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

NAME

       msgop - message operations

SYNOPSIS

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

       int msgsnd(int msqid, struct msgbuf *msgp, size_t msgsz, int msgflg);

       ssize_t msgrcv(int msqid, struct msgbuf *msgp, size_t msgsz, long msgtyp, int msgflg);

DESCRIPTION

       To send or receive a message, the calling process allocates a structure of the following general form:

	    struct msgbuf {
		 long mtype;	 /* message type, must be > 0 */
		 char mtext[1];  /* message data */
	    };

       The  mtext  field is an array (or other structure) whose size is specified by msgsz, a non-negative integer value.  Messages of zero length
       (i.e., no mtext field) are permitted.  The mtype field must have a strictly positive integer value  that  can  be  used	by  the  receiving
       process for message selection (see the section about msgrcv).

       The calling process must have write permission to send and read permission to receive a message on the queue.

       The msgsnd system call appends a copy of the message pointed to by msgp to the message queue whose identifier is specified by msqid.

       If  sufficient  space is available on the queue, msgsnd succeeds immediately.  (The queue capacity is defined by the msg_bytes field in the
       associated data structure for the message queue.  During queue creation this field is initialised to MSGMNB bytes, but this  limit  can	be
       modified  using	msgctl.)   If  insufficient  space is available on the queue, then the default behaviour of msgsnd is to block until space
       becomes available.  If IPC_NOWAIT is asserted in msgflg then the call instead fails with the error EAGAIN.

       A blocked msgsnd call may also fail if the queue is removed (in which case the system call fails with errno set to EIDRM), or a	signal	is
       caught  (in  which  case  the system call fails with errno set to EINTR).  (msgsnd and msgrcv are never automatically restarted after being
       interrupted by a signal handler, regardless of the  setting  of the SA_RESTART flag when establishing a signal handler.)

       Upon successful completion the message queue data structure is updated as follows:

	      msg_lspid is set to the process ID of the calling process.

	      msg_qnum is incremented by 1.

	      msg_stime is set to the current time.

       The system call msgrcv reads a message from the message queue specified by msqid into the msgbuf pointed to by the msgp argument,  removing
       the read message from the queue.

       The  argument msgsz specifies the maximum size in bytes for the member mtext of the structure pointed to by the msgp argument.  If the mes-
       sage text has length greater than msgsz, then if the msgflg argument asserts MSG_NOERROR, the message text will be truncated (and the trun-
       cated part will be lost), otherwise the message isn't removed from the queue and the system call fails returning with errno set to E2BIG.

       The argument msgtyp specifies the type of message requested as follows:

	      If msgtyp is 0, then the first message in the queue is read.

	      If  msgtyp  is greater than 0, then the first message on the queue of type msgtyp is read, unless MSG_EXCEPT was asserted in msgflg,
	      in which case the first message on the queue of type not equal to msgtyp will be read.

	      If msgtyp is less than 0, then the first message on the queue with the lowest type less than or equal to the absolute value of  msg-
	      typ will be read.

       The msgflg argument asserts none, one or more (or-ing them) of the following flags:

	      IPC_NOWAIT  For  immediate  return  if  no  message  of the requested type is on the queue.  The system call fails with errno set to
	      ENOMSG.

	      MSG_EXCEPT Used with msgtyp greater than 0 to read the first message on the queue with message type that differs from msgtyp.

	      MSG_NOERROR To truncate the message text if longer than msgsz bytes.

       If no message of the requested type is available and IPC_NOWAIT isn't asserted in msgflg, the calling process is blocked until one  of  the
       following conditions occurs:

	      A message of the desired type is placed on the queue.

	      The message queue is removed from the system.  In this case the system call fails with errno set to EIDRM.

	      The calling process catches a signal.  In this case the system call fails with errno set to EINTR.

       Upon successful completion the message queue data structure is updated as follows:

	      msg_lrpid is set to the process ID of the calling process.

	      msg_qnum is decremented by 1.

	      msg_rtime is set to the current time.

RETURN VALUE

       On  a  failure  both functions return -1 with errno indicating the error, otherwise msgsnd returns 0 and msgrvc returns the number of bytes
       actually copied into the mtext array.

ERRORS

       When msgsnd fails, at return errno will be set to one among the following values:

       EAGAIN	  The message can't be sent due to the msg_qbytes limit for the queue and IPC_NOWAIT was asserted in mgsflg.

       EACCES	  The calling process has no write permission on the message queue.

       EFAULT	  The address pointed to by msgp isn't accessible.

       EIDRM	  The message queue was removed.

       EINTR	  Sleeping on a full message queue condition, the process caught a signal.

       EINVAL	  Invalid msqid value, or nonpositive mtype value, or invalid msgsz value (less than 0 or greater than the system value MSGMAX).

       ENOMEM	  The system has not enough memory to make a copy of the supplied msgbuf.

       When msgrcv fails, at return errno will be set to one among the following values:

       E2BIG	  The message text length is greater than msgsz and MSG_NOERROR isn't asserted in msgflg.

       EACCES	  The calling process does not have read permission on the message queue.

       EFAULT	  The address pointed to by msgp isn't accessible.

       EIDRM	  While the process was sleeping to receive a message, the message queue was removed.

       EINTR	  While the process was sleeping to receive a message, the process received a signal that had to be caught.

       EINVAL	  Illegal msgqid value, or msgsz less than 0.

       ENOMSG	  IPC_NOWAIT was asserted in msgflg and no message of the requested type existed on the message queue.

NOTES

       The followings are system limits affecting a msgsnd system call:

       MSGMAX	  Maximum size for a message text: the implementation set this value to 8192 bytes.

       MSGMNB	  Default maximum size in bytes of a message queue: 16384 bytes.  The super-user can increase the size of a message  queue  beyond
		  MSGMNB by a msgctl system call.

       The  implementation  has no intrinsic limits for the system wide maximum number of message headers (MSGTQL) and for the system wide maximum
       size in bytes of the message pool (MSGPOOL).

CONFORMING TO

       SVr4, SVID.

NOTE

       The pointer argument is declared as struct msgbuf * with libc4, libc5, glibc 2.0, glibc 2.1. It is declared as void *  (const  void  *  for
       msgsnd()) with glibc 2.2, following the SUSv2.

SEE ALSO

       ipc(5), msgctl(2), msgget(2), msgrcv(2), msgsnd(2)

Linux 2.4.18							    2002-06-01								  MSGOP(2)