msgctl(2)                                               msgctl(2)



     NAME
          msgctl - message control operations

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

          int msgctl(id, cmd, buf)
          int id, cmd;
          struct msqid_ds *buf;

     DESCRIPTION
          msgctl provides a variety of message control operations as
          specified by cmd.  The following cmds are available:

          IPC_STAT       Place the current value of each member of the
                         data structure associated with id into the
                         structure referenced by buf.  The contents of
                         this structure are defined in intro(2).

          IPC_SET        Set the value of the following members of the
                         data structure associated with id to the
                         corresponding value found in the structure
                         referenced by buf:

                            msg_perm.uid
                            msg_perm.gid
                            msg_perm.mode (only low 9 bits)
                            msg_qbytes

                         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
                         msg_perm.uid in the data structure associated
                         with id.  Only the superuser can raise the
                         value of msg_qbytes.

          IPC_RMID       Remove the message queue identifier specified
                         by id from the system and destroy the message
                         queue 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 super user or to the value of
                         msg_perm.uid in the data structure associated
                         with id.  The identifier and its associated
                         data structure are not actually removed until
                         there are no more referencing processes.  See
                         ipcrm(1), and ipcs(1).






     April, 1990                                                     1

     msgctl(2)                                               msgctl(2)



     RETURN VALUE
          Upon successful completion, a value of 0 is returned.  Oth-
          erwise, a value of -1 is returned and errno is set to indi-
          cate the error.

     ERRORS
          msgctl will fail if one or more of the following is true.

          [EINVAL]       id is not a valid message queue identifier.

          [EINVAL]       cmd is not a valid command.

          [EACCES]       cmd is equal to IPC_STAT and operation per-
                         mission is denied to the calling process (see
                         intro(2)).

          [EPERM]        cmd is equal to IPC_RMID or IPC_SET.  The ef-
                         fective user ID of the calling process is not
                         equal to that of superuser and it is not
                         equal to the value of msg_perm.uid in the
                         data structure associated with id.

          [EPERM]        cmd is equal to IPC_SET, an attempt is being
                         made to increase to the value of msg_qbytes,
                         and the effective user ID of the calling pro-
                         cess is not equal to that of superuser.

          [EFAULT]       buf points to an illegal address.

     SEE ALSO
          intro(2), msgget(2), msgop(2).
























     2                                                     April, 1990