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Message Too Long Error Mq_send With Struct

- 1 answer

I am trying to set up message queue IPC example.

Problem: I am trying to make a working test case where I use mqueue to send a struct with various fields (I am referencing this post). However, I am running into Message too long error. Most examples send a char* message but I want to send a struct with multiple fields. In this toy example it just has a filepath but I would like to add other fields into the struct as well and send the struct over the message queue.

Working example:

#include <fcntl.h>
#include <sys/stat.h>
#include <mqueue.h> // mq_open(), mq_send(), mq_receive()

//#include <errno.h>
//#include <time.h>
#include <string.h> // strncpy(), strlen()

#include <stdio.h>  // printf(), perror()
#include <stdlib.h> // exit(), EXIT_FAILURE

#define MSG_SIZE  (300)

static mqd_t mqdes = -1;
static struct mq_attr attr;


 typedef struct{
     char filepath[MSG_SIZE];
 } obj;

void sendQueue()
{
    obj obj1;
    strncpy(obj1.filepath, "this_is_a_test", MSG_SIZE);

    if( -1 == mq_send(mqdes, (const char *) &obj1, sizeof(obj1), 0) )
    {
        perror( "mq_send failed" );
        exit( EXIT_FAILURE );
    }

    else
    {
        printf("msg sent successfully");
    }
}

void recvQueue()
{
    obj recv_obj;
    ssize_t res = mq_receive(mqdes, (char*) &recv_obj, sizeof(recv_obj), NULL);
    if (res == -1)
    {
        perror("mq_receive failed");
    }

    else
    {
        printf("Message: %s\n", recv_obj.filepath);
    }
}

int main( void )
{
    mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH;

    memset( &attr, 0x00, sizeof(struct mq_attr) );
    attr.mq_flags = 0;
    attr.mq_maxmsg = 10;
    attr.mq_msgsize = 1024;
    attr.mq_curmsgs = 0;

    char *queueName = "/test";

    mqdes = mq_open( queueName, O_RDWR|O_CREAT, mode, &attr);
    if( -1 == mqdes )
    {
        perror( "mq_open failed");
        exit( EXIT_FAILURE );
    }

    // implied else, mq_open successful


    sendQueue();
    recvQueue();
    return 0;
}

compiled with

gcc -std=gnu99 test.c -lrt -o test

On linux 18.04.3 LTS.

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Answer

The 3rd argument in mq_receive should be 1024 and not the size of the message sent. According to man page of mq_receive "The msg_len argument specifies the size of the buffer pointed to by msg_ptr; this must be greater than or equal to the mq_msgsize attribute of the queue (see mq_getattr(3))."

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source: stackoverflow.com
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