Why msgrcv does not save recieved values in structure?

Question

I have two programs: server.c, and c1.c. Server should be running infinitely, and other programs, like c1.c, should send the number to the server, and get the square of that number as a server answer. The problem with my two programs is that server calculates number correctlly, c1 reads from the message queue without errors, but it does not save the number to the struct.

server.c:

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

#define LAST_MESSAGE 255 
int main()
{
    int msqid; 
    char pathname[] = "server.c";
    key_t key; 
    int len, maxlen;
    struct mymsgbuf
    { 
        long mtype;
        int pid;
        int num;
    } mybuf;
    if((key = ftok(pathname,0)) < 0){
        printf("Can\'t generate key\n");
        exit(-1);
    }
    if((msqid = msgget(key, 0666 | IPC_CREAT)) < 0){
        printf("Can\'t get msqid\n");
        exit(-1);
    } 

    //SERVER

    while(1){
        maxlen = sizeof(mybuf.num) + sizeof(mybuf.pid);
        if( len = msgrcv(msqid, (struct msgbuf *) &mybuf, maxlen, 1, 0) < 0){
            printf("Can\'t receive message from queue\n");
            exit(-1);
        }
        printf("Got number from process (#%d): '%d'", mybuf.pid, mybuf.num);

        mybuf.mtype=mybuf.pid;
        int answer = (mybuf.num * mybuf.num);
        printf("Calculating... %d*%d = %d\n", mybuf.num, mybuf.num, answer);
        mybuf.num = answer;
        printf("mybuf.num=%d\n", mybuf.num);
        printf("Sending answer to process (#%d).\n", mybuf.pid);
        if (msgsnd(msqid, (struct msgbuf *) &mybuf, len, 0) < 0){
            printf("Can\'t send message to queue\n");
            msgctl(msqid, IPC_RMID, (struct msqid_ds *) NULL);
            exit(-1);
        }
        printf("Sent answer to process (#%d).\n", mybuf.pid);
    }
    return 0; 
}

c1.c:

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define LAST_MESSAGE 255 
int main()
{
    int msqid; 
    char pathname[] = "server.c";
    key_t key; 
    int len, maxlen;
    struct mymsgbuf
    { 
        long mtype;
        int pid;
        int num;
    } mybuf;
    

    int c1_pid = getpid();
    mybuf.pid = c1_pid;

    if((key = ftok(pathname,0)) < 0){
        printf("Can\'t generate key\n");
        exit(-1);
    }
    if((msqid = msgget(key, 0666 | IPC_CREAT)) < 0){
        printf("Can\'t get msqid\n");
        exit(-1);
    } 

    for (int i=1; i<=5; i++)
    {
        mybuf.num = i;
        mybuf.mtype = 1;
        maxlen = sizeof(mybuf.num) + sizeof(mybuf.pid);
        if (msgsnd(msqid, (struct msgbuf *) &mybuf, maxlen, 0) < 0){
            printf("Can\'t send message to queue\n");
            msgctl(msqid, IPC_RMID, (struct msqid_ds *) NULL);
            exit(-1);
        }
        printf("Process with pid=%d sent the message to the server. number = %d \n", c1_pid, i);
        printf("num=%d pid=%d\n", mybuf.num, mybuf.pid);
        if( len = msgrcv(msqid, (struct msgbuf *) &mybuf, maxlen, c1_pid, 0) < 0){
            printf("Can\'t receive message from queue\n");
            exit(-1);
        }
        printf("Process with pid=%d recieved answer from server. Answer=%d \n", c1_pid, mybuf.num);
        sleep(0.4);
    }
    
}

server.c results:

c1.c results:

What am i doing wrong, and how should i prevent this from happening? Thanks in advance!