sem_init(3) — Linux manual page

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)

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Linux man-pages 6.9.1          2024-05-02                    sem_init(3)

Pages that refer to this page: sem_destroy(3), sem_overview(7)

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