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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  Defi-
       nitions  volume  of  IEEE Std 1003.1-2001, Section 3.340, Shared Memory
       Object). It is unspecified whether this function interoperates with the
       realtime interprocess communication 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 seg-
          ment 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 attach-
              ing 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  inter-
       process  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(), shm-
       ctl(),  shmdt(),  shmget(),  shm_open(), shm_unlink(), the Base Defini-
       tions 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,  Copyright  (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.opengroup.org/unix/online.html .

IEEE/The Open Group                  2003                            SHMAT(3P)