sem_init(3) - Linux manual page (original) (raw)

seminit(3) Library Functions Manual seminit(3)

NAME top

   sem_init - initialize an unnamed semaphore

LIBRARY top

   POSIX threads library (_libpthread_, _-lpthread_)

SYNOPSIS top

   **#include <semaphore.h>**

   **int sem_init(sem_t ***_sem_**, int** _pshared_**, unsigned int** _value_**);**

DESCRIPTION top

   **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)](../man3/shm%5Fopen.3.html), [mmap(2)](../man2/mmap.2.html), and [shmget(2)](../man2/shmget.2.html)).  (Since a child created by
   [fork(2)](../man2/fork.2.html) 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)](../man3/sem%5Fpost.3.html),
   [sem_wait(3)](../man3/sem%5Fwait.3.html), 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](../man3/errno.3.html)_ 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)](../man7/sem%5Foverview.7.html)).

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌──────────────────────────────────────┬───────────────┬─────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├──────────────────────────────────────┼───────────────┼─────────┤
   │ **sem_init**()                           │ Thread safety │ MT-Safe │
   └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

   POSIX.1-2008.

HISTORY top

   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 top

   See [shm_open(3)](../man3/shm%5Fopen.3.html) and [sem_wait(3)](../man3/sem%5Fwait.3.html).

SEE ALSO top

   [sem_destroy(3)](../man3/sem%5Fdestroy.3.html), [sem_post(3)](../man3/sem%5Fpost.3.html), [sem_wait(3)](../man3/sem%5Fwait.3.html), [sem_overview(7)](../man7/sem%5Foverview.7.html)

COLOPHON top

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Linux man-pages 6.10 2024-07-23 seminit(3)

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