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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.

Debian                                            July 28, 2016                                        MSGRCV(2)