Message queue is giving me an invalid argument - c

Question

I'm having some trouble with my code. It should create a message queue and send a message, than wait some time for another program to receive that message and answer. The problem is, when I run it, I get an invalid argument both on the msgsnd and on the msgrcv.
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/msg.h>
typedef struct my_msg{
long type;
char text[100];
char sqdr;
}message;
static void score(int messagge_id, char* A_B){
message send;
send.type=1;
strcpy(send.text, "Try to score");
send.sqdr = *A_B;
if((msgsnd(messagge_id, &send, sizeof(send), 0))<0)perror("Error msgsnd\n");
sleep(3);
if((msgrcv(messagge_id, &send, sizeof(send), 4, 0))==-1)perror("Error msgrcv 1\n");
int test=atoi(send.text);
printf("%d\n", test);
}
int main(){
int caso, key;
char team= 'A';
key=1234;
int msg_id=msgget(key, S_IRUSR|S_IWUSR);
printf("Try function score\n");
score(msg_id, &team);
printf("After score\n");
return 0;
}

Answer 1

You need to ensure that the message queue is created. You either use the key IPC_PRIVATE or you add IPC_CREAT to the flags. You also need to try to read the message correctly. You sent a 'type 1' message and attempted to read a 'type 4' message, so the read hangs.
This code also removes the message queue. That's not critical if it is a private queue (such queues are deleted when the program terminates), but it is important for queues using IPC_CREAT and a user-defined key. (I also changed the message text so that atoi() returned something more interesting — and convincing — than zero. The code also uses separate send and receive buffers so that we know the code is not cheating and reusing data already in the buffer.)
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <sys/msg.h>
#include <fcntl.h>
typedef struct my_msg
{
long type;
char text[100];
char sqdr;
} message;
static void score(int messagge_id)
{
message send;
message recv;
send.type = 1;
strcpy(send.text, "47 tries to score");
send.sqdr = 'A';
if ((msgsnd(messagge_id, &send, sizeof(send), 0)) < 0)
perror("Error msgsnd");
printf("Dozing...\n");
sleep(3);
printf("Unslumbering...\n");
if ((msgrcv(messagge_id, &recv, sizeof(recv), -4, 0)) == -1)
perror("Error msgrcv");
int test = atoi(recv.text);
printf("%d\n", test);
}
int main(void)
{
int key = 1234;
int flags = S_IRUSR|S_IWUSR|IPC_CREAT;
// int key = IPC_PRIVATE;
// int flags = S_IRUSR|S_IWUSR;
int msg_id = msgget(key, flags);
if (msg_id < 0)
perror("Error msgget");
else
{
printf("Try function score\n");
score(msg_id);
printf("After score\n");
if (msgctl(msg_id, IPC_RMID, 0) < 0)
perror("Error msgctl");
}
return 0;
}
Sample output:
Try function score
Dozing...
Unslumbering...
47
After score
There's a 3 second pause between 'Dozing' and 'Unslumbering', of course.

Answer 2

use like this:-
if((msgsnd(messagge_id, (void *)&send, sizeof(send), 0))<0)perror("Error msgsnd\n");
if((msgrcv(messagge_id, (void *)&send, sizeof(send), 4, 0))==-1)perror("Error msgrcv 1\n");