shmctl(2)                                               shmctl(2)



NAME
     shmctl - shared memory control operations

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

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

DESCRIPTION
     shmctl provides a variety of shared memory control
     operations as specified by cmd.  (Structure definitions and
     permissions are described in intro(2).)  The following cmds
     are available:

     IPC_STAT
          Place the current value of each member of the data
          structure associated with shmid into the structure
          pointed to by buf.

     IPC_SET
          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:

             shm_perm.uid
             shm_perm.gid
             shm_perm.mode /*only low 9 bits*/


          This cmd can only be executed by a process that has an
          effective user ID equal to either that of superuser or
          to the value of shm_perm.uid in the 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 data structure associated with it.  This cmd can
          only be executed by a process that has an effective
          user ID equal to either that of superuser or to the
          value of shm_perm.uid in the data structure associated
          with shmid.  The identifier and its associated data
          structure are not actually removed until there are no
          more referencing processes.  See ipcrm(1), and ipcs(1).

STATUS MESSAGES AND VALUES
     Upon successful completion, a value of 0 is returned.
     Otherwise, a value of -1 is returned and errno is set to



January 1992                                                    1

shmctl(2)                                               shmctl(2)



     indicate the error.

     The shmctl command will fail if one or more of the following
     is true:

     EINVAL
          shmid is not a valid shared memory identifier.

     EINVAL
          cmd is not a valid command.

     EACCES
          cmd is equal to IPC_STAT and READ operation permission
          is denied to the calling process (see intro(2)).

     EAGAIN
          The system has temporarily exhausted its available
          memory or swap space.

     EPERM
          cmd is equal to IPC_RMID or IPC_SET and the effective
          user ID of the calling process is not equal to that of
          superuser nor equal to the value of shm_perm.uid in the
          data structure associated with shmid.

     EFAULT
          buf points to an illegal address.

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

























2                                                    January 1992