huschi.net

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  Defini-
       tions volume of POSIX.1-2008, Section 4.16, Semaphore).  It is unspeci-
       fied 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 asso-
           ciated with it and (semflg &IPC_CREAT) is non-zero.
       Upon creation, the semid_ds data structure associated with the new sem-
       aphore 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 inte-
       ger, 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 inter-
       process 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, sem-
       ncnt, and semzcnt members of all semaphores in a semaphore set be  ini-
       tialized  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.1-2008, Section 4.16, Semaphore,
       <sys_sem.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) 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.unix.org/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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                           SEMGET(3P)