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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SHMGET(3P) POSIX Programmer's Manual SHMGET(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.
shmget — get an XSI shared memory segment
#include <sys/shm.h>
int shmget(key_t key, size_t size, int shmflg);
The shmget() 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 shmget() function shall return the shared memory identifier
associated with key.
A shared memory identifier, associated data structure, and shared
memory segment of at least size bytes (see <sys/shm.h>) are
created for key if one of the following is true:
* The argument key is equal to IPC_PRIVATE.
* The argument key does not already have a shared memory
identifier associated with it and (shmflg &IPC_CREAT) is non-
zero.
Upon creation, the data structure associated with the new shared
memory identifier shall be initialized as follows:
* The values of shm_perm.cuid, shm_perm.uid, shm_perm.cgid, and
shm_perm.gid are set to the effective user ID and effective
group ID, respectively, of the calling process.
* The low-order nine bits of shm_perm.mode are set to the low-
order nine bits of shmflg.
* The value of shm_segsz is set to the value of size.
* The values of shm_lpid, shm_nattch, shm_atime, and shm_dtime
are set to 0.
* The value of shm_ctime is set to the current time, as
described in Section 2.7.1, IPC General Description.
When the shared memory segment is created, it shall be
initialized with all zero values.
Upon successful completion, shmget() shall return a non-negative
integer, namely a shared memory identifier; otherwise, it shall
return -1 and set errno to indicate the error.
The shmget() function shall fail if:
EACCES A shared memory identifier exists for key but operation
permission as specified by the low-order nine bits of
shmflg would not be granted; see Section 2.7, XSI
Interprocess Communication.
EEXIST A shared memory identifier exists for the argument key but
(shmflg &IPC_CREAT) &&(shmflg &IPC_EXCL) is non-zero.
EINVAL A shared memory segment is to be created and the value of
size is less than the system-imposed minimum or greater
than the system-imposed maximum.
EINVAL No shared memory segment is to be created and a shared
memory segment exists for key but the size of the segment
associated with it is less than size.
ENOENT A shared memory identifier does not exist for the argument
key and (shmflg &IPC_CREAT) is 0.
ENOMEM A shared memory identifier and associated shared memory
segment are to be created, but the amount of available
physical memory is not sufficient to fill the request.
ENOSPC A shared memory identifier is to be created, but the
system-imposed limit on the maximum number of allowed
shared memory identifiers system-wide would be exceeded.
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, ftok(3p), shmat(3p), shmctl(3p), shmdt(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 SHMGET(3P)
Pages that refer to this page: sys_shm.h(0p), ipcs(1p), _Exit(3p), ftok(3p), shmat(3p), shmctl(3p), shmdt(3p)
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HTML rendering created 2024-06-26 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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