IPC System V - Message Queue creation

Question

I have miss understood problem about multiple message queue creation in IPC System V.

My problem is: A main process creates NB_fils child process. Each process (include the main process) possesses a message queue. The child process i (0 <= i < NB_fils) possesses the message queue mq[i]. The main process possesses the message queue mq[NB_Fils]. The children generate a message and send to the message queue of the main process. The main process back to the child process exactly max_msg_i value.

Here is my work:

#define SVID_SOURCE 1
#define NB_FILS 4
#define MSG_SIZE 128 
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/ipc.h>
#include <sys/msg.h>
/* Message structure */
typedef struct msgbuf{
 long mtype; 
 int msg_val; 
 int mq_index; 
 } message_buf; 
int main(int argc, char **argv){
 int mq[NB_FILS + 1]; /* There are totally NB_FILS + 1 message queues */
 int i = 0;
 int j = 0; 
 int proc_index = -1; 
 char path[14] = "File_msg";
 key_t cle; 
 int max_msg_i = 0; 
 message_buf msg_send; 
 message_buf msg_rcv; 
 /* Creation of NB_FILS + 1 message queues */
 for(i = 0; i < NB_FILS + 1; i++){
 /* cle = ftok(".", i); */ <=======================================
 cle = ftok(path, i); <=======================================
 mq[i] = msgget(cle, 0666|IPC_CREAT);
 }
 /* Creation of NB_FILS child process */
 for(i = 0; i < NB_FILS; i++){
 if(fork() > 0)
 break; 
 } 
 /* The child process */
 if(i != NB_FILS){
 int somme = 0; 
 proc_index = i; 
 printf("(Pid=%d) My index is %d\n", getpid(), proc_index);
 /* Message creation */ 
 srand(getpid()); 
 max_msg_i = (int) (NB_FILS*(float)rand()/RAND_MAX);
 msg_send.msg_val = max_msg_i; 
 msg_send.mtype = 1L; 
 msg_send.mq_index = proc_index; 
 printf("(Pid=%d) I'm waiting for %d messages from the main process\n", getpid(), msg_send.msg_val);
 /* Send message to message queue associated with the main process */
 msgsnd(mq[NB_FILS], &msg_send, 2*sizeof(int) + sizeof(long), 1L); 
 /* At the owned message queue, child process waits for max_msg_i message sent from the main process */
 for(j = 0; j < max_msg_i; j++){
 msgrcv(mq[proc_index], &msg_rcv, 2*sizeof(int) + sizeof(long), 1L, 0);
 printf("(Pid=%d) I have received the message containing value %d of from the main process \n", getpid(), msg_rcv.msg_val); 
 somme += msg_rcv.msg_val; 
 }
 printf("(Pid=%d) Sum of %d values received: %d\n", getpid(), max_msg_i, somme);
 /* Drop the queue message */ 
 msgctl(mq[proc_index], IPC_RMID, 0);
 }
 /* The main process */
 else{
 srand(time(NULL)); 
 /* At the owned message queue, the main process wait for values max_msg_i (1 <= max_msg_i <= NB_FILS) sent by 
 * the child process and send back to them max_msg_i messages. */
 for(i = 0; i < NB_FILS; i++){
 msgrcv(mq[NB_FILS], &msg_rcv,2*sizeof(int) + sizeof(long), 1L, 0); 
 printf("(P)J'ai reçu le message: msg_val = %d, mq_index = %d\n", msg_rcv.msg_val, msg_rcv.mq_index); 
 /* Creat max_msg_i messages and send to process i */ 
 for(j = 0; j < msg_rcv.msg_val; j++){
 msg_send.msg_val = (int) (100*(float)rand()/RAND_MAX);
 msg_send.mtype = 1L; 
 msg_send.mq_index = msg_rcv.mq_index; 
 msgsnd(mq[msg_rcv.mq_index], &msg_send, 2*sizeof(int) + sizeof(long), 1L); 
 }
 }
 /* Drop the queue message */
 msgctl(mq[NB_FILS], IPC_RMID, 0); 
 }
 return EXIT_SUCCESS; 
 }

My problem is focus in 2 lines:

 cle = ftok(".", i); <=======================================
 /* cle = ftok(path, i); */ <=======================================

When I tried to use cle = ftok(".", i), it's worked fine. But for the other, it's always show
like:

(Pid=3813) My index is 0
(Pid=3813) I'm waiting for 1 messages from the main process
(Pid=3813) I have received the message containing value 1 of from the main process 
(Pid=3813) Sum of 1 values received: 1
(Pid=3814) My index is 1
(Pid=3814) I'm waiting for 3 messages from the main process
(Pid=3814) I have received the message containing value 3 of from the main process 
(Pid=3815) My index is 2
(Pid=3815) I'm waiting for 2 messages from the main process
(Pid=3814) I have received the message containing value 2 of from the main process 
(Pid=3816) My index is 3
(Pid=3816) I'm waiting for 0 messages from the main process
(Pid=3814) I have received the message containing value 0 of from the main process 
(Pid=3814) Sum of 3 values received: 5
(Pid=3815) I have received the message containing value -1080789062 of from the main process 
(Pid=3815) I have received the message containing value -1080789062 of from the main process 
(Pid=3815) Sum of 2 values received: 2133389172
(P)I have received: msg_val = -1080789062, mq_index = 47
(P)I have received: msg_val = -1080789062, mq_index = 47
(P)I have received: msg_val = -1080789062, mq_index = 47
(P)I have received: msg_val = -1080789062, mq_index = 47
(Pid=3816) Sum of 0 values received: 0

It like the message is not sent exactly.

Could any one can tell me what is the problem.

Thanks very much.

• whoan: You may start checking the return value of ftok to see whether has failed. Try giving the full path of File_msg too.

Answer 1

You need to specify an existing file for the first parameter when calling ftok(). As "." points to the local directly (which exists), this is valid in your example. But I doubt that a file named "File_msg" exists in the current directory, therefore you get an error. Use a relative or absolute path to an existing file instead.