05-13-2005
You would be best off having two programs, but not in the same way that you have created. Have one program that is the "server". This will create the message queue when it starts up, and destroy the queue when it shuts down. The second program will be the client program that will just connect to the message queue (no IPC_CREAT).
The client program will put messages with a type flag set to some unique value ( using the program's pid would be a good idea ); while msgrcv will be done with type set to the the unique value, but with the flag set to MSG_EXCEPT - this will pick up messages with any type that is not the value set as msgtyp in the msgrcv call.
A single program will probably complicate things unnecessarily.
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LEARN ABOUT DEBIAN
msgrcv
MSGRCV(2) BSD 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>
int
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:
o The msgtyp argument is greater than 0. The first message of type msgtyp will be received.
o The msgtyp argument is equal to 0. The first message on the queue will be received.
o 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:
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
Upon successful completion, msgrcv() returns the number of bytes received into the mtext field of the structure pointed to by msgp. Other-
wise, -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.
BSD
July 9, 2009 BSD