redhat man page for msgctl

MSGCTL(2)						     Linux Programmer's Manual							 MSGCTL(2)

NAME
       msgctl - message control operations

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

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

DESCRIPTION
       This function performs the control operation specified by cmd on the message queue with identifier msqid.  Legal values for cmd are:

       IPC_STAT
	      Copy  info  from	the message queue data structure associated with msqid into the structure pointed to by buf.  The caller must have
	      read permission on the message queue.

       IPC_SET
	      Write the values of some members of the msqid_ds structure pointed to by buf to the message queue data structure, updating also  its
	      msg_ctime member.  The following members of the structure can be updated:

		   msg_perm.uid
		   msg_perm.gid
		   msg_perm.mode  /* only lowest 9-bits */
		   msg_qbytes

	      The  calling  process  must  have  appropriate  (probably,  root)  privileges  or  its effective user-ID must be that of the creator
	      (msg_perm.cuid) or owner (msg_perm.uid) of the message queue.  Appropriate (probably, root) privileges are  required  to	raise  the
	      msg_qbytes value beyond the system parameter MSGMNB.

       IPC_RMID
	      Immediately  remove  the message queue and its associated data structure, awakening all waiting reader and writer processes (with an
	      error return and errno set to EIDRM).  The calling process must have  appropriate  (probably,  root)  privileges	or  its  effective
	      user-ID must be either that of the creator or owner of the message queue.

RETURN VALUE
       On success, the return value will be 0, otherwise -1 with errno indicating the error.

ERRORS
       On failure, errno is set to one of the following:

       EACCES	  The argument cmd is equal to IPC_STAT but the calling process does not have read permission on the message queue msqid.

       EFAULT	  The argument cmd has the value IPC_SET or IPC_STAT, but the address pointed to by buf isn't accessible.

       EIDRM	  The message queue was removed.

       EINVAL	  Invalid value for cmd or msqid.

       EPERM	  The argument cmd has the value IPC_SET or IPC_RMID, but the calling process has insufficient privileges to execute the command.

NOTES
       The  IPC_INFO,  MSG_STAT  and MSG_INFO control calls are used by the ipcs(8) program to provide information on allocated resources.  In the
       future these may be modified as needed or moved to a proc file system interface.

       Various fields in a struct msqid_ds were shorts under Linux 2.2 and have become longs under Linux 2.4. To take advantage of this, a  recom-
       pilation under glibc-2.1.91 or later should suffice.  (The kernel distinguishes old and new calls by a IPC_64 flag in cmd.)

CONFORMING TO
       SVr4, SVID.  SVID does not document the EIDRM error condition.

SEE ALSO
       ipc(5), msgget(2), msgsnd(2), msgrcv(2)

Linux 2.4.1							    2001-02-18								 MSGCTL(2)