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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 POSIX.1-2008, Section 3.342, 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 -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 Sec-
              tion 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 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 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.1-2008, Section 3.342, Shared Mem-
       ory Object, <sys_shm.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) 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.unix.org/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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                            SHMAT(3P)