msgop(2)							System Calls Manual							  msgop(2)

Name
       msgsnd, msgrcv - message operations

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

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

       int msgrcv (msqid, msgp, msgsz, msgtyp, msgflg)
       int msqid;
       void *msgp;
       size_t msgsz;
       long msgtyp;
       int msgflg;

Description
       There are two message operations system calls, and

   msgsnd
       The system call is used to send a message to the queue associated with the message queue identifier specified by msqid.	The msgp parameter
       points to a structure containing the message.  This structure is composed of the following members:
       long mtype;    /* message type */
       char mtext[];  /* message text */
       The mtype parameter is a positive integer that can be used by the receiving process for message selection.  For more information,  see  the
       section	of this reference page.  The mtext parameter is any text of length msgsz bytes.  The msgsz parameter can range from 0 to a system-
       imposed maximum.

       The msgflg parameter specifies the action to be taken if the number of bytes already on the queue is equal  to  msg_qbytes.   (For  further
       information,  see  )  The parameter also specifies what happens when the total number of messages on all queues system-wide is equal to the
       system-imposed limit.

       If either of these conditions is true, and if (msgflg & IPC_NOWAIT) is true, the message is not sent and the calling process returns  imme-
       diately.  However, if either of the conditions is true and (msgflg & IPC_NOWAIT) is false, the calling process suspends execution until one
       of the following occurs:

       o    The condition responsible for the suspension no longer exists, in which case the message is sent.

       o    The msqid parameter is removed from the system.  For further information, see When this occurs, errno is set equal	to  EIDRM,  and  a
	    value of -1 is returned.

       o    The  calling  process  receives  a signal that is to be caught.  In this case, the message is not sent and the calling process resumes
	    execution in the manner prescribed in

       The system call fails and no message is sent under the following conditions:

       [EINVAL]       The msqid parameter is not a valid message queue identifier.

       [EACCES]       Operation permission is denied to the calling process.  For more information, see

       [EINVAL]       The mtype parameter is less than 1.

       [EAGAIN]       The message cannot be sent for one of the reasons cited above and (msgflg & IPC_NOWAIT) is true.

       [EINVAL]       The msgsz parameter is less than zero or greater than the system-imposed limit.

       [EFAULT]       The msgp parameter points to an illegal address.

       Upon successful completion, the following actions are taken with respect to the data structure associated with msqid (for more information,
       see ):

       o    The msg_qnum is incremented by 1.

       o    The msg_lspid is set equal to the process ID of the calling process.

       o    The msg_stime is set equal to the current time.

   msgrcv
       The  system call reads a message from the queue associated with the message queue identifier specified by msqid and places it in the struc-
       ture pointed to by msgp.  This structure is composed of the following members:

       long    mtype;	   /* message type */
       char    mtext[];    /* message text */
       The mtype parameter is the received message's type, as specified by the sending process.  The mtext parameter is the text of  the  message.
       The  msgsz  parameter  specifies the size, in bytes, of mtext.  The received message is truncated to msgsz bytes if it is larger than msgsz
       and (msgflg & MSG_NOERROR) is true.  The truncated part of the message is lost and no indication of the truncation is given to the  calling
       process.

       The msgtyp parameter specifies the type of message requested, as follows:

       o    If msgtyp is equal to 0, the first message on the queue is received.

       o    If msgtyp is greater than 0, the first message of type msgtyp is received.

       o    If msgtyp is less than 0, the first message of the lowest type that is less than or equal to the absolute value of msgtyp is received.

       The msgflg parameter specifies the action to be taken if a message of the desired type is not on the queue.  These specified actions are as
       follows:

       o    If (msgflg & IPC_NOWAIT) is true, the calling process returns immediately with a value of -1 and errno set to ENOMSG.

       o    If (msgflg & IPC_NOWAIT) is false, the calling process suspends execution until one of the following occurs:

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

	    -	 The msqid parameter is removed from the system.  When this occurs, errno is set equal to EIDRM, and a value of -1 is returned.

	    -	 The calling process receives a signal that is to be caught.  In this case, a message is not  received	and  the  calling  process
		 resumes execution in the manner prescribed in

       The system call fails and no message is received under the following conditions:

       [EINVAL]       The msqid parameter is not a valid message queue identifier.

       [EACCES]       Operation permission is denied to the calling process.

       [EINVAL]       The msgsz parameter is less than 0.

       [E2BIG]	      The mtext parameter is greater than msgsz and (msgflg & MSG_NOERROR) is false.

       [ENOMSG]       The queue does not contain a message of the desired type and (msgtyp & IPC_NOWAIT) is true.

       [EFAULT]       The msgp parameter points to an illegal address.

       Upon successful completion, the following actions are taken with respect to the data structure associated with msqid:

       o    The msg_qnum is decremented by 1.

       o    The msg_lrpid is set equal to the process ID of the calling process.

       o    The msg_rtime is set equal to the current time.

Return Values
       If  the or system calls return due to the receipt of a signal, a value of -1 is returned to the calling process, and errno is set to EINTR.
       If they return due to removal of msqid from the system, a value of -1 is returned, and errno is set to EIDRM.

       Upon successful completion, the return value is as follows:

       o    The system call returns a value of 0.

       o    The system call returns a value equal to the number of bytes actually placed into mtext.

       Otherwise, a value of -1 is returned, and errno is set to indicate the error.

See Also
       errno(2), intro(2), msgctl(2), msgget(2), signal(3)

																	  msgop(2)