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MSGRCV(2) System Calls Manual MSGRCV(2)

msgrcvreceive a message from a message queue

#include <sys/msg.h>

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

The () function receives a message from the message queue specified in msqid, and places it into the structure pointed to by msgp. This structure should consist of the following members:

    long mtype;    /* message type */
    char mtext[1]; /* body of message */

mtype is an integer greater than 0 that can be used for selecting messages, mtext is an array of bytes, with a size up to that of the system limit (MSGMAX).

The value of msgtyp has one of the following meanings:

msgsz specifies the maximum length of the requested message. If the received message has a length greater than msgsz it will be silently truncated if the MSG_NOERROR flag is set in msgflg, otherwise an error will be returned.

If no matching message is present on the message queue specified by msqid, the behavior of () depends on whether the IPC_NOWAIT flag is set in msgflg or not. If IPC_NOWAIT is set, msgrcv() will immediately return a value of -1, and set errno to EAGAIN. If IPC_NOWAIT is not set, the calling process will be blocked until:

If a message is successfully received, the data structure associated with msqid is updated as follows:

Upon successful completion, msgrcv() returns the number of bytes received into the mtext field of the structure pointed to by msgp. Otherwise, -1 is returned, and errno set to indicate the error.

msgrcv() will fail if:

msqid is not a valid message queue identifier.

msgsz is less than 0.

A matching message was received, but its size was greater than msgsz and the MSG_NOERROR flag was not set in msgflg.
The calling process does not have read access to the message queue.
msgp points to an invalid address.
The system call was interrupted by the delivery of a signal.
There is no message of the requested type available on the message queue, and IPC_NOWAIT is set in msgflg.
The message queue was removed while msgrcv() was waiting for a message of the requested type to become available on it.

msgctl(2), msgget(2), msgsnd(2)

Message queues appeared in AT&T System V Release 1 UNIX.

November 15, 2014 OpenBSD-6.4