NAME

shmctl — XSI shared memory control operations

SYNOPSIS

[XSI] #include <sys/shm.h>

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

DESCRIPTION

The shmctl() function operates on XSI shared memory (see XBD 3.332 Shared Memory Object). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in 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 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. The shared memory segment and shmid_ds data structure associated with it shall be destroyed when all processes with the segment attached have either detached the segment or terminated. If the segment is not attached to any process, it shall be destroyed immediately. 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 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.

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 2.7 XSI Interprocess Communication can be easily modified to use the alternative interfaces.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

2.7 XSI Interprocess Communication, 2.8 Realtime, shmat, shmdt, shmget, shm_open, shm_unlink

XBD 3.332 Shared Memory Object, <sys/shm.h>

CHANGE HISTORY

First released in Issue 2. Derived from Issue 2 of the SVID.

Issue 5

Moved from SHARED MEMORY to BASE.

The note about use of POSIX Realtime Extension IPC routines has been moved from FUTURE DIRECTIONS to a new APPLICATION USAGE section.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0573 [345] is applied.

Issue 8

Austin Group Defect 1240 is applied, clarifying the description of IPC_RMID.