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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SHMAT(3P) POSIX Programmer's Manual SHMAT(3P)
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.
shmat — XSI shared memory attach operation
#include <sys/shm.h>
void *shmat(int shmid, const void *shmaddr, int shmflg);
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.
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.
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.
None.
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.
None.
None.
Section 2.7, XSI Interprocess Communication, Section 2.8,
Realtime, exec(1p), exit(3p), fork(3p), shmctl(3p), shmdt(3p),
shmget(3p), shm_open(3p), shm_unlink(3p)
The Base Definitions volume of POSIX.1‐2017, Section 3.346, Shared
Memory Object, sys_shm.h(0p)
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 .
IEEE/The Open Group 2017 SHMAT(3P)
Pages that refer to this page: sys_shm.h(0p), ipcs(1p), exec(3p), mmap(3p), shmctl(3p), shmdt(3p), shmget(3p), shm_open(3p), shm_unlink(3p)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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