SHMCTL(2)

NAME
shmctl - shared memory control operations

LIBRARY
libc

SYNOPSIS
#include <sys/shm.h>
int
shmctl(int shmid, int cmd, struct shmid_ds *buf);

DESCRIPTION
The shmctl() system call performs control operations on the shared memory segment specified by shmid.

Each shared memory segment has a shmid_ds structure associated with it which contains the following members:

        struct ipc_perm shm_perm;   /* operation permissions */
        size_t          shm_segsz;  /* size of segment in bytes */
        pid_t           shm_lpid;   /* pid of last shm op */
        pid_t           shm_cpid;   /* pid of creator */
        shmatt_t        shm_nattch; /* # of current attaches */
        time_t          shm_atime;  /* last shmat() time*/
        time_t          shm_dtime;  /* last shmdt() time */
        time_t          shm_ctime;  /* last change by shmctl() */

The ipc_perm structure used inside the shmid_ds structure is defined in <sys/ipc.h> and contains the following members:

        uid_t cuid;  /* creator user id */
        gid_t cgid;  /* creator group id */
        uid_t uid;	 /* user id */
        gid_t gid;	 /* group id */
        mode_t mode; /* permission (lower 9 bits) */

The operation to be performed by shmctl() is specified in cmd and is one of:

IPC_STAT
    Gather information about the shared memory segment and place it in the structure pointed to by buf.

IPC_SET
    Set the value of the shm_perm.uid, shm_perm.gid and shm_perm.mode fields in the structure associated with shmid. The values are taken from the corresponding fields in the structure pointed to by buf. This operation can only be executed by the super-user, or a process that has an effective user id equal to either shm_perm.cuid or shm_perm.uid in the data structure associated with the shared memory segment.

IPC_RMID
    Remove the shared memory segment specified by shmid and destroy the data associated with it. Only the super-user or a process with an effective uid equal to the shm_perm.cuid or shm_perm.uid values in the data structure associated with the queue can do this.

The read and write permissions on a shared memory identifier are determined by the shm_perm.mode field in the same way as is done with files (see chmod(2) ), but the effective uid can match either the shm_perm.cuid field or the shm_perm.uid field, and the effective gid can match either shm_perm.cgid or shm_perm.gid.

RETURN VALUES
Upon successful completion, a value of 0 is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error.

ERRORS
shmctl() will fail if:

[EPERM]
    cmd is equal to IPC_SET or IPC_RMID and the caller is not the super-user, nor does the effective uid match either the shm_perm.uid or shm_perm.cuid fields of the data structure associated with the shared memory segment.

    An attempt was made to increase the value of shm_qbytes through IPC_SET but the caller is not the super-user.

[EACCES]
    The command is IPC_STAT and the caller has no read permission for this shared memory segment.

[EINVAL]
    shmid is not a valid shared memory segment identifier.

    cmd is not a valid command.

[EFAULT]
    buf specifies an invalid address.

SEE ALSO
shmat(2), shmget(2)

STANDARDS
The shmctl system call conforms to xsh5.

HISTORY
Shared memory segments appeared in the first release of AT&T UNIX.