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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.1‐2008,
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 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.1‐2008, Section 3.342, Shared Memory 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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .