sem_init(3)                Library Functions Manual                sem_init(3)

NAME
       sem_init - initialize an unnamed semaphore

LIBRARY
       POSIX threads library (libpthread, -lpthread)

SYNOPSIS
       #include <semaphore.h>

       int sem_init(sem_t *sem, int pshared, unsigned int value);

DESCRIPTION
       sem_init() initializes the unnamed semaphore at the address pointed to
       by sem.  The value argument specifies the initial value for the
       semaphore.

       The pshared argument indicates whether this semaphore is to be shared
       between the threads of a process, or between processes.

       If pshared has the value 0, then the semaphore is shared between the
       threads of a process, and should be located at some address that is
       visible to all threads (e.g., a global variable, or a variable
       allocated dynamically on the heap).

       If pshared is nonzero, then the semaphore is shared between processes,
       and should be located in a region of shared memory (see shm_open(3),
       mmap(2), and shmget(2)).  (Since a child created by fork(2) inherits
       its parent's memory mappings, it can also access the semaphore.)  Any
       process that can access the shared memory region can operate on the
       semaphore using sem_post(3), sem_wait(3), and so on.

       Initializing a semaphore that has already been initialized results in
       undefined behavior.

RETURN VALUE
       sem_init() returns 0 on success; on error, -1 is returned, and errno is
       set to indicate the error.

ERRORS
       EINVAL value exceeds SEM_VALUE_MAX.

       ENOSYS pshared is nonzero, but the system does not support process-
              shared semaphores (see sem_overview(7)).

ATTRIBUTES
       For an explanation of the terms used in this section, see
       attributes(7).

       +--------------------------------------------+---------------+---------+
       |Interface                                   | Attribute     | Value   |
       +--------------------------------------------+---------------+---------+
       |sem_init ()                                 | Thread safety | MT-Safe |
       +--------------------------------------------+---------------+---------+
STANDARDS
       POSIX.1-2008.

HISTORY
       POSIX.1-2001.

       Bizarrely, POSIX.1-2001 does not specify the value that should be
       returned by a successful call to sem_init().  POSIX.1-2008 rectifies
       this, specifying the zero return on success.

EXAMPLES
       See shm_open(3) and sem_wait(3).

SEE ALSO
       sem_destroy(3), sem_post(3), sem_wait(3), sem_overview(7)

Linux man-pages 6.7               2023-10-31                       sem_init(3)