POSIX mq_receive issue: Message too long


 
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# 1  
Old 09-10-2009
POSIX mq_receive issue: Message too long

Hello,

I am trying to implement posix message queue application. I am faced with an error on the mq_receive section. It says "Message too long". I've tried couple of small tweeks, but to no result. Please do suggest any rectificaitons.

mq_send section-works successfully
Code:
#include <mqueue.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <sys/stat.h>
#include <string.h>
#include <signal.h>

#define MSG_SIZE 16
#define MQ_NAME "/msgqueue"
int main(void)
{
      
      mqd_t    mqPXId;          
        char *msg = "test";
         
        
        if ((mqPXId = mq_open (MQ_NAME, O_RDWR | O_CREAT, S_IWUSR | S_IRUSR, NULL)) ==-1) 
            { 
            printf ("sendTask: mq_open failed: %s\n", strerror(errno)); 
            return 0; 
            }

           
        if (mq_send (mqPXId, msg, MSG_SIZE, NULL) == -1) 
            { 
            printf ("sendTask: mq_send failed\n"); 
            return 0; 
            } 
        else 
        {
            printf ("sendTask: mq_send succeeded, msg sent: %s\n",msg); 
         }
            return 0;
}

mq_receive section: error-Message too long

Code:
#include <stdio.h>
#include <mqueue.h>
#include <pthread.h>
#include <string.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <errno.h>
#define MSG_SIZE 16
#define MQ_NAME "/msgqueue"

int main(void)
{
    mqd_t    mqPXId;         /* msg queue descriptor */ 
        char     *msg;  /* msg buffer */ 
       // unsigned int      prio;           /* priority of message */

     
        /* open message queue using default attributes */ 
        if ((mqPXId = mq_open (MQ_NAME, O_RDWR , S_IWUSR | S_IRUSR, NULL))  
            ==  -1) 
            { 
            printf ("receiveTask: mq_open failed\n"); 
            return 0; 
            }

     
        /* try reading from queue */ 
        if (mq_receive (mqPXId,(char *)&msg,MSG_SIZE, NULL) == -1) 
            { 
            printf ("receiveTask: mq_receive failed: %s\n",strerror(errno)); 
            return 0; 
            } 
        else 
            { 
            printf ("receiveTask: Msg of priority received:\n\t\t%s\n", 
                    msg); 
            } 
return 0;
        }

# 2  
Old 09-12-2009
Have you checked the value of mq_msgsize?
# 3  
Old 09-13-2009
Your receive is not likely to work. You're not putting the message into a buffer that "char *msg" points to, your putting it into the bytes (probably 4) that make up the "char *msg" variable itself along with randomly overwriting your stack, because sizeof( msg ) is probably 4, but you're telling the mq_receive() call that the address you're passing points to 16 bytes.

Try something like this:

Code:
ssize_t bytes_received;
char buffer[ MSG_SIZE ];

bytes_received = mq_receive( mxPXId, buffer, sizeof( buffer ), NULL );
...

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