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SHMCTL(2)                     System Calls Manual                    SHMCTL(2)

NAME
     shmctl -- shared memory control operations

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <&lt;sys/shm.h>&gt;

     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
                segment can do this.

     SHM_LOCK   Lock the shared memory segment specified by shmid in memory.
                This operation can only be executed by the super-user.

     SHM_UNLOCK
                Unlock the shared memory segment specified by shmid.  This
                operation can only be executed by the super-user.

     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:

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

     [EFAULT]           buf specifies an invalid address.

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

                        cmd is not a valid command.

     [ENOMEM]           The cmd is equal to SHM_LOCK and there is not enough
                        physical memory.

     [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.

                        The cmd is equal to SHM_LOCK or SHM_UNLOCK and the
                        caller is not the super-user.

SEE ALSO
     ipcrm(1), ipcs(1), shmat(2), shmget(2)

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

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

NetBSD 6.1.5                   November 25, 2006                  NetBSD 6.1.5