PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

       shmat — XSI shared memory attach operation

SYNOPSIS         top

       #include <sys/shm.h>

       void *shmat(int shmid, const void *shmaddr, int shmflg);

DESCRIPTION         top

       The shmat() function operates on XSI shared memory (see the Base
       Definitions volume of POSIX.1‐2008, Section 3.342, Shared Memory
       Object).  It is unspecified whether this function interoperates with
       the realtime interprocess communication facilities defined in Section
       2.8, Realtime.

       The shmat() function attaches the shared memory segment associated
       with the shared memory identifier specified by shmid to the address
       space of the calling process. The segment is attached at the address
       specified by one of the following criteria:

        *  If shmaddr is a null pointer, the segment is attached at the
           first available address as selected by the system.

        *  If shmaddr is not a null pointer and (shmflg &SHM_RND) is non-
           zero, the segment is attached at the address given by (shmaddr
           −((uintptr_t)shmaddr %SHMLBA)). The character '%' is the C-
           language remainder operator.

        *  If shmaddr is not a null pointer and (shmflg &SHM_RND) is 0, the
           segment is attached at the address given by shmaddr.

        *  The segment is attached for reading if (shmflg &SHM_RDONLY) is
           non-zero and the calling process has read permission; otherwise,
           if it is 0 and the calling process has read and write permission,
           the segment is attached for reading and writing.

RETURN VALUE         top

       Upon successful completion, shmat() shall increment the value of
       shm_nattch in the data structure associated with the shared memory ID
       of the attached shared memory segment and return the segment's start
       address.  Also, the shm_atime timestamp shall be set to the current
       time, as described in Section 2.7.1, IPC General Description.

       Otherwise, the shared memory segment shall not be attached, shmat()
       shall return −1, and errno shall be set to indicate the error.

ERRORS         top

       The shmat() function shall fail if:

       EACCES Operation permission is denied to the calling process; see
              Section 2.7, XSI Interprocess Communication.

       EINVAL The value of shmid is not a valid shared memory identifier,
              the shmaddr is not a null pointer, and the value of (shmaddr
              −((uintptr_t)shmaddr %SHMLBA)) is an illegal address for
              attaching shared memory; or the shmaddr is not a null pointer,
              (shmflg &SHM_RND) is 0, and the value of shmaddr is an illegal
              address for attaching shared memory.

       EMFILE The number of shared memory segments attached to the calling
              process would exceed the system-imposed limit.

       ENOMEM The available data space is not large enough to accommodate
              the shared memory segment.

       The following sections are informative.

EXAMPLES         top

       None.

APPLICATION USAGE         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 interfaces.

RATIONALE         top

       None.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
       exec(1p), exit(3p), fork(3p), shmctl(3p), shmdt(3p), shmget(3p),
       shm_open(3p), shm_unlink(3p)

       The Base Definitions volume of POSIX.1‐2008, Section 3.342, 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, 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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                           SHMAT(3P)

Pages that refer to this page: sys_shm.h(0p)ipcs(1p)exec(3p)mmap(3p)shmctl(3p)shmdt(3p)shmget(3p)shm_open(3p)shm_unlink(3p)