sem_init(3) — Linux manual page


SEM_INIT(3)               Linux Programmer's Manual              SEM_INIT(3)

NAME         top

       sem_init - initialize an unnamed semaphore

SYNOPSIS         top

       #include <semaphore.h>

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

       Link with -pthread.

DESCRIPTION         top

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

       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         top

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

ERRORS         top

       EINVAL value exceeds SEM_VALUE_MAX.

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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface  Attribute     Value   │
       │sem_init() │ Thread safety │ MT-Safe │

CONFORMING TO         top


NOTES         top

       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         top

       See shm_open(3) and sem_wait(3).

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-08-13                      SEM_INIT(3)

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