msgrcv - receive message from queue

Syntax	#include <sys/msg.h> int msgrcv(int msqid, void *msgp, size_t msgsz, long int msgtyp, int msgflg);
Description	msgrcv() reads a message from the queue associated with the message queue identifier specified by msqid and places it in the user-defined buffer pointed to by msgp. msgp points to a user-defined buffer that must contain first a field of type long int that will specify the type of the message, and then a data portion that will hold the data bytes of the message. The structure below is an example of what this user-defined buffer might look like:  struct mymsg { long int mtype; /* Message type */ char mtext[1]; /* Message text */ } The structure member mtype is the type of the received message, as specified by the sending process. The structure member mtext is the text of the message. msgsz 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 non-zero. The truncated part of the message is lost and no indication of the truncation is given to the calling process. msgtyp specifies the type of message requested as follows: If msgtyp is 0, the first message on the queue is received. If msgtyp is greater than 0, the first message of type msgtyp is received. If msgtyp is less than 0, the first message that is less than or equal to the absolute value of msgtyp is received. msgflg specifies the action to be taken if a message of the desired type is not on the queue. The following actions are possible: If ( msgflg & IPC_NOWAIT) is non-zero, the calling process will return immediately with a return value of -1, and errno set to ENOMSG. If ( msgflg & IPC_NOWAIT) is 0, the calling process will suspend execution until one of the following events occurs: A message of the desired type is placed on the queue. The message queue identifier msqid is removed from the system; when this occurs, errno is set equal to EIDRM, and -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 sigaction(). Upon successful completion, the following actions are taken with respect to the data structure associated with msqid: msg_qnum is decremented by 1. msg_lrpid is set equal to the process ID of the calling process. msg_rtime is set equal to the current time. If threads are used, then the function affects the process or a thread in the following manner: The msgflg parameter refers to the calling thread.
Return val.	number of bytes placed in mtext
		if successful.
	-1	if an error occurs. errno is set to indicate the error.
Errors	msgrcv() will fail if:
	E2BIG	The value of mtext is greater than msgsz and (msgflg & MSG_NOERROR) is 0.
	EACCES	Operation permission is denied to the calling process.
	Extension
	EFAULT	msgp points to an invalid address. (End)
	EIDRM	The message queue identifier msqid is removed from the system.
	EINTR	msgrcv() was interrupted by a signal.
	EINVAL	msqid is not a valid message queue identifier or the value of msgsz is less than 0.
	ENOMSG	The queue does not contain a message of the desired type and (msgtyp & IPC_NOWAIT) is non-zero.
Notes	msgp should be converted to type void *. The IEEE 1003.4 Standards Committee is developing alternative interfaces for interprocess communication. Application developers who need to use interprocess communication (IPC) should design their applications so that modules using the IPC routines described here can be easily modified at a later date.
See also	msgctl(), msgget(), msgsnd(), sigaction(), sys/msg.h, section “Interprocesscommunication” .