Content-type: text/html Man page of shmctl

shmctl

Section: System Calls (2)
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NAME

shmctl - Performs shared memory control operations  

SYNOPSIS

#include <sys/shm.h>

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

Application developers may want to specify #include statements for <sys/types.h> and <sys/ipc.h> before the one for <sys/shm.h> if programs are being developed for multiple platforms. The additional #include statements are not required on Tru64 UNIX systems or by ISO or X/Open standards, but may be required on other vendors' systems that conform to these standards.  

STANDARDS

Interfaces documented on this reference page conform to industry standards as follows:

shmctl():  XPG4, XPG4-UNIX

Refer to the standards(5) reference page for more information about industry standards and associated tags.  

PARAMETERS

Specifies the ID of the shared memory region. Specifies the type of command. The possible commands are: IPC_STAT, IPC_SET, IPC_RMID, SHM_LOCK, and SHM_UNLOCK. Specifies the address of a shmid_ds structure.  

DESCRIPTION

The shmctl() function provides a variety of shared memory control operations as specified by the cmd parameter.

The cmd values and their operations are as follows: Queries the shared memory region ID by copying the contents of its associated shmid_ds data structure into the buf structure. Sets the shared memory region ID by copying values found in the buf structure into corresponding fields in the shmid_ds structure associated with the shared memory region ID. The fields are set as follows: The shm_perm.uid field is set to the owner's user ID. The shm_perm.gid field is set to the owner's group ID. The shm_perm.mode field is set to the access modes for the shared memory region. Only the low-order nine bits are set. Removes the shared memory region ID and deallocates its associated shmid_ds structure. [Digital]  Locks the shared memory segment specified by shmid in memory. [Digital]  Unlocks the shared memory segment specified by shmid in memory.

The shmid_ds structure is used only with the IPC_STAT and IPC_SET commands. In either case, the calling process must have allocated the structure before making the call.

[Digital]  The SHM_LOCK and SHM_UNLOCK commands can be used to lock (wire) down a shared segment in memory to prevent it from being paged out.  

NOTES

[Digital]   When using the SHM_LOCK command, make sure that the system has enough physical memory available for the shared segment to be wired without exceeding the system-wide limit or otherwise severely impacting system performance.

[Digital]  The SHM_LOCK operation wires all the pages in a global shared segment and prevents the pageout daemon from reclaiming any of the pages. This can potentially lead to thrashing.  

RESTRICTIONS

The following restrictions apply to the shared memory commands: For the IPC_SET and IPC_RMID commands, the effective user ID of the calling process must be equal to that of superuser or equal to the value of shm_perm.cuid or shm_perm.uid in the associated shmid_ds structure. [Digital]  For the SHM_LOCK and SHM_UNLOCK commands, the effective user ID of the calling process must be equal to that of superuser.  

RETURN VALUES

Upon successful completion, a value of 0 (zero) is returned. If the shmctl() function fails, a value of -1 is returned and errno is set to indicate the error.  

ERRORS

The shmctl() function sets errno to the specified values for the following conditions: The cmd parameter is IPC_STAT, but the calling process does not have read permission. [Digital]  The cmd parameter is SHM_LOCK, and the system-wide wire limit has been exceeded. [Digital]  The cmd parameter is IPC_STAT or IPC_SET. An error occurred in accessing the buf structure. The shmid parameter does not specify a valid shared memory region ID, or cmd is not a valid command.

[Digital]  The cmd parameter is SHM_UNLOCK, and the segment was not locked. The cmd parameter is equal to either IPC_RMID or IPC_SET, and the calling process does not have appropriate privilege.
 

RELATED INFORMATION

Functions: shmat(2), shmdt(2), shmget(2)

Data structures: shmid_ds(4)

Standards: standards(5) delim off


 

Index

NAME
SYNOPSIS
STANDARDS
PARAMETERS
DESCRIPTION
NOTES
RESTRICTIONS
RETURN VALUES
ERRORS
RELATED INFORMATION

This document was created by man2html, using the manual pages.
Time: 02:40:18 GMT, October 02, 2010