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MSGRCV(2) System Calls Manual MSGRCV(2)

msgrcvreceive a message from a message queue

library “libc”

#include <sys/msg.h>

ssize_t
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 user-defined structure pointed to by msgp. This structure must contain a first field of type that will indicate the user-defined type of the message. The remaining fields will contain the contents of the message. The following is an example of what this user-defined structure might look like:
struct mymsg {
    long mtype;    /* message type */
    char mtext[1]; /* body of message */
};

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

The value of msgtyp has one of the following meanings:

The argument msgsz specifies the size in bytes of mtext. 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 behaviour of () depends on whether the IPC_NOWAIT flag is set in msgflg or not. If IPC_NOWAIT is set, then msgrcv() will immediately return a value of -1 and set errno to EAGAIN. If IPC_NOWAIT is not set, the calling process will block 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 and placed 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:

[]
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.
[]
There is no message of the requested type available on the message queue, and IPC_NOWAIT is set in msgflg.
[]
msgp points to an invalid address.
[]
The message queue identifier msqid is removed from the system.
[]
The system call was interrupted by the delivery of a signal.
[]
msqid is not a valid message queue identifier

The message queue was removed while msgrcv() was waiting for a message of the requested type to become available in it.

msgsz is greater than SSIZE_MAX.

[]
The queue does not contain a message of the desired type and IPC_NOWAIT is set.

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

The msgrcv system call conforms to X/Open System Interfaces and Headers Issue 5 (“XSH5”).

Message queues appeared in the first release of AT&T System V UNIX.

July 24, 2013 NetBSD-9.2