msgsnd - send message to queue

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Syntax	#include <sys/msg.h> int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
Description	msgsnd() sends a message to the queue associated with the message queue identifier specified by msqid. 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 a non-zero positive type long int that can be used by the receiving process for message selection. The structure member mtext is any text of length msgsz bytes. The argument msgsz can range from 0 to a system-imposed maximum. msgflg specifies the action to be taken if one or more of the following conditions are true: The number of bytes already on the queue is equal to msg_qbytes (see sys/msg.h). The total number of messages on all queues system-wide is already equal to the system-imposed limit. These actions are as follows: If ( msgflg & IPC_NOWAIT) is non-zero, the message will not be sent, and the calling process will return immediately. If ( msgflg & IPC_NOWAIT) is 0, the calling process will suspend execution until one of the following events occurs: The condition responsible for the suspension no longer exists, in which case the message is sent. 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, the message is not sent, 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 incremented by 1. msg_lspid is set equal to the process ID of the calling process. msg_stime 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.	0	if successful.
	-1	if an error occurs. errno is set to indicate the error.
Errors	msgsnd() will fail if:
	EACCES	Operation permission is denied to the calling process.
	EAGAIN	The message cannot be sent for one of the reasons cited above and ( msgflg & IPC_NOWAIT) is non-zero.
	Extension
	EFAULT	msgp points to an invalid address. (End)
	EIDRM	The message queue identifier msgid is removed from the system.
	EINTR	msgsnd() was interrupted by a signal.
	EINVAL	msqid is not a valid message queue identifier, or the value of mtype is less than 1; or the value of msgsz is less than 0 or greater than the system-imposed limit.
Notes	The value of the msgp argument 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(), msgrcv(), sigaction(), sys/msg.h, section “Interprocesscommunication”.