struct and release it later, or release it immediately and rely solely on the
garbage collector to eventually deallocate the resource object when it
collects the term.
Another usage of resource objects is to create binary terms with user defined
memory management. enif_make_resource_binary will create a binary term
that is connected to a resource object. The destructor of the resource will be
called when the binary is garbage collected, at which time the binary data can
be released. An example of this can be a binary term consisting of data from a
mmap'ed file. The destructor can then do munmap to release the
memory region.
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.
- Threads and concurrency:
- 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 enif_priv_data you need to supply your own explicit synchronization. This includes terms in process independent environments that are shared between threads. Resource objects will also require synchronization if you treat them as mutable.
The library initialization callbacks load, reload and
upgrade are all thread-safe even for shared state data.
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.
INITIALIZATION¶
- ERL_NIF_INIT(MODULE, ErlNifFunc funcs[], load, reload, upgrade, unload):
- This is the magic macro to initialize a NIF library. It should be evaluated in global file scope.
MODULE is the name of the Erlang module as an identifier without string
quotations. It will be stringified by the macro.
funcs is a static array of function descriptors for all the implemented
NIFs in this library.
load, reload, upgrade and unload are pointers to
functions. One of load, reload or upgrade will be called
to initialize the library. unload is called to release the library.
They are all described individually below.
- int (*load)(ErlNifEnv* env, void** priv_data, ERL_NIF_TERM load_info):
- load is called when the NIF library is loaded and there is no previously loaded library for this module.
*priv_data can be set to point to some private data that the library
needs in order to keep a state between NIF calls. enif_priv_data will
return this pointer. *priv_data will be initialized to NULL when
load is called.
load_info is the second argument to erlang:load_nif/2.
The library will fail to load if load returns anything other than 0.
load can be NULL in case no initialization is needed.
- int (*upgrade)(ErlNifEnv* env, void** priv_data, void** old_priv_data, ERL_NIF_TERM load_info):
- upgrade is called when the NIF library is loaded and there is old code of this module with a loaded NIF library.
Works the same as load. The only difference is that *old_priv_data
already contains the value set by the last call to load or
reload for the old module code. *priv_data will be initialized
to NULL when upgrade is called. It is allowed to write to both
*priv_data and *old_priv_data.
The library will fail to load if upgrade returns anything other than 0 or
if upgrade is NULL.
- void (*unload)(ErlNifEnv* env, void* priv_data):
- unload is called when the module code that the NIF library belongs to is purged as old. New code of the same module may or may not exist. Note that unload is not called for a replaced library as a consequence of reload.
- int (*reload)(ErlNifEnv* env, void** priv_data, ERL_NIF_TERM load_info):
- reload is called when the NIF library is loaded and there is already a previously loaded library for this module code.
Works the same as load. The only difference is that *priv_data
already contains the value set by the previous call to load or
reload.
The library will fail to load if reload returns anything other than 0 or
if reload is NULL.
DATA TYPES¶
- ERL_NIF_TERM:
- Variables of type ERL_NIF_TERM can refer to any Erlang term. This is an opaque type and values of it can only by used either as arguments to API functions or as return values from NIFs. All ERL_NIF_TERM's belong to an environment (ErlNifEnv). A term can not be destructed individually, it is valid until its environment is destructed.
- ErlNifEnv:
- ErlNifEnv represents an environment that can host Erlang terms. All terms in an environment are valid as long as the environment is valid. ErlNifEnv is an opaque type and pointers to it can only be passed on to API functions. There are two types of environments; process bound and process independent.
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
enif_alloc_env. It can be used to store terms between NIF calls and to
send terms with enif_send. A process independent environment with all
its terms is valid until you explicitly invalidates it with
enif_free_env or enif_send.
All elements of a list/tuple must belong to the same environment as the
list/tuple itself. Terms can be copied between environments with
enif_make_copy.
- ErlNifFunc:
-
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. fptr is a pointer
to the function that implements the NIF. The argument argv of a NIF
will contain the function arguments passed to the NIF and argc is the
length of the array, i.e. the function arity. argv[N-1] will thus
denote the Nth argument to the NIF. Note that the argc argument allows
for the same C function to implement several Erlang functions with different
arity (but same name probably).
- ErlNifBinary:
-
typedef struct { unsigned size; unsigned char* data; } ErlNifBinary;
ErlNifBinary contains transient information about an inspected binary
term. data is a pointer to a buffer of size bytes with the raw
content of the binary.
Note that ErlNifBinary is a semi-opaque type and you are only allowed to
read fields size and data.
- ErlNifPid:
- ErlNifPid is a process identifier (pid). In contrast to pid terms (instances of ERL_NIF_TERM), ErlNifPid's are self contained and not bound to any environment. ErlNifPid is an opaque type.
- ErlNifResourceType:
- Each instance of ErlNifResourceType represent a class of memory managed resource objects that can be garbage collected. Each resource type has a unique name and a destructor function that is called when objects of its type are released.
- ErlNifResourceDtor:
-
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.
- ErlNifCharEncoding:
-
typedef enum { ERL_NIF_LATIN1 }ErlNifCharEncoding;
The character encoding used in strings and atoms. The only supported encoding is
currently ERL_NIF_LATIN1 for iso-latin-1 (8-bit ascii).
- ErlNifSysInfo:
- Used by enif_system_info to return information about the runtime system. Contains currently the exact same content as ErlDrvSysInfo.
- ErlNifSInt64:
- A native signed 64-bit integer type.
- ErlNifUInt64:
- A native unsigned 64-bit integer type.
EXPORTS¶
void*enif_alloc(size_t size)
Allocate memory of size bytes. Return NULL if allocation failed.
Allocate a new binary of size size bytes. Initialize the structure
pointed to by bin to refer to the allocated binary. The binary must
either be released by enif_release_binary or ownership transferred to
an Erlang term with enif_make_binary. An allocated (and owned)
ErlNifBinary can be kept between NIF calls.
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 enif_make_copy or be sent to a process as a
message with enif_send.
Return pointer to the new environment.
Allocate a memory managed resource object of type type and size
size bytes.
Free all terms in an environment and clear it for reuse. The environment must
have been allocated with enif_alloc_env.
Return an integer less than, equal to, or greater than zero if lhs is
found, respectively, to be less than, equal, or greater than rhs.
Corresponds to the Erlang operators ==, /=, =<,
<, >= and > (but not =:= or
=/=).
Same as erl_drv_cond_broadcast.
Same as erl_drv_cond_create.
Same as erl_drv_cond_destroy.
Same as erl_drv_cond_signal.
Same as erl_drv_cond_wait.
Same as erl_drv_equal_tids.
Free memory allocated by enif_alloc.
Free an environment allocated with enif_alloc_env. All terms created in
the environment will be freed as well.
Write a null-terminated string, in the buffer pointed to by buf of size
size, consisting of the string representation of the atom term
with encoding encode. Return the number of bytes written (including
terminating null character) or 0 if term is not an atom with maximum
length of size-1.
Set *len to the length (number of bytes excluding terminating null
character) of the atom term with encoding encode. Return true on
success or false if term is not an atom.
Set *dp to the floating point value of term. Return true on
success or false if term is not a float.
Set *ip to the integer value of term. Return true on success or
false if term is not an integer or is outside the bounds of type
int.
Set *ip to the integer value of term. Return true on success or
false if term is not an integer or is outside the bounds of a signed
64-bit integer.
If term is the pid of a node local process, initialize the pid variable
*pid from it and return true. Otherwise return false. No check if the
process is alive is done.
Set *head and *tail from list and return true, or return
false if list is not a non-empty list.
Set *len to the length of list term and return true, or return
false if term is not a list.
Set *ip to the long integer value of term and return true, or
return false if term is not an integer or is outside the bounds of type
long int.
Set *objp to point to the resource object referred to by term.
Return true on success or false if term is not a handle to a resource
object of type type.
Write a null-terminated string, in the buffer pointed to by buf with size
size, consisting of the characters in the string list. The
characters are written using encoding encode. Return the number of
bytes written (including terminating null character), or -size if the
string was truncated due to buffer space, or 0 if list is not a string
that can be encoded with encode or if size was less than 1. The
written string is always null-terminated unless buffer size is less
than 1.
If term is a tuple, set *array to point to an array containing the
elements of the tuple and set *arity to the number of elements. Note
that the array is read-only and (*array)[N-1] will be the Nth element
of the tuple. *array is undefined if the arity of the tuple is zero.
Return true on success or false if term is not a tuple.
Set *ip to the unsigned integer value of term and return true, or
return false if term is not an unsigned integer or is outside the
bounds of type unsigned int.
Set *ip to the unsigned integer value of term and return true, or
return false if term is not an unsigned integer or is outside the
bounds of an unsigned 64-bit integer.
Set *ip to the unsigned long integer value of term and return
true, or return false if term is not an unsigned integer or is outside
the bounds of type unsigned long.
Initialize the structure pointed to by bin with information about the
binary term bin_term. Return true on success or false if
bin_term is not a binary.
Initialize the structure pointed to by bin with one continuous buffer
with the same byte content as iolist. As with inspect_binary, the data
pointed to by bin is transient and does not need to be released. Return
true on success or false if iolist is not an iolist.
Return true if term is an atom.
Return true if term is a binary
Return true if term is an empty list.
Return true if term is an exception.
Return true if term is a number.
Return true if term is a fun.
Return true if the two terms are identical. Corresponds to the Erlang operators
=:= and =/=.
Return true if term is a pid.
Return true if term is a port.
Return true if term is a reference.
Return true if term is a tuple.
Return true if term is a list.
Add a reference to resource object obj obtained from
enif_alloc_resource. Each call to enif_keep_resource for an
object must be balanced by a call to enif_release_resource before the
object will be destructed.
Create an atom term from the null-terminated C-string name with
iso-latin-1 encoding.
Create an atom term from the string name with length len.
Null-characters are treated as any other characters.
Make a badarg exception to be returned from a NIF, and set an associated
exception reason in env. If enif_make_badarg is called, the term
it returns must be returned from the function that called it. No other
return value is allowed. Also, the term returned from enif_make_badarg
may be passed only to enif_is_exception and not to any other NIF API
function.
Make a binary term from bin. Any ownership of the binary data will be
transferred to the created term and bin should be considered read-only
for the rest of the NIF call and then as released.
Make a copy of term src_term. The copy will be created in environment
dst_env. The source term may be located in any environment.
Create a floating-point term from a double.
Try to create the term of an already existing atom from the null-terminated
C-string name with encoding encode. If the atom already exists
store the term in *atom and return true, otherwise return false.
Try to create the term of an already existing atom from the string name
with length len and encoding encode. Null-characters are treated
as any other characters. If the atom already exists store the term in
*atom and return true, otherwise return false.
Create an integer term.
Create an integer term from a signed 64-bit integer.
Create an ordinary list term of length cnt. Expects cnt number of
arguments (after cnt) of type ERL_NIF_TERM as the elements of the list.
An empty list is returned if cnt is 0.
Create an ordinary list term with length indicated by the function name. Prefer
these functions (macros) over the variadic enif_make_list to get a
compile time error if the number of arguments does not match.
Create a list cell [head | tail].
Create an ordinary list containing the elements of array arr of length
cnt. An empty list is returned if cnt is 0.
Set *list to the reverse list of the list term and return true, or
return false if term is not a list. This function should only be used
on short lists as a copy will be created of the list which will not be
released until after the nif returns.
Create an integer term from a long int.
Allocate a binary of size size bytes and create an owning term. The
binary data is mutable until the calling NIF returns. This is a quick way to
create a new binary without having to use ErlNifBinary. The drawbacks
are that the binary can not be kept between NIF calls and it can not be
reallocated.
Return a pointer to the raw binary data and set *termp to the binary
term.
Make a pid term from *pid.
Create a reference like erlang:make_ref/0.
Create an opaque handle to a memory managed resource object obtained by
enif_alloc_resource. No ownership transfer is done, as the resource
object still needs to be released by enif_release_resource, but note
that the call to enif_release_resource can occur immediately after
obtaining the term from enif_make_resource, in which case the resource
object will be deallocated when the term is garbage collected. See the
example of creating and returning a resource object for more details.
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 term_to_binary will have unpredictable
(but harmless) results.
Create a binary term that is memory managed by a resource object obj
obtained by enif_alloc_resource. The returned binary term will consist
of size bytes pointed to by data. This raw binary data must be
kept readable and unchanged until the destructor of the resource is called.
The binary data may be stored external to the resource object in which case it
is the responsibility of the destructor to release the data.
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 enif_make_resource, no ownership transfer is done. The resource
still needs to be released with enif_release_resource.
Create a list containing the characters of the null-terminated string
string with encoding encoding.
Create a list containing the characters of the string string with length
len and encoding encoding. Null-characters are treated as any
other characters.
Make a subbinary of binary bin_term, starting at zero-based position
pos with a length of size bytes. bin_term must be a
binary or bitstring and pos+size must be less or equal to the number of
whole bytes in bin_term.
Create a tuple term of arity cnt. Expects cnt number of arguments
(after cnt) of type ERL_NIF_TERM as the elements of the tuple.
Create a tuple term with length indicated by the function name. Prefer these
functions (macros) over the variadic enif_make_tuple to get a compile
time error if the number of arguments does not match.
Create a tuple containing the elements of array arr of length
cnt.
Create an integer term from an unsigned int.
Create an integer term from an unsigned 64-bit integer.
Create an integer term from an unsigned long int.
Same as erl_drv_mutex_create.
Same as erl_drv_mutex_destroy.
Same as erl_drv_mutex_lock.
Same as erl_drv_mutex_trylock.
Same as erl_drv_mutex_unlock.
Create or takeover a resource type identified by the string name and give
it the destructor function pointed to by dtor. Argument flags
can have the following values:
The two flag values can be combined with bitwise-or. The name of the resource
type is local to the calling module. Argument module_str is not (yet)
used and must be NULL. The dtor may be NULL in case no
destructor is needed.
On success, return a pointer to the resource type and *tried will be set
to either ERL_NIF_RT_CREATE or ERL_NIF_RT_TAKEOVER to indicate
what was actually done. On failure, return NULL and set *tried
to flags. It is allowed to set tried to NULL.
Note that enif_open_resource_type is only allowed to be called in the
three callbacks load, reload and upgrade.
- ERL_NIF_RT_CREATE:
- Create a new resource type that does not already exist.
- ERL_NIF_RT_TAKEOVER:
- Open an existing resource type and take over ownership of all its instances. The supplied destructor dtor will be called both for existing instances as well as new instances not yet created by the calling NIF library.
Return the pointer to the private data that was set by load,
reload or upgrade.
Was previously named enif_get_data.
Change the size of a binary bin. The source binary may be read-only, in
which case it will be left untouched and a mutable copy is allocated and
assigned to *bin. Return true on success, false if memory allocation
failed.
Release a binary obtained from enif_alloc_binary.
Remove a reference to resource object objobtained from
enif_alloc_resource. The resource object will be destructed when the
last reference is removed. Each call to enif_release_resource must
correspond to a previous call to enif_alloc_resource or
enif_keep_resource. References made by enif_make_resource can
only be removed by the garbage collector.
Same as erl_drv_rwlock_create.
Same as erl_drv_rwlock_destroy.
Same as erl_drv_rwlock_rlock.
Same as erl_drv_rwlock_runlock.
Same as erl_drv_rwlock_rwlock.
Same as erl_drv_rwlock_rwunlock.
Same as erl_drv_rwlock_tryrlock.
Same as erl_drv_rwlock_tryrwlock.
Initialize the pid variable *pid to represent the calling process. Return
pid.
Send a message to a process.
Return true on success, or false if *to_pid does not refer to an alive
local process.
The message environment msg_env with all its terms (including msg)
will be invalidated by a successful call to enif_send. The environment
should either be freed with enif_free_env of cleared for reuse with
enif_clear_env.
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.
- env:
- The environment of the calling process. Must be NULL if and only if calling from a created thread.
- *to_pid:
- The pid of the receiving process. The pid should refer to a process on the local node.
- msg_env:
- The environment of the message term. Must be a process independent environment allocated with enif_alloc_env.
- msg:
- The message term to send.
Get the byte size of a resource object obj obtained by
enif_alloc_resource.
Same as driver_system_info.
Same as erl_drv_thread_create.
Same as erl_drv_thread_exit.
Same as erl_drv_thread_join .
Same as erl_drv_thread_opts_create.
Same as erl_drv_thread_opts_destroy.
Same as erl_drv_thread_self.
Same as erl_drv_tsd_key_create.
Same as erl_drv_tsd_key_destroy.
Same as erl_drv_tsd_get.
Same as erl_drv_tsd_set.
SEE ALSO¶
erlang:load_nif/2erts 5.9.1 | Ericsson AB |