Why can I not get system V message queues working with fork()?

Question

I have a program here:

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

#include "linkedlist.h"
#include "globals.h"
#include "card.h"
#include "logging.h"
#include "stack.h"
#include "servers.h"
#include "player.h"

#define VERSION "0.1"

#define ERR -1

/*
 * Main function runs, loads 5 named table objects. For each table
 * created a fork is created for that specific table object that
 * allows each table to run independently.
 *
 * Usernames details are also created and prepared. These are hardcoded
 * until I add database logic later.
 *
 * After the tables are prepared, the main function starts a server
 * listening thread that waits for UNIX sockets to connect to it. When
 * they arrive it spawns a handling thread that does a quick login check
 * and then asks the user which of the named tables they want to play on
 *
 * The socket information is then connected to the table, so the table can
 * then communicate with user. When the table is full, the table algorithm
 * is responcible for basically looping contineously, creating new "games"
 * until only one player is left. He is the winner. When he wins, the table
 * exits.
 *
 * the game loop does all the real work for the texas holdem game, hosting
 * rounds of bets, dealing cards etc.
 */

void table_process(int tables_id);
void main_game_loop(void);

int main(int argc, char **argv)
{
  int main_process_id;
  int connection_process_id;

  char table_config_name[8];
  char *table_name;
  stack *my_deck;
  card *current_card;
  int i;
  int s;

  printf("------------ PRT POKER -----------\nA texas holdem poker server\n~~~~Version: %s~~~~~~~\n", VERSION);

  config_load("poker.conf");
  logging_init();

  if (key = ftok("/mnt/mydocuments/git/texas_holdem/poker", 'a') == -1)
  {
    logging_critical("Call to ftok failed");
    exit(1);
  }

//  if ((msg_queue = msgget(key, 0644 | IPC_CREAT)) == -1)
//  {
//    logging_critical("call to msgget failed");
//    exit(1);
//  }

  //read int the list of tables from the configuration file

  config_get_int("players_per_table", &player_count);
  logging_info("players per table: %i", *player_count);

  config_get_int("table_count", &table_count);
  logging_info("number of tables: %i", *table_count);

  table_names = malloc(sizeof(char*) * (*table_count));

  logging_info("Table count: %d", *table_count);

  for (i = 0; i < *table_count; i++)
  {
    sprintf(table_config_name, "table_%d", (i + 1));  
    config_get_string(table_config_name, &table_name);

    table_names[i] = table_name;

    logging_info("Created table %d: %s", i, table_name);

    if (fork() == 0)
    {
      table_process(i);
      _exit(0);
    }
  }

  main_process_id = getpid();

  connection_process_id = fork();
  logging_info("connecton process: %d", connection_process_id);

  if (connection_process_id == 0)
  {
    wait_for_players();
    _exit(0);
  }
  else
    waitpid(connection_process_id, &s, 0);

  logging_info("Application ended");
  return 0;
}

void table_process(int table_id)
{
  linkedlist *players;
  player *p;
  int players_added;

  if ((msg_queue = msgget(key, 0644 | IPC_CREAT)) == -1)
  {
    logging_critical("call to msgget failed");
    exit(1);
  }

  logging_info("table %s (%i) running in process %d", table_names[table_id], table_id, getpid());

  players = linkedlist_new();

  players_added = *player_count;
  while (players_added > 0)
  {
    logging_info("still waiting for %i players to join %s with queue id %i", players_added, table_names[table_id], table_id + MSG_QUEUE_OFFSET);


    //THIS DOESN'T WORK!!!! IT DOESN'T WAKE UP
    if (msgrcv(msg_queue, &p, sizeof(player), table_id + MSG_QUEUE_OFFSET, 0) == -1)
    {
      logging_critical("recieving from message queue failed");
      _exit(1);
    }
    else
      logging_info("played joined %s", table_names[table_id]);

    logging_debug("player = %s, %s", p->name, p->password);

    players_added--;
  }

  logging_info("table %s ready to start!!!!!", table_names[table_id]);
}

void main_game_loop(void)
{
  logging_debug("main_game_loop");
}

The program is supposed to start and fork into 4 threads. The first three threads call msgget() to create a system V message queue.

Then they all call msgrcv() and wait for a different id to occur.

The fourth thread listens for connecting clients and then invokes msgsnd(). For some reason, none of the forks waiting for a message awake again.

1. I'm sure the message queue is correct.

2. I have a print statement before msgsrcv so I can see that see that the fork happens correctly for each statement. There is also a print statement afterwards so I can see the code doesn't pass it.

3. I have run ipcs from the command prompt.

4. There is definitely a message queue created and definetly has a message in it.

5. If I change the msgrcv to say "0" then one of the threads will wake up but that's not the behaviour I want.

6. msgget() doesn't throw an error.

7. msgsnd() didn't throw an error.

8. I'm positive the id in the structure is correct.

Any ideas what I am doing wrong?

Answer 1

There is at least one thing wrong: you have declared "p" as pointer to player, and you're again taking its address when passing it to msgrcv. What's more, you pass a size that is most probably too large for the pointer (i.e., sizeof(player) > sizeof(player*)). Stack corruption waiting to happen.

Ohterwise, try using strace (on Linux; truss on Solaris) to see what's happening in real-time.