Question

msgrcv system call. System V. message queue how to get out of loop.

Asked by: ubuntuguy

Hi, I have a message queue from which I am getting messages in a loop. The problem is that I don't know how to exit the loop. msgrcv returns type size_t so it keeps going. What value should I compare the return value so I can exit the loop?

while(msgrcv(msqid, &msgreceived, sizeof(msgreceived), BUFFER_CHANGED, 0) != -1){
        printf("%d %d %d %d ",msgreceived.value0,msgreceived.value1,msgreceived.value2,msgreceived.value3)

                                  
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Asked On
2009-11-05 at 11:00:52ID24875596
Topics

C Programming Language

,

C++ Programming Language

,

Unix Operating Systems

Participating Experts
2
Points
500
Comments
6

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Answers

 

by: evilrixPosted on 2009-11-05 at 11:10:41ID: 25752792

According to the docs...

"Upon successful completion, msgrcv() returns a value equal to the number of bytes actually placed into the buffer mtext. Otherwise, no message will be received, msgrcv() will return (ssize_t)-1 and errno will be set to indicate the error."

http://www.opengroup.org/onlinepubs/007908799/xsh/msgrcv.html

So, according to that you should test for (ssize_t)-1

hile(msgrcv(msqid, &msgreceived, sizeof(msgreceived), BUFFER_CHANGED, 0) != (ssize_t)-1)

 

by: jkrPosted on 2009-11-05 at 11:14:50ID: 25752830

I'd assume that you want to exit the loop based on a message that indicates to do so, e.g.

// commonmsgs.h
#define MYMSG_QUIT 42 
// receiver.c
#include "commonmsgs.h" 
// ... 
while(msgrcv(msqid, &msgreceived, sizeof(msgreceived), BUFFER_CHANGED, 0) != -1){ 
        if (msgreceived.mtype == MYMSG_QUIT) break; 
        printf("%d %d %d %d ",msgreceived.value0,msgreceived.value1,msgreceived.value2,msgreceived.value3)
}
                                              
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by: ubuntuguyPosted on 2009-11-05 at 11:16:33ID: 25752852

msgrcv returns 20 always. so the size must be 20.  so i i changed your code to (ssize_t)20 it still doesn't work bc even after all messages have been retrieved from the queue the value is still 20.  is there a way to figure out how many messages are in the queue. i can't do it when i add them because it is children processes that add messages so the parent doesn't read that value. unless i make a shared memory segment, but that is more work than i need to

 

by: ubuntuguyPosted on 2009-11-05 at 11:21:32ID: 25752905

I want to exit the loop once I read the last element.

 

by: jkrPosted on 2009-11-05 at 11:24:02ID: 25752937

Well, so you either need to know how many messages you are going to receive or have the sender tell you that everything was sent - like in my above example.

 

by: ubuntuguyPosted on 2009-11-05 at 11:25:58ID: 25752955

I got it I had to use IPC_NOWAIT instead of 0.

20120131-EE-VQP-002

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