Updated: 2021/Apr/14

MSGRCV(2)                     System Calls Manual                    MSGRCV(2)

     msgrcv - receive a message from a message queue

     Standard C Library (libc, -lc)

     #include <sys/msg.h>

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

     The msgrcv() 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 long 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:

        msgtyp is greater than 0.  The first message of type msgtyp will be

        msgtyp is equal to 0.  The first message on the queue will be

        msgtyp 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

     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

     If no matching message is present on the message queue specified by
     msqid, the behaviour of msgrcv() 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:

        A message of the requested type becomes available on the message

        The message queue is removed, in which case -1 will be returned and
         errno set to EIDRM.

        A signal is received and caught.  -1 is returned and errno is set to

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

        msg_lrpid is set to the pid of the caller.

        msg_lrtime is set to the current time.

        msg_qnum is decremented by 1.

     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:

     [E2BIG]            A matching message was received, but its size was
                        greater than msgsz and the MSG_NOERROR flag was not
                        set in msgflg.

     [EACCES]           The calling process does not have read access to the
                        message queue.

     [EAGAIN]           There is no message of the requested type available on
                        the message queue, and IPC_NOWAIT is set in msgflg.

     [EFAULT]           msgp points to an invalid address.

     [EIDRM]            The message queue identifier msqid is removed from the

     [EINTR]            The system call was interrupted by the delivery of a

     [EINVAL]           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.

     [ENOMSG]           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.

NetBSD 9.99                      July 24, 2013                     NetBSD 9.99