SHMAT(3)                   POSIX Programmer's Manual                  SHMAT(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
       shmat -- XSI shared memory attach operation

SYNOPSIS
       #include <sys/shm.h>

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

DESCRIPTION
       The shmat() 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 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
       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 (void **)-1, and errno shall be set to indicate the error.

ERRORS
       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
       None.

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
       None.

SEE ALSO
       Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
       exec, exit(), fork(), shmctl(), shmdt(), shmget(), shm__open(),
       shm__unlink()

       The Base Definitions volume of POSIX.1-2017, Section 3.346, Shared
       Memory Object, <sys_shm.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                             SHMAT(3)
