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 .

       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.kernel.org/doc/man-pages/reporting_bugs.html .