shmget(3p) — Linux manual page


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

PROLOG         top

       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         top

       shmget — get an XSI shared memory segment

SYNOPSIS         top

       #include <sys/shm.h>

       int shmget(key_t key, size_t size, int shmflg);

DESCRIPTION         top

       The shmget() function operates on XSI shared memory (see the Base
       Definitions volume of POSIX.1‐2017, Section 3.346, Shared Memory
       Object).  It is unspecified whether this function interoperates
       with the realtime interprocess communication facilities defined
       in Section 2.8, Realtime.

       The shmget() function shall return the shared memory identifier
       associated with key.

       A shared memory identifier, associated data structure, and shared
       memory segment of at least size bytes (see <sys/shm.h>) are
       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 shared memory
           identifier associated with it and (shmflg &IPC_CREAT) is non-

       Upon creation, the data structure associated with the new shared
       memory identifier shall be initialized as follows:

        *  The values of shm_perm.cuid, shm_perm.uid, shm_perm.cgid, and
           shm_perm.gid are set to the effective user ID and effective
           group ID, respectively, of the calling process.

        *  The low-order nine bits of shm_perm.mode are set to the low-
           order nine bits of shmflg.

        *  The value of shm_segsz is set to the value of size.

        *  The values of shm_lpid, shm_nattch, shm_atime, and shm_dtime
           are set to 0.

        *  The value of shm_ctime is set to the current time, as
           described in Section 2.7.1, IPC General Description.

       When the shared memory segment is created, it shall be
       initialized with all zero values.

RETURN VALUE         top

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

ERRORS         top

       The shmget() function shall fail if:

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

       EEXIST A shared memory identifier exists for the argument key but
              (shmflg &IPC_CREAT) &&(shmflg &IPC_EXCL) is non-zero.

       EINVAL A shared memory segment is to be created and the value of
              size is less than the system-imposed minimum or greater
              than the system-imposed maximum.

       EINVAL No shared memory segment is to be created and a shared
              memory segment exists for key but the size of the segment
              associated with it is less than size.

       ENOENT A shared memory identifier does not exist for the argument
              key and (shmflg &IPC_CREAT) is 0.

       ENOMEM A shared memory identifier and associated shared memory
              segment are to be created, but the amount of available
              physical memory is not sufficient to fill the request.

       ENOSPC A shared memory identifier is to be created, but the
              system-imposed limit on the maximum number of allowed
              shared memory identifiers system-wide would be exceeded.

       The following sections are informative.

EXAMPLES         top



       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

RATIONALE         top




SEE ALSO         top

       Section 2.7, XSI Interprocess Communication, Section 2.8,
       Realtime, ftok(3p), shmat(3p), shmctl(3p), shmdt(3p),
       shm_open(3p), shm_unlink(3p)

       The Base Definitions volume of POSIX.1‐2017, Section 3.346,
       Shared Memory Object, sys_shm.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group               2017                        SHMGET(3P)

Pages that refer to this page: sys_shm.h(0p)ipcs(1p)_Exit(3p)ftok(3p)shmat(3p)shmctl(3p)shmdt(3p)