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Manual page for msgrcv(2)

msgrcv - message receive operation

SYNOPSIS

#include <sys/msg.h>

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

DESCRIPTION

The msgrcv() function 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.

The argument 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	mtype;	/* message type */
	char	mtext[1];	/* message text */
}

The structure member mtype is the received message's type as specified by the sending process.

The structure member mtext is the text of the message.

The argument 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.

The argument 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 of the lowest type that is less than or equal to the absolute value of msgtyp is received.

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

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 occurs:

--: A message of the desired type is placed on the queue.
--: The message queue identifier msqid is removed from the system (see msgctl.2 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.2

Upon successful completion, the following actions are taken with respect to the data structure associated with msqid (see intro.2

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.

RETURN VALUES

Upon successful completion, msgrcv() returns a value equal to the number of bytes actually placed into the buffer mtext. Otherwise, no message will be received, msgrcv() will return -1 and errno will be set to indicate the error.

ERRORS

The msgrcv() function 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. See intro.2
EIDRM: The message queue identifier msqid is removed from the system.
EINTR: The msgrcv() function 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 (msgflg&IPC_NOWAIT) is non-zero.

USAGE