shmctl(2)                        UNIX System V                        shmctl(2)


NAME
      shmctl - shared memory control operations

SYNOPSIS
      #include <sys/types.h>
      #include <sys/ipc.h>
      #include <sys/shm.h>

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

DESCRIPTION
      shmctl provides a variety of shared memory control operations as
      specified by cmd.  The following cmds are available:

      IPCSTAT    Place the current value of each member of the data structure
                  associated with shmid into the structure pointed to by buf.
                  The contents of this structure are defined in intro(2).
                  {READ}

      IPCSET     Set the value of the following members of the data structure
                  associated with shmid to the corresponding value found in the
                  structure pointed to by buf:

                      shmperm.uid
                      shmperm.gid
                      shmperm.mode /* only access permission bits */

                  This command can be executed only by a process that has an
                  effective user ID equal to that of super-user, or to the
                  value of shmperm.cuid or shmperm.uid in the data structure
                  associated with shmid.

      IPCRMID    Remove the shared memory identifier specified by shmid from
                  the system and destroy the shared memory segment and data
                  structure associated with it.  This command can be executed
                  only by a process that has an effective user ID equal to that
                  of super-user, or to the value of shmperm.cuid or
                  shmperm.uid in the data structure associated with shmid.

      SHMLOCK    Lock the shared memory segment specified by shmid in memory.
                  This command can be executed only by a process that has an
                  effective user ID equal to super-user.

      SHMUNLOCK  Unlock the shared memory segment specified by shmid.  This
                  command can be executed only by a process that has an
                  effective user ID equal to super-user.

      shmctl fails if one or more of the following are true:

      EACCES      cmd is equal to IPCSTAT and {READ} operation permission is
                  denied to the calling process [see intro(2)].



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shmctl(2)                        UNIX System V                        shmctl(2)


      EINVAL      shmid is not a valid shared memory identifier.

      EINVAL      cmd is not a valid command.

      EINVAL      cmd is IPCSET and shmperm.uid or shmperm.gid is not valid.

      EOVERFLOW   cmd is IPCSTAT and uid or gid is too large to be stored in
                  the structure pointed to by buf.

      EPERM       cmd is equal to IPCRMID or IPCSET and the effective user ID
                  of the calling process is not equal to that of super-user, or
                  to the value of shmperm.cuid or shmperm.uid in the data
                  structure associated with shmid.

      EPERM       cmd is equal to SHMLOCK or SHMUNLOCK and the effective user
                  ID of the calling process is not equal to that of super-user.

      EFAULT      buf points to an illegal address.

      ENOMEM      cmd is equal to SHMLOCK and there is not enough memory.

SEE ALSO
      shmget(2), shmop(2).

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

NOTES
      The user must explicitly remove shared memory segments after the last
      reference to them has been removed.























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