NAME
msgrcv — receive a message from a message queueLIBRARY
Standard C Library (libc, -lc)SYNOPSIS
#include <sys/types.h>#include <sys/ipc.h>
#include <sys/msg.h> ssize_t
msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);
DESCRIPTION
The msgrcv() 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 */
MSGMAX).
The value of msgtyp has one of the following
meanings:
- The msgtyp argument is greater than 0. The first message of type msgtyp will be received.
- The msgtyp argument is equal to 0. The first message on the queue will be received.
- The msgtyp argument is less than 0. The first message of the lowest message type that is less than or equal to the absolute value of msgtyp will be received.
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
msgrcv() 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
ENOMSG. If
IPC_NOWAIT is not set, the calling process
will be blocked until:
- A message of the requested type becomes available on the message queue.
- The message queue is removed, in which case -1 will be
returned, and errno set to
EINVAL. - A signal is received and caught. -1 is returned, and
errno set to
EINTR.
- msg_cbytes is decremented by the size of the message.
- msg_lrpid is set to the pid of the caller.
- msg_lrtime is set to the current time.
- msg_qnum is decremented by 1.
RETURN VALUES
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.ERRORS
The msgrcv() function will fail if:- [
EINVAL] - The msqid argument 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 on it. The msgsz argument is less than 0.
- [
E2BIG] - A matching message was received, but its size was greater
than msgsz and the
MSG_NOERRORflag was not set in msgflg. - [
EACCES] - The calling process does not have read access to the message queue.
- [
EFAULT] - The msgp argument points to an invalid address.
- [
EINTR] - The system call was interrupted by the delivery of a signal.
- [
ENOMSG] - There is no message of the requested type available on the
message queue, and
IPC_NOWAITis set in msgflg.
SEE ALSO
msgctl(2), msgget(2), msgsnd(2)HISTORY
Message queues appeared in the first release of AT&T System V UNIX.| July 28, 2016 | Debian |