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, exit(), fork(), shmctl(),
       shmdt(), shmget(), shm_open(), shm_unlink()

       The Base Definitions volume of POSIX.1‐2017, Section 3.346, Shared Memory Object, <sys_shm.h>

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 .

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       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 .