NAME

shmctl - shared memory control operations

 SYNOPSIS

#include <sys/shm.h>

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

 DESCRIPTION

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

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() returns 0. Otherwise, it returns -1 and errno will be set to indicate the error.

 ERRORS

The shmctl() function will fail if:

[EACCES]

The argument cmd is equal to IPC_STAT and the calling process does not have read permission, see IPC.

[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 IPC can be easily modified to use the alternative interfaces.

 FUTURE DIRECTIONS

None.

 SEE ALSO

shmat(), shmdt(), shmget(), shm_open(), shm_unlink(), <sys/shm.h>, IPC.

DERIVATION

Derived from Issue 2 of the SVID.