NAME    [Toc]    [Back]

     msgrcv - receive a message from a message queue

SYNOPSIS    [Toc]    [Back]

     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/msg.h>

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

DESCRIPTION    [Toc]    [Back]

     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:

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

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


     +o    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
         received.

     msgsz 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  EAGAIN.   If
IPC_NOWAIT is not set,
     the calling process will be blocked until:

     +o   A message of the requested type becomes available on the
message
         queue.

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

     +o   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:

     +o   msg_cbytes is decremented by the size of the message.

     +o   msg_lrpid is set to the pid of the caller.

     +o   msg_lrtime is set to the current time.

     +o   msg_qnum is decremented by 1.

RETURN VALUES    [Toc]    [Back]

     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    [Toc]    [Back]

     msgrcv() will fail if:

     [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 on
                   it.

                   msgsz 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]      msgp points to an invalid address.

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


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

SEE ALSO    [Toc]    [Back]

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

HISTORY    [Toc]    [Back]

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

BUGS    [Toc]    [Back]

     OpenBSD does not define the EIDRM error value, which  should
be used in
     the  case  of a removed message queue, nor the ENOMSG value,
which should
     be used when no suitable message is available and IPC_NOWAIT
is set.

OpenBSD      3.6                          August     17,     1995