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



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

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().

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(), semctl(), semop(), sem__close(), sem__destroy(), sem__getvalue(),
       sem__init(), sem__open(), sem__post(), sem__trywait(), sem__unlink()

       The Base Definitions volume of POSIX.1-2017, Section 4.17, Semaphore,
       <sys_sem.h>

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(3)
