erl_nif(3)

NAME
erl_nif - API functions for an Erlang NIF library.

DESCRIPTION

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. A NIF library is built as a dynamically linked library file and loaded in runtime by calling erlang:load_nif/2.

    Warning:

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 cannot provide the same services as provided when executing Erlang code, such as pre-emptive scheduling or memory protection. If the native function does not behave well, the whole VM will misbehave.

    *   A native function that crashes will crash the whole VM.

    *   An erroneously implemented native function can cause a VM internal state inconsistency, which can 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 doing lengthy work before returning degrades responsiveness of the VM, and can 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 can occur because of lengthy work can also vary between Erlang/OTP releases.

EXAMPLE

A minimal example of a NIF library can look as follows:

/* 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)

The Erlang module can look as follows:

-module(niftest).
-export([init/0, hello/0]).
-on_load(init/0).
init() ->
      erlang:load_nif("./niftest", 0).
hello() ->
      erlang:nif_error("NIF library not loaded").

Compile and test can look as follows (on Linux):

$> gcc -fPIC -shared -o niftest.so niftest.c -I $ERL_ROOT/usr/include/
$> erl
1> c(niftest).
{ok,niftest}
2> niftest:hello().
"Hello world!"

In the example above the on_load directive is used get function init called automatically when the module is loaded. Function init in turn calls erlang:load_nif/2 which loads the NIF library and replaces the hello function with its native implementation in C. Once loaded, a NIF library is persistent. It will not be unloaded until the module code version that it belongs to is purged.

Each NIF must have an implementation in Erlang to be invoked if the function is called before the NIF library is successfully loaded. A typical such stub implementation is to call erlang:nif_error which will raise an exception. The Erlang function can also be used as a fallback implementation if the NIF library lacks implementation for some OS or hardware architecture for example.

    Note:
A NIF does not have to be exported, it can be local to the module. However, unused local stub functions will be optimized away by the compiler, causing loading of the NIF library to fail.

FUNCTIONALITY

All interaction between NIF code and the Erlang runtime system is performed by calling NIF API functions. Functions exist 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 is 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 struct type ErlNifBinary, which 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 are only to 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 operate on a binary 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. However, it does not have to occur 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 safe way to return pointers to native data structures from a NIF. A resource object is only 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 opaque in nature. It can be stored and passed between processes, 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, which is guaranteed to still be valid. A resource object is not deallocated until the last handle term is garbage collected by the VM and the resource is 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, which 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.

        The following 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;

        Notice 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 only on the garbage collector to deallocate the resource object eventually when it collects the term.

        Another use of resource objects is to create binary terms with user-defined memory management. enif_make_resource_binary creates a binary term that is connected to a resource object. The destructor of the resource is 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 take over an already existing resource type and by that "inherit" all existing objects of that type. The destructor of the new library is thereafter 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 is postponed as long as there exist resource objects with a destructor function in the library.

    Module upgrade and static data:
        A loaded NIF library is tied to the Erlang module instance that loaded it. If the module is upgraded, the new module instance needs to load its own NIF library (or maybe choose not to). The new module instance can, however, choose to load the exact same NIF library as the old code if it wants to. Sharing the dynamic library means that static data defined by the library is shared as well. To avoid unintentionally shared static data between module instances, each Erlang module version can keep its own private data. This private data can be set when the NIF library is loaded and later retrieved by calling enif_priv_data.

    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. When you write to 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 also require synchronization if you treat them as mutable.

        The library initialization callbacks load and upgrade are thread-safe even for shared state data.

    Version Management:
        When a NIF library is built, information about the NIF API version is compiled into the library. When a NIF library is loaded, the runtime system verifies that the library is of a compatible version. erl_nif.h defines the following:

        ERL_NIF_MAJOR_VERSION:
            Incremented when NIF library incompatible changes are made to the Erlang runtime system. Normally it suffices to recompile the NIF library when the ERL_NIF_MAJOR_VERSION has changed, but it can, under rare circumstances, mean that NIF libraries must be slightly modified. If so, this will of course be documented.

        ERL_NIF_MINOR_VERSION:
            Incremented when new features are added. The runtime system uses the minor version to determine what features to use.

        The runtime system normally refuses to load a NIF library if the major versions differ, or if the major versions are equal and the minor version used by the NIF library is greater than the one used by the runtime system. Old NIF libraries with lower major versions are, however, allowed after a bump of the major version during a transition period of two major releases. Such old NIF libraries can however fail if deprecated features are used.

    Time Measurement:
        Support for time measurement in NIF libraries:

        *   ErlNifTime

        *   ErlNifTimeUnit

        *   enif_monotonic_time()

        *   enif_time_offset()

        *   enif_convert_time_unit()

    I/O Queues:
        The Erlang nif library contains function for easily working with I/O vectors as used by the unix system call writev. The I/O Queue is not thread safe, so some other synchronization mechanism has to be used.

        *   SysIOVec

        *   ErlNifIOVec

        *   enif_ioq_create()

        *   enif_ioq_destroy()

        *   enif_ioq_enq_binary()

        *   enif_ioq_enqv()

        *   enif_ioq_deq()

        *   enif_ioq_peek()

        *   enif_ioq_peek_head()

        *   enif_inspect_iovec()

        *   enif_free_iovec()

        Typical usage when writing to a file descriptor looks like this:

    int writeiovec(ErlNifEnv *env, ERL_NIF_TERM term, ERL_NIF_TERM *tail,
                   ErlNifIOQueue *q, int fd) {
        ErlNifIOVec vec, *iovec = &vec;
        SysIOVec *sysiovec;
        int saved_errno;
        int iovcnt, n;
        if (!enif_inspect_iovec(env, 64, term, tail, &iovec))
            return -2;
        if (enif_ioq_size(q) > 0) {
            /* If the I/O queue contains data we enqueue the iovec and
               then peek the data to write out of the queue. */
            if (!enif_ioq_enqv(q, iovec, 0))
                return -3;
            sysiovec = enif_ioq_peek(q, &iovcnt);
        } else {
            /* If the I/O queue is empty we skip the trip through it. */
            iovcnt = iovec->iovcnt;
            sysiovec = iovec->iov;
        }
        /* Attempt to write the data */
        n = writev(fd, sysiovec, iovcnt);
        saved_errno = errno;
        if (enif_ioq_size(q) == 0) {
            /* If the I/O queue was initially empty we enqueue any
               remaining data into the queue for writing later. */
            if (n >= 0 && !enif_ioq_enqv(q, iovec, n))
                return -3;
        } else {
            /* Dequeue any data that was written from the queue. */
            if (n > 0 && !enif_ioq_deq(q, n, NULL))
                return -4;
        }
        /* return n, which is either number of bytes written or -1 if
           some error happened */
        errno = saved_errno;
        return n;
    }

    Long-running NIFs:
        As mentioned in the warning text at the beginning of this manual page, it is of vital importance that a native function returns relatively fast. It is difficult to give an exact maximum amount of time that a native function is allowed to work, but usually a well-behaving native function is to return to its caller within 1 millisecond. This can be achieved using different approaches. If you have full control over the code 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 is, however, not always possible, for example when calling third-party libraries.

        The enif_consume_timeslice() function can be used to inform the runtime system about the length of the NIF call. It is typically always to be used unless the NIF executes very fast.

        If the NIF call is too lengthy, this must be handled in one of the following ways to avoid degraded responsiveness, scheduler load balancing problems, and other strange behaviors:

        Yielding NIF:
            If the functionality of a long-running NIF can be split so that its work can be achieved through a series of shorter NIF calls, the application has two options:

            *   Make that series of NIF calls from the Erlang level.

            *   Call a NIF that first performs a chunk of the work, then invokes the enif_schedule_nif function to schedule another NIF call to perform the next chunk. The final call scheduled in this manner can then return the overall result.

            Breaking up a long-running function in this manner enables the VM to regain control between calls to the NIFs.

            This approach is always preferred over the other alternatives described below. This both from a performance perspective and a system characteristics perspective.

        Threaded NIF:
            This is accomplished by dispatching the work to another thread managed by the NIF library, return from the NIF, and wait for the result. The thread can send the result back to the Erlang process using enif_send. Information about thread primitives is provided below.

        Dirty NIF:

            Note:
        Dirty NIF support is available only when the emulator is configured with dirty scheduler support. As of ERTS version 9.0, dirty scheduler support is enabled by default on the runtime system with SMP support. The Erlang runtime without SMP support does not support dirty schedulers even when the dirty scheduler support is explicitly enabled. To check at runtime for the presence of dirty scheduler threads, code can use the enif_system_info() API function.

            A NIF that cannot be split and cannot execute in a millisecond or less is called a "dirty NIF", as it performs work that the ordinary schedulers of the Erlang runtime system 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. This is handled by executing dirty jobs on a separate set of schedulers called dirty schedulers. A dirty NIF executing on a dirty scheduler does not have the same duration restriction as a normal NIF.

            It is important to classify the dirty job correct. An I/O bound job should be classified as such, and a CPU bound job should be classified as such. If you should classify CPU bound jobs as I/O bound jobs, dirty I/O schedulers might starve ordinary schedulers. I/O bound jobs are expected to either block waiting for I/O, and/or spend a limited amount of time moving data.

            To schedule a dirty NIF for execution, the application has two options:

            *   Set the appropriate flags value for the dirty NIF in its ErlNifFunc entry.

            *   Call enif_schedule_nif, pass to it a pointer to the dirty NIF to be executed, and indicate with argument flags whether it expects the operation to be CPU-bound or I/O-bound.

            A job that alternates between I/O bound and CPU bound can be reclassified and rescheduled using enif_schedule_nif so that it executes on the correct type of dirty scheduler at all times. For more information see the documentation of the erl(1) command line arguments +SDcpu, and +SDio.

            While a process executes a dirty NIF, some operations that communicate with it can take a very long time to complete. Suspend or garbage collection of a process executing a dirty NIF cannot be done until the dirty NIF has returned. Thus, other processes waiting for such operations to complete might have to wait for a very long time. Blocking multi-scheduling, that is, calling erlang:system_flag(multi_scheduling, block), can also take a very long time to complete. This is because all ongoing dirty operations on all dirty schedulers must complete before the block operation can complete.

            Many operations communicating with a process executing a dirty NIF can, however, complete while it executes the dirty NIF. For example, retrieving information about it through process_info, setting its group leader, register/unregister its name, and so on.

            Termination of a process executing a dirty NIF can only be completed up to a certain point while it executes the dirty NIF. All Erlang resources, such as its registered name and its ETS tables, are released. All links and monitors are triggered. The execution of the NIF is, however, not stopped. The NIF can safely continue execution, allocate heap memory, and so on, but it is of course better to stop executing as soon as possible. The NIF can check whether a current process is alive using enif_is_current_process_alive. Communication using enif_send and enif_port_command is also dropped when the sending process is not alive. Deallocation of certain internal resources, such as process heap and process control block, is delayed until the dirty NIF has completed.

INITIALIZATION

    ERL_NIF_INIT(MODULE, ErlNifFunc funcs[], load, NULL, upgrade, unload):
        This is the magic macro to initialize a NIF library. It is to be evaluated in global file scope.

        MODULE is the name of the Erlang module as an identifier without string quotations. It is stringified by the macro.

        funcs is a static array of function descriptors for all the implemented NIFs in this library.

        load, upgrade and unload are pointers to functions. One of load or upgrade is called to initialize the library. unload is called to release the library. All are described individually below.

        The fourth argument NULL is ignored. It was earlier used for the deprecated reload callback which is no longer supported since OTP 20.

        If compiling a NIF for static inclusion through --enable-static-nifs, you must define STATIC_ERLANG_NIF before the ERL_NIF_INIT declaration.

    int (*load)(ErlNifEnv* caller_env, void** priv_data, ERL_NIF_TERM load_info):
        load is called when the NIF library is loaded and no previously loaded library exists for this module.

        *priv_data can be set to point to some private data if the library needs to keep a state between NIF calls. enif_priv_data returns this pointer. *priv_data is initialized to NULL when load is called.

        load_info is the second argument to erlang:load_nif/2.

        The library fails to load if load returns anything other than 0. load can be NULL if initialization is not needed.

    int (*upgrade)(ErlNifEnv* caller_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 as load, except that *old_priv_data already contains the value set by the last call to load or upgrade for the old module code. *priv_data is initialized to NULL when upgrade is called. It is allowed to write to both *priv_data and *old_priv_data.

        The library fails to load if upgrade returns anything other than 0 or if upgrade is NULL.

    void (*unload)(ErlNifEnv* caller_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.

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_TERMs belong to an environment (ErlNifEnv). A term cannot 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; pointers to it can only be passed on to API functions. Three types of environments exist:

        Process bound environment:
            Passed as the first argument to all NIFs. All function arguments passed to a NIF belong to that environment. The return value from a NIF must also be a term belonging to the same environment.

            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.

        Callback environment:
            Passed as the first argument to all the non-NIF callback functions (load, upgrade, unload, dtor, down and stop). Works like a process bound environment but with a temporary pseudo process that "terminates" when the callback has returned. Terms may be created in this environment but they will only be accessible during the callback.

        Process independent environment:
            Created by calling enif_alloc_env. This environment 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 invalidate it with enif_free_env or enif_send.

        All contained terms of a list/tuple/map must belong to the same environment as the list/tuple/map 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[]);
        unsigned flags;
    } ErlNifFunc;

        Describes a NIF by its name, arity, and implementation.

        fptr:
            A pointer to the function that implements the NIF.

        argv:
            Contains the function arguments passed to the NIF.

        argc:
            The array length, that is, the function arity. argv[N-1] thus denotes the Nth argument to the NIF. Notice that the argument argc allows for the same C function to implement several Erlang functions with different arity (but probably with the same name).

        flags:
            Is 0 for a regular NIF (and so its value can be omitted for statically initialized ErlNifFunc instances).

            flags can be used to indicate that the NIF is a dirty NIF that is to be executed on a dirty scheduler thread.

            If the dirty NIF is expected to be CPU-bound, its flags field is to be set to ERL_NIF_DIRTY_JOB_CPU_BOUND or ERL_NIF_DIRTY_JOB_IO_BOUND.

            Note:
        If one of the ERL_NIF_DIRTY_JOB_*_BOUND flags is set, and the runtime system has no support for dirty schedulers, the runtime system refuses to load the NIF library.

    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.

        Notice that ErlNifBinary is a semi-opaque type and you are only allowed to read fields size and data.

    ErlNifBinaryToTerm:
        An enumeration of the options that can be specified to enif_binary_to_term. For default behavior, use value 0.

        When receiving data from untrusted sources, use option ERL_NIF_BIN2TERM_SAFE.

    ErlNifMonitor:
        This is an opaque data type that identifies a monitor.

        The nif writer is to provide the memory for storing the monitor when calling enif_monitor_process. The address of the data is not stored by the runtime system, so ErlNifMonitor can be used as any other data, it can be copied, moved in memory, forgotten, and so on. To compare two monitors, enif_compare_monitors must be used.

    ErlNifPid:
        A process identifier (pid). In contrast to pid terms (instances of ERL_NIF_TERM), ErlNifPids are self-contained and not bound to any environment. ErlNifPid is an opaque type. It can be copied, moved in memory, forgotten, and so on.

    ErlNifPort:
        A port identifier. In contrast to port ID terms (instances of ERL_NIF_TERM), ErlNifPorts are self-contained and not bound to any environment. ErlNifPort is an opaque type. It can be copied, moved in memory, forgotten, and so on.

    ErlNifResourceType:
        Each instance of ErlNifResourceType represents 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.

    ErlNifResourceTypeInit:

    typedef struct {
        ErlNifResourceDtor* dtor;
        ErlNifResourceStop* stop;
        ErlNifResourceDown* down;
    } ErlNifResourceTypeInit;

        Initialization structure read by enif_open_resource_type_x.

    ErlNifResourceDtor:

    typedef void ErlNifResourceDtor(ErlNifEnv* caller_env, void* obj);

        The function prototype of a resource destructor function.

        The obj argument is a pointer to the resource. The only allowed use for the resource in the destructor is to access its user data one final time. The destructor is guaranteed to be the last callback before the resource is deallocated.

    ErlNifResourceDown:

    typedef void ErlNifResourceDown(ErlNifEnv* caller_env, void* obj, ErlNifPid* pid, ErlNifMonitor* mon);

        The function prototype of a resource down function, called on the behalf of enif_monitor_process. obj is the resource, pid is the identity of the monitored process that is exiting, and mon is the identity of the monitor.

    ErlNifResourceStop:

    typedef void ErlNifResourceStop(ErlNifEnv* caller_env, void* obj, ErlNifEvent event, int is_direct_call);

        The function prototype of a resource stop function, called on the behalf of enif_select. obj is the resource, event is OS event, is_direct_call is true if the call is made directly from enif_select or false if it is a scheduled call (potentially from another thread).

    ErlNifCharEncoding:

    typedef enum {
        ERL_NIF_LATIN1
    }ErlNifCharEncoding;

        The character encoding used in strings and atoms. The only supported encoding is ERL_NIF_LATIN1 for ISO Latin-1 (8-bit ASCII).

    ErlNifSysInfo:
        Used by enif_system_info to return information about the runtime system. Contains the same content as ErlDrvSysInfo.

    ErlNifSInt64:
        A native signed 64-bit integer type.

    ErlNifUInt64:
        A native unsigned 64-bit integer type.

    ErlNifTime:
        A signed 64-bit integer type for representation of time.

    ErlNifTimeUnit:
        An enumeration of time units supported by the NIF API:

        ERL_NIF_SEC:
            Seconds

        ERL_NIF_MSEC:
            Milliseconds

        ERL_NIF_USEC:
            Microseconds

        ERL_NIF_NSEC:
            Nanoseconds

    ErlNifUniqueInteger:
        An enumeration of the properties that can be requested from enif_make_unique_integer. For default properties, use value 0.

        ERL_NIF_UNIQUE_POSITIVE:
            Return only positive integers.

        ERL_NIF_UNIQUE_MONOTONIC:
            Return only strictly monotonically increasing integer corresponding to creation time.

    ErlNifHash:
        An enumeration of the supported hash types that can be generated using enif_hash.

        ERL_NIF_INTERNAL_HASH:
            Non-portable hash function that only guarantees the same hash for the same term within one Erlang VM instance.

            It takes 32-bit salt values and generates hashes within 0..2^32-1.

        ERL_NIF_PHASH2:
            Portable hash function that gives the same hash for the same Erlang term regardless of machine architecture and ERTS version.

            It ignores salt values and generates hashes within 0..2^27-1.

            Slower than ERL_NIF_INTERNAL_HASH. It corresponds to erlang:phash2/1.

    SysIOVec:
        A system I/O vector, as used by writev on Unix and WSASend on Win32. It is used in ErlNifIOVec and by enif_ioq_peek.

    ErlNifIOVec:

    typedef struct {
      int iovcnt;
      size_t size;
      SysIOVec* iov;
    } ErlNifIOVec;

        An I/O vector containing iovcnt SysIOVecs pointing to the data. It is used by enif_inspect_iovec and enif_ioq_enqv.

    ErlNifIOQueueOpts:
         Options to configure a ErlNifIOQueue.

        ERL_NIF_IOQ_NORMAL:
            Create a normal I/O Queue

EXPORTS

void *enif_alloc(size_t size)

    Allocates memory of size bytes.

    Returns NULL if the allocation fails.

    The returned pointer is suitably aligned for any built-in type that fit in the allocated memory.

int enif_alloc_binary(size_t size, ErlNifBinary* bin)

    Allocates a new binary of size size bytes. Initializes 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.

    If you do not need to reallocate or keep the data alive across NIF calls, consider using enif_make_new_binary instead as it will allocate small binaries on the process heap when possible.

    Returns true on success, or false if allocation fails.

ErlNifEnv *enif_alloc_env()

    Allocates a new process independent environment. The environment can be used to hold terms that are 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.

    Returns pointer to the new environment.

void *enif_alloc_resource(ErlNifResourceType*
        type, unsigned size)

    Allocates a memory-managed resource object of type type and size size bytes.

size_t enif_binary_to_term(ErlNifEnv *env,
        const unsigned char* data, size_t size, ERL_NIF_TERM *term,
        ErlNifBinaryToTerm opts)

    Creates a term that is the result of decoding the binary data at data, which must be encoded according to the Erlang external term format. No more than size bytes are read from data. Argument opts corresponds to the second argument to erlang:binary_to_term/2 and must be either 0 or ERL_NIF_BIN2TERM_SAFE.

    On success, stores the resulting term at *term and returns the number of bytes read. Returns 0 if decoding fails or if opts is invalid.

    See also ErlNifBinaryToTerm, erlang:binary_to_term/2, and enif_term_to_binary.

void enif_clear_env(ErlNifEnv* env)

    Frees all terms in an environment and clears it for reuse. The environment must have been allocated with enif_alloc_env.

int enif_compare(ERL_NIF_TERM lhs, ERL_NIF_TERM rhs)

    Returns an integer < 0 if lhs < rhs, 0 if lhs = rhs, and > 0 if lhs > rhs. Corresponds to the Erlang operators ==, /=, =<, <, >=, and > (but not =:= or =/=).

int enif_compare_monitors(const ErlNifMonitor
        *monitor1, const ErlNifMonitor *monitor2)

    Compares two ErlNifMonitors. Can also be used to imply some artificial order on monitors, for whatever reason.

    Returns 0 if monitor1 and monitor2 are equal, < 0 if monitor1 < monitor2, and > 0 if monitor1 > monitor2.

int enif_compare_pids(const ErlNifPid *pid1, const ErlNifPid *pid2)

    Compares two ErlNifPids according to term order.

    Returns 0 if pid1 and pid2 are equal, < 0 if pid1 < pid2, and > 0 if pid1 > pid2.

void enif_cond_broadcast(ErlNifCond *cnd)

    Same as erl_drv_cond_broadcast.

ErlNifCond *enif_cond_create(char *name)

    Same as erl_drv_cond_create.

void enif_cond_destroy(ErlNifCond *cnd)

    Same as erl_drv_cond_destroy.

char*enif_cond_name(ErlNifCond* cnd)

    Same as erl_drv_cond_name.

void enif_cond_signal(ErlNifCond *cnd)

    Same as erl_drv_cond_signal.

void enif_cond_wait(ErlNifCond *cnd, ErlNifMutex *mtx)

    Same as erl_drv_cond_wait.

int enif_consume_timeslice(ErlNifEnv *env, int percent)

    Gives the runtime system a hint about how much CPU time the current NIF call has consumed since the last hint, or since the start of the NIF if no previous hint has been specified. The time is specified as a percent of the timeslice that a process is allowed to execute Erlang code until it can be suspended to give time for other runnable processes. The scheduling timeslice is not an exact entity, but can usually be approximated to about 1 millisecond.

    Notice that it is up to the runtime system to determine if and how to use this information. Implementations on some platforms can use other means to determine consumed CPU time. Lengthy NIFs should regardless of this frequently call enif_consume_timeslice to determine if it is allowed to continue execution.

    Argument percent must be an integer between 1 and 100. This function must only be called from a NIF-calling thread, and argument env must be the environment of the calling process.

    Returns 1 if the timeslice is exhausted, otherwise 0. If 1 is returned, the NIF is to return as soon as possible in order for the process to yield.

    This function is provided to better support co-operative scheduling, improve system responsiveness, and make it easier to prevent misbehaviors of the VM because of a NIF monopolizing a scheduler thread. It can be used to divide length work into a number of repeated NIF calls without the need to create threads.

    See also the warning text at the beginning of this manual page.

ErlNifTime enif_convert_time_unit(ErlNifTime
        val, ErlNifTimeUnit from, ErlNifTimeUnit to)

    Converts the val value of time unit from to the corresponding value of time unit to. The result is rounded using the floor function.

        val:
            Value to convert time unit for.

        from:
            Time unit of val.

        to: Time unit of returned value.

    Returns ERL_NIF_TIME_ERROR if called with an invalid time unit argument.

    See also ErlNifTime and ErlNifTimeUnit.

ERL_NIF_TERM enif_cpu_time(ErlNifEnv *)

    Returns the CPU time in the same format as erlang:timestamp(). The CPU time is the time the current logical CPU has spent executing since some arbitrary point in the past. If the OS does not support fetching this value, enif_cpu_time invokes enif_make_badarg.

int enif_demonitor_process(ErlNifEnv* caller_env,
      void* obj, const ErlNifMonitor* mon)

    Cancels a monitor created earlier with enif_monitor_process. Argument obj is a pointer to the resource holding the monitor and *mon identifies the monitor.

    Argument caller_env is the environment of the calling process or callback. Must only be NULL if calling from a custom thread.

    Returns 0 if the monitor was successfully identified and removed. Returns a non-zero value if the monitor could not be identified, which means it was either

        *   never created for this resource

        *   already cancelled

        *   already triggered

        *   just about to be triggered by a concurrent thread

    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.

int enif_equal_tids(ErlNifTid tid1, ErlNifTid tid2)

    Same as erl_drv_equal_tids.

int enif_fprintf(FILE *stream, const char *format, ...)

    Similar to fprintf but this format string also accepts "%T", which formats Erlang terms of type ERL_NIF_TERM.

    This function is primarily intended for debugging purpose. It is not recommended to print very large terms with %T. The function may change errno, even if successful.

void enif_free(void* ptr)

    Frees memory allocated by enif_alloc.

void enif_free_env(ErlNifEnv* env)

    Frees an environment allocated with enif_alloc_env. All terms created in the environment are freed as well.

void enif_free_iovec(ErlNifIOvec* iov)

    Frees an io vector returned from enif_inspect_iovec. This is needed only if a NULL environment is passed to enif_inspect_iovec.

    ErlNifIOVec *iovec = NULL;
    size_t max_elements = 128;
    ERL_NIF_TERM tail;
    if (!enif_inspect_iovec(NULL, max_elements, term, &tail, &iovec))
      return 0;
    // Do things with the iovec
    /* Free the iovector, possibly in another thread or nif function call */
    enif_free_iovec(iovec);

int enif_get_atom(ErlNifEnv* env, ERL_NIF_TERM
        term, char* buf, unsigned size, ErlNifCharEncoding encode)

    Writes 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.

    Returns the number of bytes written (including terminating NULL character) or 0 if term is not an atom with maximum length of size-1.

int enif_get_atom_length(ErlNifEnv* env,
        ERL_NIF_TERM term, unsigned* len, ErlNifCharEncoding encode)

    Sets *len to the length (number of bytes excluding terminating NULL character) of the atom term with encoding encode.

    Returns true on success, or false if term is not an atom.

int enif_get_double(ErlNifEnv* env,
        ERL_NIF_TERM term, double* dp)

    Sets *dp to the floating-point value of term.

    Returns true on success, or false if term is not a float.

int enif_get_int(ErlNifEnv* env, ERL_NIF_TERM
        term, int* ip)

    Sets *ip to the integer value of term.

    Returns true on success, or false if term is not an integer or is outside the bounds of type int.

int enif_get_int64(ErlNifEnv* env, ERL_NIF_TERM
        term, ErlNifSInt64* ip)

    Sets *ip to the integer value of term.

    Returns true on success, or false if term is not an integer or is outside the bounds of a signed 64-bit integer.

int enif_get_local_pid(ErlNifEnv* env,
        ERL_NIF_TERM term, ErlNifPid* pid)

    If term is the pid of a node local process, this function initializes the pid variable *pid from it and returns true. Otherwise returns false. No check is done to see if the process is alive.

        Note:
    enif_get_local_pid will return false if argument term is the atom undefined.

int enif_get_local_port(ErlNifEnv* env,
        ERL_NIF_TERM term, ErlNifPort* port_id)

    If term identifies a node local port, this function initializes the port variable *port_id from it and returns true. Otherwise returns false. No check is done to see if the port is alive.

int enif_get_list_cell(ErlNifEnv* env,
        ERL_NIF_TERM list, ERL_NIF_TERM* head, ERL_NIF_TERM* tail)

    Sets *head and *tail from list list.

    Returns true on success, or false if it is not a list or the list is empty.

int enif_get_list_length(ErlNifEnv* env,
        ERL_NIF_TERM term, unsigned* len)

    Sets *len to the length of list term.

    Returns true on success, or false if term is not a proper list.

int enif_get_long(ErlNifEnv* env, ERL_NIF_TERM
        term, long int* ip)

    Sets *ip to the long integer value of term.

    Returns true on success, or false if term is not an integer or is outside the bounds of type long int.

int enif_get_map_size(ErlNifEnv* env,
        ERL_NIF_TERM term, size_t *size)

    Sets *size to the number of key-value pairs in the map term.

    Returns true on success, or false if term is not a map.

int enif_get_map_value(ErlNifEnv* env,
        ERL_NIF_TERM map, ERL_NIF_TERM key, ERL_NIF_TERM* value)

    Sets *value to the value associated with key in the map map.

    Returns true on success, or false if map is not a map or if map does not contain key.

int enif_get_resource(ErlNifEnv* env,
        ERL_NIF_TERM term, ErlNifResourceType* type, void** objp)

    Sets *objp to point to the resource object referred to by term.

    Returns true on success, or false if term is not a handle to a resource object of type type.

    enif_get_resource does not add a reference to the resource object. However, the pointer received in *objp is guaranteed to be valid at least as long as the resource handle term is valid.

int enif_get_string(ErlNifEnv* env,
        ERL_NIF_TERM list, char* buf, unsigned size,
        ErlNifCharEncoding encode)

    Writes 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.

    Returns one of the following:

        *   The number of bytes written (including terminating NULL character)

        *   -size if the string was truncated because of buffer space

        *   0 if list is not a string that can be encoded with encode or if size was < 1.

    The written string is always NULL-terminated, unless buffer size is < 1.

int enif_get_tuple(ErlNifEnv* env, ERL_NIF_TERM
        term, int* arity, const ERL_NIF_TERM** array)

    If term is a tuple, this function sets *array to point to an array containing the elements of the tuple, and sets *arity to the number of elements. Notice that the array is read-only and (*array)[N-1] is the Nth element of the tuple. *array is undefined if the arity of the tuple is zero.

    Returns true on success, or false if term is not a tuple.

int enif_get_uint(ErlNifEnv* env, ERL_NIF_TERM
        term, unsigned int* ip)

    Sets *ip to the unsigned integer value of term.

    Returns true on success, or false if term is not an unsigned integer or is outside the bounds of type unsigned int.

int enif_get_uint64(ErlNifEnv* env,
        ERL_NIF_TERM term, ErlNifUInt64* ip)

    Sets *ip to the unsigned integer value of term.

    Returns true on success, or false if term is not an unsigned integer or is outside the bounds of an unsigned 64-bit integer.

int enif_get_ulong(ErlNifEnv* env, ERL_NIF_TERM
        term, unsigned long* ip)

    Sets *ip to the unsigned long integer value of term.

    Returns true on success, or false if term is not an unsigned integer or is outside the bounds of type unsigned long.

int enif_getenv(const char* key, char* value,
        size_t *value_size)

    Same as erl_drv_getenv.

int enif_has_pending_exception(ErlNifEnv* env,
        ERL_NIF_TERM* reason)

    Returns true if a pending exception is associated with the environment env. If reason is a NULL pointer, ignore it. Otherwise, if a pending exception associated with env exists, set *reason to the value of the exception term. For example, if enif_make_badarg is called to set a pending badarg exception, a later call to enif_has_pending_exception(env, &reason) sets *reason to the atom badarg, then return true.

    See also enif_make_badarg and enif_raise_exception.

ErlNifUInt64 enif_hash(ErlNifHash type, ERL_NIF_TERM term, ErlNifUInt64 salt)

    Hashes term according to the specified ErlNifHash type.

    Ranges of taken salt (if any) and returned value depend on the hash type.

int enif_inspect_binary(ErlNifEnv* env,
        ERL_NIF_TERM bin_term, ErlNifBinary* bin)

    Initializes the structure pointed to by bin with information about binary term bin_term.

    Returns true on success, or false if bin_term is not a binary.

int enif_inspect_iolist_as_binary(ErlNifEnv*
        env, ERL_NIF_TERM term, ErlNifBinary* bin)

    Initializes the structure pointed to by bin with a 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.

    Returns true on success, or false if iolist is not an iolist.

int enif_inspect_iovec(ErlNifEnv*
        env, size_t max_elements, ERL_NIF_TERM iovec_term, ERL_NIF_TERM* tail,
        ErlNifIOVec** iovec)

    Fills iovec with the list of binaries provided in iovec_term. The number of elements handled in the call is limited to max_elements, and tail is set to the remainder of the list. Note that the output may be longer than max_elements on some platforms.

    To create a list of binaries from an arbitrary iolist, use erlang:iolist_to_iovec/1.

    When calling this function, iovec should contain a pointer to NULL or a ErlNifIOVec structure that should be used if possible. e.g.

    /* Don't use a pre-allocated structure */
    ErlNifIOVec *iovec = NULL;
    enif_inspect_iovec(env, max_elements, term, &tail, &iovec);
    /* Use a stack-allocated vector as an optimization for vectors with few elements */
    ErlNifIOVec vec, *iovec = &vec;
    enif_inspect_iovec(env, max_elements, term, &tail, &iovec);

    The contents of the iovec is valid until the called nif function returns. If the iovec should be valid after the nif call returns, it is possible to call this function with a NULL environment. If no environment is given the iovec owns the data in the vector and it has to be explicitly freed using enif_free_iovec.

    Returns true on success, or false if iovec_term not an iovec.

ErlNifIOQueue *enif_ioq_create(ErlNifIOQueueOpts opts)

    Create a new I/O Queue that can be used to store data. opts has to be set to ERL_NIF_IOQ_NORMAL.

void enif_ioq_destroy(ErlNifIOQueue *q)

    Destroy the I/O queue and free all of it's contents

int enif_ioq_deq(ErlNifIOQueue *q, size_t count, size_t *size)

    Dequeue count bytes from the I/O queue. If size is not NULL, the new size of the queue is placed there.

    Returns true on success, or false if the I/O does not contain count bytes. On failure the queue is left un-altered.

int enif_ioq_enq_binary(ErlNifIOQueue *q, ErlNifBinary *bin, size_t skip)

    Enqueue the bin into q skipping the first skip bytes.

    Returns true on success, or false if skip is greater than the size of bin. Any ownership of the binary data is transferred to the queue and bin is to be considered read-only for the rest of the NIF call and then as released.

int enif_ioq_enqv(ErlNifIOQueue *q, ErlNifIOVec *iovec, size_t skip)

    Enqueue the iovec into q skipping the first skip bytes.

    Returns true on success, or false if skip is greater than the size of iovec.

SysIOVec *enif_ioq_peek(ErlNifIOQueue *q, int *iovlen)

    Get the I/O queue as a pointer to an array of SysIOVecs. It also returns the number of elements in iovlen.

    Nothing is removed from the queue by this function, that must be done with enif_ioq_deq.

    The returned array is suitable to use with the Unix system call writev.

int enif_ioq_peek_head(ErlNifEnv *env, ErlNifIOQueue *q, size_t *size, ERL_NIF_TERM *bin_term)

    Get the head of the IO Queue as a binary term.

    If size is not NULL, the size of the head is placed there.

    Nothing is removed from the queue by this function, that must be done with enif_ioq_deq.

    Returns true on success, or false if the queue is empty.

size_t enif_ioq_size(ErlNifIOQueue *q)

    Get the size of q.

int enif_is_atom(ErlNifEnv* env, ERL_NIF_TERM term)

    Returns true if term is an atom.

int enif_is_binary(ErlNifEnv* env, ERL_NIF_TERM term)

    Returns true if term is a binary.

int enif_is_current_process_alive(ErlNifEnv* env)

    Returns true if the currently executing process is currently alive, otherwise false.

    This function can only be used from a NIF-calling thread, and with an environment corresponding to currently executing processes.

int enif_is_empty_list(ErlNifEnv* env,
        ERL_NIF_TERM term)

    Returns true if term is an empty list.

int enif_is_exception(ErlNifEnv* env,
        ERL_NIF_TERM term)

    Return true if term is an exception.

int enif_is_fun(ErlNifEnv* env, ERL_NIF_TERM
        term)

    Returns true if term is a fun.

int enif_is_identical(ERL_NIF_TERM lhs,
        ERL_NIF_TERM rhs)

    Returns true if the two terms are identical. Corresponds to the Erlang operators =:= and =/=.

int enif_is_list(ErlNifEnv* env, ERL_NIF_TERM term)

    Returns true if term is a list.

int enif_is_map(ErlNifEnv* env, ERL_NIF_TERM
        term)

    Returns true if term is a map, otherwise false.

int enif_is_number(ErlNifEnv* env, ERL_NIF_TERM
        term)

    Returns true if term is a number.

int enif_is_pid(ErlNifEnv* env, ERL_NIF_TERM term)

    Returns true if term is a pid.

int enif_is_pid_undefined(const ErlNifPid* pid)

    Returns true if pid has been set as undefined by enif_set_pid_undefined.

int enif_is_port(ErlNifEnv* env, ERL_NIF_TERM term)

    Returns true if term is a port.

int enif_is_port_alive(ErlNifEnv* env,
        ErlNifPort *port_id)

    Returns true if port_id is alive.

    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.

int enif_is_process_alive(ErlNifEnv* env,
        ErlNifPid *pid)

    Returns true if pid is alive.

    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.

int enif_is_ref(ErlNifEnv* env, ERL_NIF_TERM term)

    Returns true if term is a reference.

int enif_is_tuple(ErlNifEnv* env, ERL_NIF_TERM term)

    Returns true if term is a tuple.

int enif_keep_resource(void* obj)

    Adds 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 is destructed.

ERL_NIF_TERM enif_make_atom(ErlNifEnv* env, const char* name)

    Creates an atom term from the NULL-terminated C-string name with ISO Latin-1 encoding. If the length of name exceeds the maximum length allowed for an atom (255 characters), enif_make_atom invokes enif_make_badarg.

ERL_NIF_TERM enif_make_atom_len(ErlNifEnv* env,
        const char* name, size_t len)

    Create an atom term from the string name with length len. NULL characters are treated as any other characters. If len exceeds the maximum length allowed for an atom (255 characters), enif_make_atom invokes enif_make_badarg.

ERL_NIF_TERM enif_make_badarg(ErlNifEnv* env)

    Makes a badarg exception to be returned from a NIF, and associates it with environment env. Once a NIF or any function it calls invokes enif_make_badarg, the runtime ensures that a badarg exception is raised when the NIF returns, even if the NIF attempts to return a non-exception term instead.

    The return value from enif_make_badarg can be used only as the return value from the NIF that invoked it (directly or indirectly) or be passed to enif_is_exception, but not to any other NIF API function.

    See also enif_has_pending_exception and enif_raise_exception.

        Note:
    Before ERTS 7.0 (Erlang/OTP 18), the return value from enif_make_badarg had to be returned from the NIF. This requirement is now lifted as the return value from the NIF is ignored if enif_make_badarg has been invoked.

ERL_NIF_TERM enif_make_binary(ErlNifEnv* env, ErlNifBinary* bin)

    Makes a binary term from bin. Any ownership of the binary data is transferred to the created term and bin is to be considered read-only for the rest of the NIF call and then as released.

ERL_NIF_TERM enif_make_copy(ErlNifEnv* dst_env,
        ERL_NIF_TERM src_term)

    Makes a copy of term src_term. The copy is created in environment dst_env. The source term can be located in any environment.

ERL_NIF_TERM enif_make_double(ErlNifEnv* env, double d)

    Creates a floating-point term from a double. If argument double is not finite or is NaN, enif_make_double invokes enif_make_badarg.

int enif_make_existing_atom(ErlNifEnv* env,
        const char* name, ERL_NIF_TERM* atom, ErlNifCharEncoding
        encode)

    Tries to create the term of an already existing atom from the NULL-terminated C-string name with encoding encode.

    If the atom already exists, this function stores the term in *atom and returns true, otherwise false. Also returns false if the length of name exceeds the maximum length allowed for an atom (255 characters).

int enif_make_existing_atom_len(ErlNifEnv* env,
        const char* name, size_t len, ERL_NIF_TERM* atom, ErlNifCharEncoding
        encoding)

    Tries 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, this function stores the term in *atom and returns true, otherwise false. Also returns false if len exceeds the maximum length allowed for an atom (255 characters).

ERL_NIF_TERM enif_make_int(ErlNifEnv* env, int i)

    Creates an integer term.

ERL_NIF_TERM enif_make_int64(ErlNifEnv* env, ErlNifSInt64 i)

    Creates an integer term from a signed 64-bit integer.

ERL_NIF_TERM enif_make_list(ErlNifEnv* env, unsigned cnt, ...)

    Creates 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.

    Returns an empty list if cnt is 0.

ERL_NIF_TERM enif_make_list1(ErlNifEnv* env, ERL_NIF_TERM e1)
ERL_NIF_TERM enif_make_list2(ErlNifEnv* env,
        ERL_NIF_TERM e1, ERL_NIF_TERM e2)
ERL_NIF_TERM enif_make_list3(ErlNifEnv* env,
        ERL_NIF_TERM e1, ERL_NIF_TERM e2, ERL_NIF_TERM e3)
ERL_NIF_TERM enif_make_list4(ErlNifEnv* env,
        ERL_NIF_TERM e1, ..., ERL_NIF_TERM e4)
ERL_NIF_TERM enif_make_list5(ErlNifEnv* env,
        ERL_NIF_TERM e1, ..., ERL_NIF_TERM e5)
ERL_NIF_TERM enif_make_list6(ErlNifEnv* env,
        ERL_NIF_TERM e1, ..., ERL_NIF_TERM e6)
ERL_NIF_TERM enif_make_list7(ErlNifEnv* env,
        ERL_NIF_TERM e1, ..., ERL_NIF_TERM e7)
ERL_NIF_TERM enif_make_list8(ErlNifEnv* env,
        ERL_NIF_TERM e1, ..., ERL_NIF_TERM e8)
ERL_NIF_TERM enif_make_list9(ErlNifEnv* env,
        ERL_NIF_TERM e1, ..., ERL_NIF_TERM e9)

    Creates 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.

ERL_NIF_TERM enif_make_list_cell(ErlNifEnv*
        env, ERL_NIF_TERM head, ERL_NIF_TERM tail)

    Creates a list cell [head | tail].

ERL_NIF_TERM enif_make_list_from_array(ErlNifEnv* env, const ERL_NIF_TERM
          arr[], unsigned cnt)

    Creates an ordinary list containing the elements of array arr of length cnt.

    Returns an empty list if cnt is 0.

ERL_NIF_TERM enif_make_long(ErlNifEnv* env, long int i)

    Creates an integer term from a long int.

int enif_make_map_put(ErlNifEnv* env,
        ERL_NIF_TERM map_in, ERL_NIF_TERM key, ERL_NIF_TERM value,
        ERL_NIF_TERM* map_out)

    Makes a copy of map map_in and inserts key with value. If key already exists in map_in, the old associated value is replaced by value.

    If successful, this function sets *map_out to the new map and returns true. Returns false if map_in is not a map.

    The map_in term must belong to environment env.

int enif_make_map_remove(ErlNifEnv* env,
        ERL_NIF_TERM map_in, ERL_NIF_TERM key, ERL_NIF_TERM* map_out)

    If map map_in contains key, this function makes a copy of map_in in *map_out, and removes key and the associated value. If map map_in does not contain key, *map_out is set to map_in.

    Returns true on success, or false if map_in is not a map.

    The map_in term must belong to environment env.

int enif_make_map_update(ErlNifEnv* env,
        ERL_NIF_TERM map_in, ERL_NIF_TERM key, ERL_NIF_TERM new_value,
        ERL_NIF_TERM* map_out)

    Makes a copy of map map_in and replace the old associated value for key with new_value.

    If successful, this function sets *map_out to the new map and returns true. Returns false if map_in is not a map or if it does not contain key.

    The map_in term must belong to environment env.

int enif_make_map_from_arrays(ErlNifEnv* env, ERL_NIF_TERM keys[],
        ERL_NIF_TERM values[], size_t cnt, ERL_NIF_TERM *map_out)

    Makes a map term from the given keys and values.

    If successful, this function sets *map_out to the new map and returns true. Returns false there are any duplicate keys.

    All keys and values must belong to env.

ERL_NIF_TERM enif_make_monitor_term(ErlNifEnv* env, const ErlNifMonitor* mon)

    Creates a term identifying the given monitor received from enif_monitor_process.

    This function is primarily intended for debugging purpose.

unsigned char *enif_make_new_binary(ErlNifEnv*
        env, size_t size, ERL_NIF_TERM* termp)

    Allocates a binary of size size bytes and creates 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 cannot be kept between NIF calls and it cannot be reallocated.

    Returns a pointer to the raw binary data and sets *termp to the binary term.

ERL_NIF_TERM enif_make_new_map(ErlNifEnv* env)

    Makes an empty map term.

ERL_NIF_TERM enif_make_pid(ErlNifEnv* env, const ErlNifPid* pid)

    Makes a pid term or the atom undefined from *pid.

ERL_NIF_TERM enif_make_ref(ErlNifEnv* env)

    Creates a reference like erlang:make_ref/0.

ERL_NIF_TERM enif_make_resource(ErlNifEnv* env, void* obj)

    Creates 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. However, notice that the call to enif_release_resource can occur immediately after obtaining the term from enif_make_resource, in which case the resource object is deallocated when the term is garbage collected. For more details, see the example of creating and returning a resource object in the User's Guide.

        Note:
    Since ERTS 9.0 (OTP-20.0), resource terms have a defined behavior when compared and serialized through term_to_binary or passed between nodes.

        *   Two resource terms will compare equal if and only if they would yield the same resource object pointer when passed to enif_get_resource.

        *   A resource term can be serialized with term_to_binary and later be fully recreated if the resource object is still alive when binary_to_term is called. A stale resource term will be returned from binary_to_term if the resource object has been deallocated. enif_get_resource will return false for stale resource terms.

            The same principles of serialization apply when passing resource terms in messages to remote nodes and back again. A resource term will act stale on all nodes except the node where its resource object is still alive in memory.

    Before ERTS 9.0 (OTP-20.0), all resource terms did compare equal to each other and to empty binaries (<<>>). If serialized, they would be recreated as plain empty binaries.

ERL_NIF_TERM enif_make_resource_binary(ErlNifEnv* env, void* obj, const
        void* data, size_t size)

    Creates a binary term that is memory-managed by a resource object obj obtained by enif_alloc_resource. The returned binary term consists 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 can be stored external to the resource object, in which case the destructor is responsible for releasing the data.

    Several binary terms can be managed by the same resource object. The destructor is not called until the last binary is garbage collected. This can be useful 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.

int enif_make_reverse_list(ErlNifEnv* env, ERL_NIF_TERM list_in,
        ERL_NIF_TERM *list_out)

    Sets *list_out to the reverse list of the list list_in and returns true, or returns false if list_in is not a list.

    This function is only to be used on short lists, as a copy is created of the list, which is not released until after the NIF returns.

    The list_in term must belong to environment env.

ERL_NIF_TERM enif_make_string(ErlNifEnv* env,
        const char* string, ErlNifCharEncoding encoding)

    Creates a list containing the characters of the NULL-terminated string string with encoding encoding.

ERL_NIF_TERM enif_make_string_len(ErlNifEnv*
        env, const char* string, size_t len, ErlNifCharEncoding
        encoding)

    Creates a list containing the characters of the string string with length len and encoding encoding. NULL characters are treated as any other characters.

ERL_NIF_TERM enif_make_sub_binary(ErlNifEnv*
        env, ERL_NIF_TERM bin_term, size_t pos, size_t size)

    Makes 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. pos+size must be less or equal to the number of whole bytes in bin_term.

ERL_NIF_TERM enif_make_tuple(ErlNifEnv* env,
        unsigned cnt, ...)

    Creates a tuple term of arity cnt. Expects cnt number of arguments (after cnt) of type ERL_NIF_TERM as the elements of the tuple.

ERL_NIF_TERM enif_make_tuple1(ErlNifEnv* env,
        ERL_NIF_TERM e1)
ERL_NIF_TERM enif_make_tuple2(ErlNifEnv* env,
        ERL_NIF_TERM e1, ERL_NIF_TERM e2)
ERL_NIF_TERM enif_make_tuple3(ErlNifEnv* env,
        ERL_NIF_TERM e1, ERL_NIF_TERM e2, ERL_NIF_TERM e3)
ERL_NIF_TERM enif_make_tuple4(ErlNifEnv* env,
        ERL_NIF_TERM e1, ..., ERL_NIF_TERM e4)
ERL_NIF_TERM enif_make_tuple5(ErlNifEnv* env,
        ERL_NIF_TERM e1, ..., ERL_NIF_TERM e5)
ERL_NIF_TERM enif_make_tuple6(ErlNifEnv* env,
        ERL_NIF_TERM e1, ..., ERL_NIF_TERM e6)
ERL_NIF_TERM enif_make_tuple7(ErlNifEnv* env,
        ERL_NIF_TERM e1, ..., ERL_NIF_TERM e7)
ERL_NIF_TERM enif_make_tuple8(ErlNifEnv* env,
        ERL_NIF_TERM e1, ..., ERL_NIF_TERM e8)
ERL_NIF_TERM enif_make_tuple9(ErlNifEnv* env,
        ERL_NIF_TERM e1, ..., ERL_NIF_TERM e9)

    Creates 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.

ERL_NIF_TERM enif_make_tuple_from_array(ErlNifEnv* env, const ERL_NIF_TERM
        arr[], unsigned cnt)

    Creates a tuple containing the elements of array arr of length cnt.

ERL_NIF_TERM enif_make_uint(ErlNifEnv* env, unsigned int i)

    Creates an integer term from an unsigned int.

ERL_NIF_TERM enif_make_uint64(ErlNifEnv* env, ErlNifUInt64 i)

    Creates an integer term from an unsigned 64-bit integer.

ERL_NIF_TERM enif_make_ulong(ErlNifEnv* env, unsigned long i)

    Creates an integer term from an unsigned long int.

ERL_NIF_TERM enif_make_unique_integer(ErlNifEnv
        *env, ErlNifUniqueInteger properties)

    Returns a unique integer with the same properties as specified by erlang:unique_integer/1.

    env is the environment to create the integer in.

    ERL_NIF_UNIQUE_POSITIVE and ERL_NIF_UNIQUE_MONOTONIC can be passed as the second argument to change the properties of the integer returned. They can be combined by OR:ing the two values together.

    See also ErlNifUniqueInteger.

int enif_map_iterator_create(ErlNifEnv *env,
        ERL_NIF_TERM map, ErlNifMapIterator *iter, ErlNifMapIteratorEntry
        entry)

    Creates an iterator for the map map by initializing the structure pointed to by iter. Argument entry determines the start position of the iterator: ERL_NIF_MAP_ITERATOR_FIRST or ERL_NIF_MAP_ITERATOR_LAST.

    Returns true on success, or false if map is not a map.

    A map iterator is only useful during the lifetime of environment env that the map belongs to. The iterator must be destroyed by calling enif_map_iterator_destroy:

    ERL_NIF_TERM key, value;
    ErlNifMapIterator iter;
    enif_map_iterator_create(env, my_map, &iter, ERL_NIF_MAP_ITERATOR_FIRST);
    while (enif_map_iterator_get_pair(env, &iter, &key, &value)) {
        do_something(key,value);
        enif_map_iterator_next(env, &iter);
    }
    enif_map_iterator_destroy(env, &iter);

        Note:
    The key-value pairs of a map have no defined iteration order. The only guarantee is that the iteration order of a single map instance is preserved during the lifetime of the environment that the map belongs to.

void enif_map_iterator_destroy(ErlNifEnv *env,
        ErlNifMapIterator *iter)

    Destroys a map iterator created by enif_map_iterator_create.

int enif_map_iterator_get_pair(ErlNifEnv *env,
        ErlNifMapIterator *iter, ERL_NIF_TERM *key, ERL_NIF_TERM
        *value)

    Gets key and value terms at the current map iterator position.

    On success, sets *key and *value and returns true. Returns false if the iterator is positioned at head (before first entry) or tail (beyond last entry).

int enif_map_iterator_is_head(ErlNifEnv *env,
        ErlNifMapIterator *iter)

    Returns true if map iterator iter is positioned before the first entry.

int enif_map_iterator_is_tail(ErlNifEnv *env,
        ErlNifMapIterator *iter)

    Returns true if map iterator iter is positioned after the last entry.

int enif_map_iterator_next(ErlNifEnv *env,
        ErlNifMapIterator *iter)

    Increments map iterator to point to the next key-value entry.

    Returns true if the iterator is now positioned at a valid key-value entry, or false if the iterator is positioned at the tail (beyond the last entry).

int enif_map_iterator_prev(ErlNifEnv *env,
        ErlNifMapIterator *iter)

    Decrements map iterator to point to the previous key-value entry.

    Returns true if the iterator is now positioned at a valid key-value entry, or false if the iterator is positioned at the head (before the first entry).

int enif_monitor_process(ErlNifEnv* caller_env,
      void* obj, const ErlNifPid* target_pid, ErlNifMonitor* mon)

    Starts monitoring a process from a resource. When a process is monitored, a process exit results in a call to the provided down callback associated with the resource type.

    Argument obj is pointer to the resource to hold the monitor and *target_pid identifies the local process to be monitored.

    If mon is not NULL, a successful call stores the identity of the monitor in the ErlNifMonitor struct pointed to by mon. This identifier is used to refer to the monitor for later removal with enif_demonitor_process or compare with enif_compare_monitors. A monitor is automatically removed when it triggers or when the resource is deallocated.

    Argument caller_env is the environment of the calling process or callback. Must only be NULL if calling from a custom thread.

    Returns 0 on success, < 0 if no down callback is provided, and > 0 if the process is no longer alive or if target_pid is undefined.

    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.

ErlNifTime enif_monotonic_time(ErlNifTimeUnit time_unit)

    Returns the current Erlang monotonic time. Notice that it is not uncommon with negative values.

    time_unit is the time unit of the returned value.

    Returns ERL_NIF_TIME_ERROR if called with an invalid time unit argument, or if called from a thread that is not a scheduler thread.

    See also ErlNifTime and ErlNifTimeUnit.

ErlNifMutex *enif_mutex_create(char *name)

    Same as erl_drv_mutex_create.

void enif_mutex_destroy(ErlNifMutex *mtx)

    Same as erl_drv_mutex_destroy.

void enif_mutex_lock(ErlNifMutex *mtx)

    Same as erl_drv_mutex_lock.

char*enif_mutex_name(ErlNifMutex* mtx)

    Same as erl_drv_mutex_name.

int enif_mutex_trylock(ErlNifMutex *mtx)

    Same as erl_drv_mutex_trylock.

void enif_mutex_unlock(ErlNifMutex *mtx)

    Same as erl_drv_mutex_unlock.

ERL_NIF_TERM enif_now_time(ErlNifEnv *env)

    Returns an erlang:now() time stamp.

    This function is deprecated.

ErlNifResourceType *enif_open_resource_type(ErlNifEnv* env, const char*
        module_str, const char* name, ErlNifResourceDtor* dtor,
        ErlNifResourceFlags flags, ErlNifResourceFlags* tried)

    Creates or takes over a resource type identified by the string name and gives it the destructor function pointed to by dtor. Argument flags can have the following values:

        ERL_NIF_RT_CREATE:
            Creates a new resource type that does not already exist.

        ERL_NIF_RT_TAKEOVER:
            Opens an existing resource type and takes over ownership of all its instances. The supplied destructor dtor is called both for existing instances and new instances not yet created by the calling NIF library.

    The two flag values can be combined with bitwise OR. The resource type name is local to the calling module. Argument module_str is not (yet) used and must be NULL. dtor can be NULL if no destructor is needed.

    On success, the function returns a pointer to the resource type and *tried is set to either ERL_NIF_RT_CREATE or ERL_NIF_RT_TAKEOVER to indicate what was done. On failure, returns NULL and sets *tried to flags. It is allowed to set tried to NULL.

    Notice that enif_open_resource_type is only allowed to be called in the two callbacks load and upgrade.

    See also enif_open_resource_type_x.

ErlNifResourceType *enif_open_resource_type_x(ErlNifEnv* env, const char* name,
    const ErlNifResourceTypeInit* init,
        ErlNifResourceFlags flags, ErlNifResourceFlags* tried)

    Same as enif_open_resource_type except it accepts additional callback functions for resource types that are used together with enif_select and enif_monitor_process.

    Argument init is a pointer to an ErlNifResourceTypeInit structure that contains the function pointers for destructor, down and stop callbacks for the resource type.

int enif_port_command(ErlNifEnv* env, const
       ErlNifPort* to_port, ErlNifEnv *msg_env, ERL_NIF_TERM msg)

    Works as erlang:port_command/2, except that it is always completely asynchronous.

        env:
            The environment of the calling process. Must not be NULL.

        *to_port:
            The port ID of the receiving port. The port ID is to refer to a port on the local node.

        msg_env:
            The environment of the message term. Can be a process independent environment allocated with enif_alloc_env or NULL.

        msg:
            The message term to send. The same limitations apply as on the payload to erlang:port_command/2.

    Using a msg_env of NULL is an optimization, which groups together calls to enif_alloc_env, enif_make_copy, enif_port_command, and enif_free_env into one call. This optimization is only useful when a majority of the terms are to be copied from env to msg_env.

    Returns true if the command is successfully sent. Returns false if the command fails, for example:

        *   *to_port does not refer to a local port.

        *   The currently executing process (that is, the sender) is not alive.

        *   msg is invalid.

    See also enif_get_local_port.

void *enif_priv_data(ErlNifEnv* env)

    Returns the pointer to the private data that was set by load or upgrade.

ERL_NIF_TERM enif_raise_exception(ErlNifEnv*
        env, ERL_NIF_TERM reason)

    Creates an error exception with the term reason to be returned from a NIF, and associates it with environment env. Once a NIF or any function it calls invokes enif_raise_exception, the runtime ensures that the exception it creates is raised when the NIF returns, even if the NIF attempts to return a non-exception term instead.

    The return value from enif_raise_exception can only be used as the return value from the NIF that invoked it (directly or indirectly) or be passed to enif_is_exception, but not to any other NIF API function.

    See also enif_has_pending_exception and enif_make_badarg.

void *enif_realloc(void* ptr, size_t size)

    Reallocates memory allocated by enif_alloc to size bytes.

    Returns NULL if the reallocation fails.

    The returned pointer is suitably aligned for any built-in type that fit in the allocated memory.

int enif_realloc_binary(ErlNifBinary* bin, size_t size)

    Changes the size of a binary bin. The source binary can be read-only, in which case it is left untouched and a mutable copy is allocated and assigned to *bin.

    Returns true on success, or false if memory allocation failed.

void enif_release_binary(ErlNifBinary* bin)

    Releases a binary obtained from enif_alloc_binary.

void enif_release_resource(void* obj)

    Removes a reference to resource object obj obtained from enif_alloc_resource. The resource object is 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.

    There are no guarantees exactly when the destructor of an unreferenced resource is called. It could be called directly by enif_release_resource but it could also be scheduled to be called at a later time possibly by another thread.

ErlNifRWLock *enif_rwlock_create(char *name)

    Same as erl_drv_rwlock_create.

void enif_rwlock_destroy(ErlNifRWLock *rwlck)

    Same as erl_drv_rwlock_destroy.

char*enif_rwlock_name(ErlNifRWLock* rwlck)

    Same as erl_drv_rwlock_name.

void enif_rwlock_rlock(ErlNifRWLock *rwlck)

    Same as erl_drv_rwlock_rlock.

void enif_rwlock_runlock(ErlNifRWLock *rwlck)

    Same as erl_drv_rwlock_runlock.

void enif_rwlock_rwlock(ErlNifRWLock *rwlck)

    Same as erl_drv_rwlock_rwlock.

void enif_rwlock_rwunlock(ErlNifRWLock *rwlck)

    Same as erl_drv_rwlock_rwunlock.

int enif_rwlock_tryrlock(ErlNifRWLock *rwlck)

    Same as erl_drv_rwlock_tryrlock.

int enif_rwlock_tryrwlock(ErlNifRWLock *rwlck)

    Same as erl_drv_rwlock_tryrwlock.

ERL_NIF_TERM enif_schedule_nif(ErlNifEnv* env,
        const char* fun_name, int flags, ERL_NIF_TERM (*fp)(ErlNifEnv* env, int
        argc, const ERL_NIF_TERM argv[]), int argc, const ERL_NIF_TERM
        argv[])

    Schedules NIF fp to execute. This function allows an application to break up long-running work into multiple regular NIF calls or to schedule a dirty NIF to execute on a dirty scheduler thread.

        fun_name:
            Provides a name for the NIF that is scheduled for execution. If it cannot be converted to an atom, enif_schedule_nif returns a badarg exception.

        flags:
            Must be set to 0 for a regular NIF. If the emulator was built with dirty scheduler support enabled, flags can be set to either ERL_NIF_DIRTY_JOB_CPU_BOUND if the job is expected to be CPU-bound, or ERL_NIF_DIRTY_JOB_IO_BOUND for jobs that will be I/O-bound. If dirty scheduler threads are not available in the emulator, an attempt to schedule such a job results in a notsup exception.

        argc and argv:
            Can either be the originals passed into the calling NIF, or can be values created by the calling NIF.

    The calling NIF must use the return value of enif_schedule_nif as its own return value.

    Be aware that enif_schedule_nif, as its name implies, only schedules the NIF for future execution. The calling NIF does not block waiting for the scheduled NIF to execute and return. This means that the calling NIF cannot expect to receive the scheduled NIF return value and use it for further operations.

int enif_select(ErlNifEnv* env, ErlNifEvent event, enum ErlNifSelectFlags mode,
    void* obj, const ErlNifPid* pid, ERL_NIF_TERM ref)

    This function can be used to receive asynchronous notifications when OS-specific event objects become ready for either read or write operations.

    Argument event identifies the event object. On Unix systems, the functions select/poll are used. The event object must be a socket, pipe or other file descriptor object that select/poll can use.

    Argument mode describes the type of events to wait for. It can be ERL_NIF_SELECT_READ, ERL_NIF_SELECT_WRITE or a bitwise OR combination to wait for both. It can also be ERL_NIF_SELECT_STOP or ERL_NIF_SELECT_CANCEL which are described further below. When a read or write event is triggered, a notification message like this is sent to the process identified by pid:

    {select, Obj, Ref, ready_input | ready_output}

    ready_input or ready_output indicates if the event object is ready for reading or writing.

        Note:
    For complete control over the message format use the newer functions enif_select_read or enif_select_write introduced in erts-11.0 (OTP-22.0).

    Argument pid may be NULL to indicate the calling process. It must not be set as undefined.

    Argument obj is a resource object obtained from enif_alloc_resource. The purpose of the resource objects is as a container of the event object to manage its state and lifetime. A handle to the resource is received in the notification message as Obj.

    Argument ref must be either a reference obtained from erlang:make_ref/0 or the atom undefined. It will be passed as Ref in the notifications. If a selective receive statement is used to wait for the notification then a reference created just before the receive will exploit a runtime optimization that bypasses all earlier received messages in the queue.

    The notifications are one-shot only. To receive further notifications of the same type (read or write), repeated calls to enif_select must be made after receiving each notification.

    ERL_NIF_SELECT_CANCEL can be used to cancel previously selected events. It must be used in a bitwise OR combination with ERL_NIF_SELECT_READ and/or ERL_NIF_SELECT_WRITE to indicate which type of event to cancel. Arguments pid and ref are ignored when ERL_NIF_SELECT_CANCEL is specified. The return value will tell if the event was actualy cancelled or if a notification may already have been sent.

    Use ERL_NIF_SELECT_STOP as mode in order to safely close an event object that has been passed to enif_select. The stop callback of the resource obj will be called when it is safe to close the event object. This safe way of closing event objects must be used even if all notifications have been received (or cancelled) and no further calls to enif_select have been made. ERL_NIF_SELECT_STOP will first cancel any selected events before it calls or schedules the stop callback. Arguments pid and ref are ignored when ERL_NIF_SELECT_STOP is specified.

    The first call to enif_select for a specific OS event will establish a relation between the event object and the containing resource. All subsequent calls for an event must pass its containing resource as argument obj. The relation is dissolved when enif_select has been called with mode as ERL_NIF_SELECT_STOP and the corresponding stop callback has returned. A resource can contain several event objects but one event object can only be contained within one resource. A resource will not be destructed until all its contained relations have been dissolved.

        Note:
    Use enif_monitor_process together with enif_select to detect failing Erlang processes and prevent them from causing permanent leakage of resources and their contained OS event objects.

    Returns a non-negative value on success where the following bits can be set:

        ERL_NIF_SELECT_STOP_CALLED:
            The stop callback was called directly by enif_select.

        ERL_NIF_SELECT_STOP_SCHEDULED:
            The stop callback was scheduled to run on some other thread or later by this thread.

        ERL_NIF_SELECT_READ_CANCELLED:
            A read event was cancelled by ERL_NIF_SELECT_CANCEL or ERL_NIF_SELECT_STOP and is guaranteed not to generate a ready_input notification message.

        ERL_NIF_SELECT_WRITE_CANCELLED:
            A write event was cancelled by ERL_NIF_SELECT_CANCEL or ERL_NIF_SELECT_STOP and is guaranteed not to generate a ready_output notification message.

    Returns a negative value if the call failed where the following bits can be set:

        ERL_NIF_SELECT_INVALID_EVENT:
            Argument event is not a valid OS event object.

        ERL_NIF_SELECT_FAILED:
            The system call failed to add the event object to the poll set.

        Note:
    Use bitwise AND to test for specific bits in the return value. New significant bits may be added in future releases to give more detailed information for both failed and successful calls. Do NOT use equality tests like ==, as that may cause your application to stop working.

    Example:

    retval = enif_select(env, fd, ERL_NIF_SELECT_STOP, resource, ref);
    if (retval < 0) {
        /* handle error */
    }
    /* Success! */
    if (retval & ERL_NIF_SELECT_STOP_CALLED) {
        /* ... */
    }

        Note:
    The mode flag ERL_NIF_SELECT_CANCEL and the return flags ERL_NIF_SELECT_READ_CANCELLED and ERL_NIF_SELECT_WRITE_CANCELLED were introduced in erts-11.0 (OTP-22.0).

int enif_select_read(ErlNifEnv* env, ErlNifEvent event, void* obj,
      const ErlNifPid* pid, ERL_NIF_TERM msg, ErlNifEnv* msg_env)
int enif_select_write(ErlNifEnv* env, ErlNifEvent event, void* obj,
      const ErlNifPid* pid, ERL_NIF_TERM msg, ErlNifEnv* msg_env)

    These are variants of enif_select where you can supply your own message term msg that will be sent to the process instead of the predefined tuple {select,_,_,_}.

    Argument msg_env must either be NULL or the environment of msg allocated with enif_alloc_env. If argument msg_env is NULL the term msg will be copied, otherwise both msg and msg_env will be invalidated by a successful call to enif_select_read or enif_select_write. The environment is then to either be freed with enif_free_env or cleared for reuse with enif_clear_env. An unsuccessful call will leave msg and msg_env still valid.

    Apart from the message format enif_select_read and enif_select_write behaves exactly the same as enif_select with argument mode as either ERL_NIF_SELECT_READ or ERL_NIF_SELECT_WRITE. To cancel or close events use enif_select.

ErlNifPid *enif_self(ErlNifEnv* caller_env, ErlNifPid* pid)

    Initializes the ErlNifPid variable at *pid to represent the calling process.

    Returns pid if successful, or NULL if caller_env is not a process bound environment.

int enif_send(ErlNifEnv* caller_env,
      ErlNifPid* to_pid, ErlNifEnv* msg_env, ERL_NIF_TERM msg)

    Sends a message to a process.

        caller_env:
            The environment of the calling process or callback. Must be NULL only if calling from a custom thread not spawned by ERTS.

        *to_pid:
            The pid of the receiving process. The pid is to 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 or NULL.

        msg:
            The message term to send.

    Returns true if the message is successfully sent. Returns false if the send operation fails, that is:

        *   *to_pid does not refer to an alive local process.

        *   The currently executing process (that is, the sender) is not alive.

    The message environment msg_env with all its terms (including msg) is invalidated by a successful call to enif_send. The environment is to either be freed with enif_free_env or cleared for reuse with enif_clear_env. An unsuccessful call will leave msg and msg_env still valid.

    If msg_env is set to NULL, the msg term is copied and the original term and its environment is still valid after the call.

    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.

        Note:
    Passing msg_env as NULL is only supported as from ERTS 8.0 (Erlang/OTP 19).

void enif_set_pid_undefined(ErlNifPid* pid)

    Sets an ErlNifPid variable as undefined. See enif_is_pid_undefined.

unsigned enif_sizeof_resource(void* obj)

    Gets the byte size of resource object obj obtained by enif_alloc_resource.

int enif_snprintf(char *str, size_t size, const
        char *format, ...)

    Similar to snprintf but this format string also accepts "%T", which formats Erlang terms of type ERL_NIF_TERM.

    This function is primarily intended for debugging purpose. It is not recommended to print very large terms with %T. The function may change errno, even if successful.

void enif_system_info(ErlNifSysInfo
        *sys_info_ptr, size_t size)

    Same as driver_system_info.

int enif_term_to_binary(ErlNifEnv *env,
        ERL_NIF_TERM term, ErlNifBinary *bin)

    Allocates a new binary with enif_alloc_binary and stores the result of encoding term according to the Erlang external term format.

    Returns true on success, or false if the allocation fails.

    See also erlang:term_to_binary/1 and enif_binary_to_term.

ErlNifTermType enif_term_type(ErlNifEnv *env, ERL_NIF_TERM term)

    Determines the type of the given term. The term must be an ordinary Erlang term and not one of the special terms returned by enif_raise_exception, enif_schedule_nif, or similar.

    The following types are defined at the moment:

        ERL_NIF_TERM_TYPE_ATOM:

        ERL_NIF_TERM_TYPE_BITSTRING:
            A bitstring or binary

        ERL_NIF_TERM_TYPE_FLOAT:

        ERL_NIF_TERM_TYPE_FUN:

        ERL_NIF_TERM_TYPE_INTEGER:

        ERL_NIF_TERM_TYPE_LIST:
            A list, empty or not

        ERL_NIF_TERM_TYPE_MAP:

        ERL_NIF_TERM_TYPE_PID:

        ERL_NIF_TERM_TYPE_PORT:

        ERL_NIF_TERM_TYPE_REFERENCE:

        ERL_NIF_TERM_TYPE_TUPLE:

    Note that new types may be added in the future, so the caller must be prepared to handle unknown types.

int enif_thread_create(char *name,ErlNifTid
        *tid,void * (*func)(void *),void *args,ErlNifThreadOpts
        *opts)

    Same as erl_drv_thread_create.

void enif_thread_exit(void *resp)

    Same as erl_drv_thread_exit.

int enif_thread_join(ErlNifTid, void **respp)

    Same as erl_drv_thread_join.

char*enif_thread_name(ErlNifTid tid)

    Same as erl_drv_thread_name.

ErlNifThreadOpts *enif_thread_opts_create(char *name)

    Same as erl_drv_thread_opts_create.

void enif_thread_opts_destroy(ErlNifThreadOpts *opts)

    Same as erl_drv_thread_opts_destroy.

ErlNifTid enif_thread_self(void)

    Same as erl_drv_thread_self.

int enif_thread_type(void)

    Determine the type of currently executing thread. A positive value indicates a scheduler thread while a negative value or zero indicates another type of thread. Currently the following specific types exist (which may be extended in the future):

        ERL_NIF_THR_UNDEFINED:
            Undefined thread that is not a scheduler thread.

        ERL_NIF_THR_NORMAL_SCHEDULER:
            A normal scheduler thread.

        ERL_NIF_THR_DIRTY_CPU_SCHEDULER:
            A dirty CPU scheduler thread.

        ERL_NIF_THR_DIRTY_IO_SCHEDULER:
            A dirty I/O scheduler thread.

ErlNifTime enif_time_offset(ErlNifTimeUnit time_unit)

    Returns the current time offset between Erlang monotonic time and Erlang system time converted into the time_unit passed as argument.

    time_unit is the time unit of the returned value.

    Returns ERL_NIF_TIME_ERROR if called with an invalid time unit argument or if called from a thread that is not a scheduler thread.

    See also ErlNifTime and ErlNifTimeUnit.

void *enif_tsd_get(ErlNifTSDKey key)

    Same as erl_drv_tsd_get.

int enif_tsd_key_create(char *name, ErlNifTSDKey *key)

    Same as erl_drv_tsd_key_create.

void enif_tsd_key_destroy(ErlNifTSDKey key)

    Same as erl_drv_tsd_key_destroy.

void enif_tsd_set(ErlNifTSDKey key, void *data)

    Same as erl_drv_tsd_set.

int enif_vfprintf(FILE *stream, const char *format, va_list ap)

    Equivalent to enif_fprintf except that its called with a va_list instead of a variable number of arguments.

int enif_vsnprintf(char *str, size_t size, const char *format, va_list ap)

    Equivalent to enif_snprintf except that its called with a va_list instead of a variable number of arguments.

int enif_whereis_pid(ErlNifEnv *env,
          ERL_NIF_TERM name, ErlNifPid *pid)

    Looks up a process by its registered name.

        env:
            The environment of the calling process. Must be NULL only if calling from a created thread.

        name:
            The name of a registered process, as an atom.

        *pid:
            The ErlNifPid in which the resolved process id is stored.

    On success, sets *pid to the local process registered with name and returns true. If name is not a registered process, or is not an atom, false is returned and *pid is unchanged.

    Works as erlang:whereis/1, but restricted to processes. See enif_whereis_port to resolve registered ports.

int enif_whereis_port(ErlNifEnv *env,
          ERL_NIF_TERM name, ErlNifPort *port)

    Looks up a port by its registered name.

        env:
            The environment of the calling process. Must be NULL only if calling from a created thread.

        name:
            The name of a registered port, as an atom.

        *port:
            The ErlNifPort in which the resolved port id is stored.

    On success, sets *port to the port registered with name and returns true. If name is not a registered port, or is not an atom, false is returned and *port is unchanged.

    Works as erlang:whereis/1, but restricted to ports. See enif_whereis_pid to resolve registered processes.

SEE ALSO

erlang:load_nif/2