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.
NAMEshmat — 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)
