SHMCTL(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.

NAME
shmctl — XSI shared memory control operations

SYNOPSIS

#include <sys/shm.h>

int shmctl(int shmid, int cmd, struct shmid_ds *buf);

DESCRIPTION
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.

RETURN VALUE
Upon successful completion, shmctl() shall return 0; otherwise, it shall return -1 and set errno to indicate the error.

ERRORS
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.

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, shmat(), shmdt(), shmget(), shm_open(), shm_unlink()

The Base Definitions volume of POSIX.1-2017, Section 3.346, Shared Memory Object, <sys_shm.h>

COPYRIGHT
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 .