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

       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() ,
       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, 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 .