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NAME    [Toc]    [Back]

     shmctl - shared memory control operations

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #include <sys/shm.h>

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

DESCRIPTION    [Toc]    [Back]

     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

     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    [Toc]    [Back]

     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    [Toc]    [Back]

     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

     [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    [Toc]    [Back]

     shmat(2), shmget(2)

STANDARDS    [Toc]    [Back]

     The shmctl system call conforms to X/Open System Interfaces and Headers
     Issue 5 (``XSH5'').

HISTORY    [Toc]    [Back]

     Shared memory segments appeared in the first release of AT&T System V

BSD                             August 25, 1999                            BSD
[ Back ]
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