IPC using msgsnd and msgrcv

Question

For learning, I wrote 2 IPC programs, a server and a client that use message sending. For some reason, the server does not seem to be receiving the data it has been send. Could you give me some hints on how I could go about debugging such a problem?

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

#define CENTRAL_MAILBOX 1200
#define CLIENT_MAILBOX 1201

struct {
long priority;
int value;
int pid;
} msgp, cmbox;

int main(int argc, char **argv) {

int uid = 0; // Process id of the server(this process) set to 0
int length = -1; // Length of the structure we are going to send over
int result = -1; // Result of the msgrcv operation
int status = -1;

if ( argc != 2 ) {
    fprintf(stderr, "Usage: ./server <temperature>\n");
    exit(-1);
}

printf("[+] Starting server\n");

printf("[+] Creating the mailbox\n");
int server_msgid = msgget(CENTRAL_MAILBOX, 0600 | IPC_CREAT);

printf("[+] Creating a mailbox for the client process\n");
int client_msgid = msgget(CLIENT_MAILBOX, 0600 | IPC_CREAT);

printf("[+] Initializing data to be sent across\n");
msgp.priority = 1;
msgp.value = 31337;
msgp.pid = uid;

length = ( sizeof(msgp) > sizeof(long) ? sizeof(msgp)-sizeof(long) : sizeof(long)-sizeof(msgp) );
printf("[+] Calculating the size of the message we are about to send across=%d\n", length);

// msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);
result = msgrcv(server_msgid, &cmbox, length, 1, 0);
printf("Result = %d\n", result);
printf("[+] Received message pid=%d, value=%d, priority=%ld\n", cmbox.pid, cmbox.value, cmbox.priority);

printf("[+] Sending message\n");
// int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
result = msgsnd(client_msgid, &msgp, length, 0);

printf("[+] Shutting down server\n");
status = msgctl(server_msgid, IPC_RMID, 0);

if ( status != 0 ) {
fprintf(stderr, "[*] ERROR: closing mailbox failed\n");
}

}

My Client :-

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

#define CENTRAL_MAILBOX 1200
#define CLIENT_MAILBOX 1201

struct {
long priority;
int value;
int pid;
} msgp, cmbox;

int main(int argc, char **argv) {

int temp = atoi(argv[1]);
int uid = getpid(); //7171;
int length, result, status;

if ( argc != 2 ) {
// TODO find actual process id
fprintf(stderr, "Usage: ./client <temperature>\n");
exit(-1);
}

printf("[+] Creating server mailbox\n");
int server_msgid = msgget(CENTRAL_MAILBOX, 0600 | IPC_CREAT);

printf("[+] Creating client mailbox\n");
int client_msgid = msgget(CLIENT_MAILBOX, 0600 | IPC_CREAT);

printf("[+] Initializing the data to be sent across\n");
cmbox.priority = 2;
cmbox.value = 1337;
cmbox.pid = uid;

length = ( sizeof(msgp) > sizeof(long) ? sizeof(msgp)-sizeof(long) : sizeof(long)-sizeof(msgp) );
printf("[+] Calculating the size of the message we are about to send across=%d\n", length);

printf("[+] Sending message to server\n");
result = msgsnd(server_msgid, &cmbox, length, 0);
printf("Result = %d\n", result);

result = msgrcv(client_msgid, &msgp, length, 1, 0);
printf("[+] Received message pid=%d, value=%d, priority=%ld\n", msgp.pid, msgp.value, msgp.priority);

printf("[+] Removing the mailbox\n");
status = msgctl(client_msgid, IPC_RMID, 0);

if ( status != 0 ) {
printf("Error when closing mailbox\n");
}

}

Check the return values of your msg*() function calls. Also, is there some reason you are using Sys V message queues rather than POSIX message queues? — Markku K., May 28 '13 at 14:26
No particular reason no. I'll try POSIX next. the return value of msgsnd is 0 — , May 28 '13 at 17:17
What about the return value of msgrcv? I asked about POSIX message queues because you are more likely to find helpful resources about those than the older kind. — Markku K., May 28 '13 at 17:29
msgrcv does not return. The call is blocked as it does not seem to be receiving any input — , May 29 '13 at 01:52

score 1 · Answer 1 · edited Jun 20 '20 at 09:12

Your server blocks on the 1st msgrcv() because it expects message of type 1 (what you called priority, in your msgp, cmbox structures):

// msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);
result = msgrcv(server_msgid, &cmbox, length, 1, 0);

When the client send a message of type 2:

printf("[+] Initializing the data to be sent across\n");
cmbox.priority = 2;

// [...]

printf("[+] Sending message to server\n");
result = msgsnd(server_msgid, &cmbox, length, 0);

Have a look at how to use the msgtyp value in the man page:

int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);
[...]

The msgp argument is a pointer to caller-defined structure of the following general form:
struct msgbuf {
    long mtype;       /* message type, must be > 0 */
    char mtext[1];    /* message data */
};
[...]

msgrcv()

[...]

The argument msgtyp specifies the type of message requested as follows:

If msgtyp is 0, then the first message in the queue is read.

If msgtyp is greater than 0, then the first message in the queue of type msgtyp is read, unless MSG_EXCEPT was specified in msgflg, in which case the first message in the queue of type not equal to msgtyp will be read.

If msgtyp is less than 0, then the first message in the queue with the lowest type less than or equal to the absolute value of msgtyp will be read.

IPC using msgsnd and msgrcv

1 Answers1