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 correspond-
ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
shmctl - XSI shared memory control operations
int shmctl(int shmid, int cmd, struct shmid_ds *buf);
The shmctl() function operates on XSI shared memory (see the Base Definitions volume of IEEE Std 1003.1-2001, Section 3.340, Shared Memory
Object). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Realtime
The shmctl() function provides a variety of shared memory control operations as specified by cmd. The following values for cmd are avail-
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>.
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.mode Low-order nine bits.
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.
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
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 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
The shmctl() function may fail if:
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.
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 XSI Interprocess Communication can be easily modified
to use the alternative interfaces.
XSI Interprocess Communication, Realtime, shmat(), shmdt(), shmget(), shm_open(), shm_unlink(), the Base Definitions volume of
IEEE Std 1003.1-2001, <sys/shm.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .
/The Open Group 2003 SHMCTL(3P)