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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SHMCTL(3P) POSIX Programmer's Manual SHMCTL(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.
shmctl — XSI shared memory control operations
#include <sys/shm.h>
int shmctl(int shmid, int cmd, struct shmid_ds *buf);
The shmctl() 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 shmctl() function provides a variety of shared memory control
operations as specified by cmd. The following values for cmd are
available:
IPC_STAT Place the current value of each member of the shmid_ds
data structure associated with shmid into the
structure pointed to by buf. The contents of the
structure are defined in <sys/shm.h>.
IPC_SET Set the value of the following members of the shmid_ds
data structure associated with shmid to the
corresponding value found in the structure pointed to
by buf:
shm_perm.uid
shm_perm.gid
shm_perm.mode Low-order nine bits.
Also, the shm_ctime timestamp shall be set to the
current time, as described in Section 2.7.1, IPC
General Description.
IPC_SET can only be executed by a process that has an
effective user ID equal to either that of a process
with appropriate privileges or to the value of
shm_perm.cuid or shm_perm.uid in the shmid_ds data
structure associated with shmid.
IPC_RMID Remove the shared memory identifier specified by shmid
from the system and destroy the shared memory segment
and shmid_ds data structure associated with it.
IPC_RMID can only be executed by a process that has an
effective user ID equal to either that of a process
with appropriate privileges or to the value of
shm_perm.cuid or shm_perm.uid in the shmid_ds data
structure associated with shmid.
Upon successful completion, shmctl() shall return 0; otherwise, it
shall return -1 and set errno to indicate the error.
The shmctl() function shall fail if:
EACCES The argument cmd is equal to IPC_STAT and the calling
process does not have read permission; see Section 2.7, XSI
Interprocess Communication.
EINVAL The value of shmid is not a valid shared memory identifier,
or the value of cmd is not a valid command.
EPERM The argument cmd is equal to IPC_RMID or IPC_SET and the
effective user ID of the calling process is not equal to
that of a process with appropriate privileges and it is not
equal to the value of shm_perm.cuid or shm_perm.uid in the
data structure associated with shmid.
The shmctl() function may fail if:
EOVERFLOW
The cmd argument is IPC_STAT and the gid or uid value is
too large to be stored in the structure pointed to by the
buf argument.
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, shmat(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 SHMCTL(3P)
Pages that refer to this page: sys_shm.h(0p), ipcrm(1p), shmat(3p), shmdt(3p), shmget(3p), shm_open(3p), shm_unlink(3p)
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HTML rendering created 2025-09-06 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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