MSGRCV(2)                 FreeBSD System Calls Manual                MSGRCV(2)

NAME
     msgrcv - receive a message from a message queue

LIBRARY
     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 */

     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:

     •   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.

     The msgsz argument 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 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.

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

     •   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_NOERROR flag 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_NOWAIT is 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.

FreeBSD 13.1-RELEASE-p6          July 28, 2016         FreeBSD 13.1-RELEASE-p6