semget(3p) — Linux manual page

SEMGET(3P)              POSIX Programmer's Manual             SEMGET(3P)

PROLOG

       This manual page is part of the POSIX Programmer's Manual.  The
       Linux implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior),
       or the interface may not be implemented on Linux.

NAME

       semget — get set of XSI semaphores

SYNOPSIS

       #include <sys/sem.h>

       int semget(key_t key, int nsems, int semflg);

DESCRIPTION

       The semget() function operates on XSI semaphores (see the Base
       Definitions volume of POSIX.1‐2017, Section 4.17, Semaphore).  It
       is unspecified whether this function interoperates with the
       realtime interprocess communication facilities defined in Section
       2.8, Realtime.

       The semget() function shall return the semaphore identifier
       associated with key.

       A semaphore identifier with its associated semid_ds data
       structure and its associated set of nsems semaphores (see
       <sys/sem.h>) is created for key if one of the following is true:

        *  The argument key is equal to IPC_PRIVATE.

        *  The argument key does not already have a semaphore identifier
           associated with it and (semflg &IPC_CREAT) is non-zero.

       Upon creation, the semid_ds data structure associated with the
       new semaphore identifier is initialized as follows:

        *  In the operation permissions structure sem_perm.cuid,
           sem_perm.uid, sem_perm.cgid, and sem_perm.gid shall be set to
           the effective user ID and effective group ID, respectively,
           of the calling process.

        *  The low-order 9 bits of sem_perm.mode shall be set to the
           low-order 9 bits of semflg.

        *  The variable sem_nsems shall be set to the value of nsems.

        *  The variable sem_otime shall be set to 0 and sem_ctime shall
           be set to the current time, as described in Section 2.7.1,
           IPC General Description.

        *  The data structure associated with each semaphore in the set
           need not be initialized. The semctl() function with the
           command SETVAL or SETALL can be used to initialize each
           semaphore.

RETURN VALUE

       Upon successful completion, semget() shall return a non-negative
       integer, namely a semaphore identifier; otherwise, it shall
       return -1 and set errno to indicate the error.

ERRORS

       The semget() function shall fail if:

       EACCES A semaphore identifier exists for key, but operation
              permission as specified by the low-order 9 bits of semflg
              would not be granted; see Section 2.7, XSI Interprocess
              Communication.

       EEXIST A semaphore identifier exists for the argument key but
              ((semflg &IPC_CREAT) &&(semflg &IPC_EXCL)) is non-zero.

       EINVAL The value of nsems is either less than or equal to 0 or
              greater than the system-imposed limit, or a semaphore
              identifier exists for the argument key, but the number of
              semaphores in the set associated with it is less than
              nsems and nsems is not equal to 0.

       ENOENT A semaphore identifier does not exist for the argument key
              and (semflg &IPC_CREAT) is equal to 0.

       ENOSPC A semaphore identifier is to be created but the system-
              imposed limit on the maximum number of allowed semaphores
              system-wide would be exceeded.

       The following sections are informative.

EXAMPLES

       Refer to semop(3p).

APPLICATION USAGE

       The POSIX Realtime Extension defines alternative interfaces for
       interprocess communication. Application developers who need to
       use IPC should design their applications so that modules using
       the IPC routines described in Section 2.7, XSI Interprocess
       Communication can be easily modified to use the alternative
       interfaces.

RATIONALE

       None.

FUTURE DIRECTIONS

       A future version may require that the value of the semval,
       sempid, semncnt, and semzcnt members of all semaphores in a
       semaphore set be initialized to zero when a call to semget()
       creates a semaphore set. Many semaphore implementations already
       do this and it greatly simplifies what an application must do to
       initialize a semaphore set.

SEE ALSO

       Section 2.7, XSI Interprocess Communication, Section 2.8,
       Realtime, ftok(3p), semctl(3p), semop(3p), sem_close(3p),
       sem_destroy(3p), sem_getvalue(3p), sem_init(3p), sem_open(3p),
       sem_post(3p), sem_trywait(3p), sem_unlink(3p)

       The Base Definitions volume of POSIX.1‐2017, Section 4.17,
       Semaphore, sys_sem.h(0p)

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any
       discrepancy between this version and the original IEEE and The
       Open Group Standard, the original IEEE and The Open Group
       Standard is the referee document. The original Standard can be
       obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group               2017                        SEMGET(3P)

Pages that refer to this page: sys_sem.h(0p), ipcs(1p), ftok(3p), sem_close(3p), semctl(3p), sem_destroy(3p), sem_getvalue(3p), semop(3p), sem_open(3p), sem_post(3p), sem_timedwait(3p), sem_trywait(3p), sem_unlink(3p), sigaction(3p)

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