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 (con-
       sult 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 IEEE Std 1003.1-2001, Section
       3.340, Shared Memory Object). It is unspecified whether this function interoperates with the realtime interprocess commu-
       nication facilities defined in 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.

       Otherwise,  the  shared memory segment shall not be attached, shmat() shall return -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 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 XSI Interprocess
       Communication can be easily modified to use the alternative interfaces.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       XSI Interprocess Communication, Realtime, exec() , exit(), fork(), shmctl(), shmdt(), shmget(), shm_open(), shm_unlink(),
       the Base Definitions volume of IEEE Std 1003.1-2001, <sys/shm.h>

COPYRIGHT
       Portions  of  this  text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for
       Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6,  Copy-
       right (C) 2001-2003 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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                     SHMAT(3P)