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

       msgctl() performs the control operation specified by cmd on the message queue with identifier msqid.

       The msqid_ds data structure is defined in <sys/msg.h> as follows:

	   struct msqid_ds {
	       struct ipc_perm msg_perm;     /* Ownership and permissions */
	       time_t	       msg_stime;    /* Time of last msgsnd(2) */
	       time_t	       msg_rtime;    /* Time of last msgrcv(2) */
	       time_t	       msg_ctime;    /* Time of last change */
	       unsigned long   __msg_cbytes; /* Current number of bytes in
						queue (nonstandard) */
	       msgqnum_t       msg_qnum;     /* Current number of messages
						in queue */
	       msglen_t        msg_qbytes;   /* Maximum number of bytes
						allowed in queue */
	       pid_t	       msg_lspid;    /* PID of last msgsnd(2) */
	       pid_t	       msg_lrpid;    /* PID of last msgrcv(2) */
	   };

       The ipc_perm structure is defined in <sys/ipc.h> as follows (the highlighted fields are settable using IPC_SET):

	   struct ipc_perm {
	       key_t	      __key;	   /* Key supplied to msgget(2) */
	       uid_t	      uid;	   /* Effective UID of owner */
	       gid_t	      gid;	   /* Effective GID of owner */
	       uid_t	      cuid;	   /* Effective UID of creator */
	       gid_t	      cgid;	   /* Effective GID of creator */
	       unsigned short mode;	   /* Permissions */
	       unsigned short __seq;	   /* Sequence number */
	   };

       Valid values for cmd are:

       IPC_STAT
	      Copy  information  from  the  kernel data structure associated with msqid into the msqid_ds 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 kernel data structure associated with this  mes-
	      sage  queue,  updating  also  its  msg_ctime  member.  The following members of the structure are updated: msg_qbytes, msg_perm.uid,
	      msg_perm.gid, and (the least significant 9 bits of) msg_perm.mode.  The effective UID of the calling process must  match	the  owner
	      (msg_perm.uid) or creator (msg_perm.cuid) of the message queue, or the caller must be privileged.  Appropriate privilege (Linux: the
	      CAP_IPC_RESOURCE capability) is required to raise the msg_qbytes value beyond the system parameter MSGMNB.

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

       IPC_INFO (Linux-specific)
	      Returns information about system-wide message queue limits and parameters in the structure pointed to by buf.  This structure is	of
	      type msginfo (thus, a cast is required), defined in <sys/msg.h> if the _GNU_SOURCE feature test macro is defined:

		  struct msginfo {
		      int msgpool; /* Size in kibibytes of buffer pool
				      used to hold message data;
				      unused within kernel */
		      int msgmap;  /* Maximum number of entries in message
				      map; unused within kernel */
		      int msgmax;  /* Maximum number of bytes that can be
				      written in a single message */
		      int msgmnb;  /* Maximum number of bytes that can be
				      written to queue; used to initialize
				      msg_qbytes during queue creation
				      (msgget(2)) */
		      int msgmni;  /* Maximum number of message queues */
		      int msgssz;  /* Message segment size;
				      unused within kernel */
		      int msgtql;  /* Maximum number of messages on all queues
				      in system; unused within kernel */
		      unsigned short int msgseg;
				   /* Maximum number of segments;
				      unused within kernel */
		  };

	      The msgmni, msgmax, and msgmnb settings can be changed via /proc files of the same name; see proc(5) for details.

       MSG_INFO (Linux-specific)
	      Returns  a  msginfo  structure  containing  the same information as for IPC_INFO, except that the following fields are returned with
	      information about system resources consumed by message queues: the msgpool field returns the number of message queues that currently
	      exist on the system; the msgmap field returns the total number of messages in all queues on the system; and the msgtql field returns
	      the total number of bytes in all messages in all queues on the system.

       MSG_STAT (Linux-specific)
	      Returns a msqid_ds structure as for IPC_STAT.  However, the msqid argument is not a queue identifier, but instead an index into  the
	      kernel's internal array that maintains information about all message queues on the system.

RETURN VALUE

       On  success,  IPC_STAT,	IPC_SET, and IPC_RMID return 0.  A successful IPC_INFO or MSG_INFO operation returns the index of the highest used
       entry in the kernel's internal array recording information about all message queues.  (This information can be used with repeated  MSG_STAT
       operations  to  obtain  information  about  all queues on the system.)  A successful MSG_STAT operation returns the identifier of the queue
       whose index was given in msqid.

       On error, -1 is returned 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 or MSG_STAT, but the calling process does not have read permission on the message queue msqid,
	      and does not have the CAP_IPC_OWNER capability.

       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.	Or: for a MSG_STAT operation, the index value specified in msqid referred to an array slot that is
	      currently unused.

       EPERM  The argument cmd has the value IPC_SET or IPC_RMID, but the effective user ID of the calling process is not the creator (as found in
	      msg_perm.cuid)  or  the owner (as found in msg_perm.uid) of the message queue, and the process is not privileged (Linux: it does not
	      have the CAP_SYS_ADMIN capability).

CONFORMING TO

       SVr4, POSIX.1-2001.

NOTES

       The IPC_INFO, MSG_STAT and MSG_INFO operations are used by the ipcs(1) program to provide  information  on  allocated  resources.   In  the
       future these may modified or moved to a /proc file system interface.

       Various fields in the struct msqid_ds were typed as short under Linux 2.2 and have become long under Linux 2.4.	To take advantage of this,
       a recompilation under glibc-2.1.91 or later should suffice.  (The kernel distinguishes old and new calls by an IPC_64 flag in cmd.)

SEE ALSO

       msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7), svipc(7)

COLOPHON

       This page is part of release 3.25 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2008-08-06								 MSGCTL(2)