Technical Reference: Base Operating System and Extensions, Volume 2

shmctl Subroutine

Purpose

Controls shared memory operations.

Library

Standard C Library (libc.a)

Syntax

#include <sys/shm.h> int shmctl (SharedMemoryID, Command, Buffer) int SharedMemoryID, Command; struct shmid_ds * Buffer;

Description

The shmctl subroutine performs a variety of shared-memory control operations as specified by the Command parameter.

The following limits apply to shared memory:

Maximum shared-memory segment size is:
- 256M bytes before AIX 4.3.1
- 2G bytes for AIX 4.3.1 through AIX 5.1
- 64G bytes for 64-bit applications for AIX 5.1 and later
Minimum shared-memory segment size is 1 byte.
Maximum number of shared memory IDs is 4096 for operating system releases before AIX 4.3.2 and 131072 for AIX 4.3.2 and following.

Parameters

SharedMemoryID	Specifies an identifier returned by the shmget subroutine.
Buffer	Indicates a pointer to the shmid_ds structure. The shmid_ds structure is defined in the sys/shm.h file.
Command	The following commands are available: IPC_STAT Obtains status information about the shared memory segment identified by the SharedMemoryID parameter. This information is stored in the area pointed to by the Buffer parameter. The calling process must have read permission to run this command. IPC_ SET Sets the user and group IDs of the owner as well as the access permissions for the shared memory segment identified by the SharedMemoryID parameter. This command sets the following fields: shm_perm.uid /* owning user ID / shm_perm.gid / owning group ID / shm_perm.mode / permission bits only */ You must have an effective user ID equal to root or to the value of the `shm_perm.cuid` or `shm_perm.uid` field in the shmid_ds data structure identified by the SharedMemoryID parameter. IPC_RMID Removes the shared memory identifier specified by the SharedMemoryID parameter from the system and erases the shared memory segment and data structure associated with it. This command is only executed by a process that has an effective user ID equal either to that of `superuser` or to the value of the `shm_perm.uid` or `shm_perm.cuid` field in the data structure identified by the SharedMemoryID parameter. SHM_SIZE Sets the size of the shared memory segment to the value specified by the `shm_segsz` field of the structure specified by the Buffer parameter. This value can be larger or smaller than the current size. The limit is the maximum shared-memory segment size. This command is only executed by a process that has an effective user ID equal either to that of a process with the appropriate privileges or to the value of the `shm_perm.uid` or `shm_perm.cuid` field in the data structure identified by the SharedMemoryID parameter. This command is not supported for regions created with the environment variable EXTSHM=ON. This results in a return value of -1 with errno set to EINVAL. Attempting to use the SHM_SIZE on a shared memory region larger than 256MB or attempting to increase the size of a shared memory region larger than 256MB results in a return value of -1 with errno set to EINVAL.

Return Values

When completed successfully, the shmctl subroutine returns a value of 0. Otherwise, it returns a value of -1 and the errno global variable is set to indicate the error.

Error Codes

The shmctl subroutine is unsuccessful if one or more of the following are true:

EACCES	The Command parameter is equal to the IPC_STAT value and read permission is denied to the calling process.
EFAULT	The Buffer parameter points to a location outside the allocated address space of the process.
EINVAL	The SharedMemoryID parameter is not a valid shared memory identifier.
EINVAL	The Command parameter is not a valid command.
EINVAL	The Command parameter is equal to the SHM_SIZE value and the value of the `shm_segsz` field of the structure specified by the Buffer parameter is not valid.
EINVAL	The Command parameter is equal to the SHM_SIZE value and the shared memory region was created with the environment variable EXTSHM=ON.
ENOMEM	The Command parameter is equal to the SHM_SIZE value, and the attempt to change the segment size is unsuccessful because the system does not have enough memory.
EOVERFLOW	The Command parameter is IPC_STAT and the size of the shared memory region is greater than or equal to 4G bytes. This only happens with 32-bit programs.
EPERM	The Command parameter is equal to the IPC_RMID or SHM_SIZE value, and the effective user ID of the calling process is not equal to the value of the `shm_perm.uid` or `shm_perm.cuid` field in the data structure identified by the SharedMemoryID parameter. The effective user ID of the calling process is not the root user ID.

Related Information

The disclaim subroutine, shmat subroutine, shmdt subroutine, shmget subroutine.

The ipcs command and ipcrm command.

List of Memory Manipulation Services, Subroutines Overview, Understanding Memory Mapping in AIX 5L Version 5.2 General Programming Concepts: Writing and Debugging Programs.