shmat(3p) — Linux manual page

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

       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(1p), exit(3p), fork(3p), shmctl(3p), shmdt(3p),
       shmget(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

       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(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)

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