mq_send fails with Message too long when sending a struct

Question

I am trying the following code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <mqueue.h>
#include "sctp.h"
#include "s1ap.h"
#include "messages.h"

void send_to_s1ap(void);

int main()
{
    pthread_t tid;

    pthread_create(&tid, NULL, s1ap, (void *)&tid);
    pthread_create(&tid, NULL, sctp, (void *)&tid);

    send_to_s1ap();

    pthread_exit(NULL);
    return 0;
}

void send_to_s1ap(void)
{
    int status = 0;
    mqd_t mqfd;
    struct mq_attr attr;
    int open_flags = 0;
    int priority_of_msg;
    message msg;

    attr.mq_maxmsg = 100;
    attr.mq_msgsize = 100;
    attr.mq_flags   = 0;
    open_flags = O_WRONLY|O_CREAT;
    mqfd = mq_open("/s1ap_queue",open_flags,0660,&attr);
    if (mqfd == -1)
    {
        printf("mq_open failure from main\n");
        return;
    }
    /* Fill in a test message buffer to send */

    msg.msg_id = 1;
    msg.data[0] = 'P';
    msg.data[1] = 'R';
    msg.data[2] = 'I';
    msg.data[3] = 'O';
    msg.data[4] = 'R';
    msg.data[5] = 'I';
    msg.data[6] = 'T';
    msg.data[7] = 'Y';
    msg.data[8] = '1';
    msg.data[9] = 'a';
    priority_of_msg = 1;

    status = mq_send(mqfd,(const char *) &msg,sizeof(msg),priority_of_msg);

    if (status == -1)
    {
        perror("mq_send failure on mqfd\n");
    }
    else
    {
        printf("successful call to mq_send\n");
    }
    if (mq_close(mqfd) == -1)
    {
        printf("mq_close failure on mqfd");
    }
}

where in messages.h I declare

#define max_msg_size 10

typedef struct message {
    int msg_id;
   char data[max_msg_size];
message;

I get the following error code: mq_send failure on mqfd : Message too long

I have commented out all the code in the client side (which is doing mq_receive) , so the error doesnt come from there. Any ideas what is causing the problem?

I tried changing

attr.mq_maxmsg = 100; attr.mq_msgsize = 100;

but it didnt help

Edit:

I did an example with

void send_to_s1ap(void)
{
    int status = 0;
    mqd_t mqfd;
    struct mq_attr attr;
    int open_flags = 0;
    int priority_of_msg;
    char buffer2[12] = "Hello world";
    
    attr.mq_maxmsg = 10;
    attr.mq_msgsize = 100;
    attr.mq_flags   = 0;
    attr.mq_curmsgs = 0;
    open_flags = O_WRONLY|O_CREAT;
    mqfd = mq_open("/s1ap_queue",open_flags,0660,&attr);
    if (mqfd == -1)
    {
        printf("mq_open failure from main\n");
        return;
    }
    /* Fill in a test message buffer to send */


    
    priority_of_msg = 1;

    status = mq_send(mqfd,buffer2,sizeof(buffer2),priority_of_msg);

    if (status == -1)
    {
        perror("mq_send failure on mqfd\n");
    }
    else
    {
        printf("successful call to mq_send\n");
    }
    if (mq_close(mqfd) == -1)
    {
        printf("mq_close failure on mqfd");
    }
}

and its still failing with

mq_send failure on mqfd : Message too long

So I guess its not the struct that is causing the issue, but indeed for some reason its not sending big structs/strings. Any idea why?