Question

I have been working on semaphores and shared memory for a week now, and have some difficulties yet,so i tried to make this program which the children are supposed to write to a memory shared multidimensional integer array and the father is suppose to read that array from the memory that is shared.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ipc.h> 
#include <sys/shm.h>
#include <stdio.h>
#include <sys/fcntl.h>
#include <semaphore.h>
#include <sys/wait.h>

#define MAXCHILDS 1
#define MAX_SIZE 10
#define MAX_WRITES 100

typedef struct{
    int m[MAX_SIZE][MAX_SIZE];
}matrix;


/*fork variables*/
pid_t child[MAXCHILDS];
/*semphores variables */
sem_t *empty, *full, * mutex;
/*share memory id*/
int shmid;
/*shared memory array pointer */
matrix * sh_mem;




void init(){

     /*semaphores unlink and creation */    
     sem_unlink("EMPTY");
     empty=sem_open("EMPTY",O_CREAT|O_EXCL,0700,50);
     sem_unlink("FULL");
     full=sem_open("FULL",O_CREAT|O_EXCL,0700,0);
     sem_unlink("MUTEX");
     mutex=sem_open("MUTEX",O_CREAT|O_EXCL,0700,1);
    /*initialize shared memory */
    shmid = shmget(IPC_PRIVATE,sizeof(matrix),IPC_CREAT|0777);
    /*map shared memory*/
    sh_mem = (matrix*)shmat(shmid,NULL,0);
    if(sh_mem == (matrix*)(-1)){
        perror("shmat");
    }
}

void writer(int m[MAX_SIZE][MAX_SIZE],int n_child){
    int i,k;
    for(i = 0;i<MAX_SIZE;i++){
        for(k= 0;k<MAX_SIZE;k++){
            m[i][k] = 0;
            if(i==(k+1)){
                m[i][k] = 1;
            }
        }
    }

}
void reader(int m[MAX_SIZE][MAX_SIZE]){
    int i = 0;
    int k = 0;
    int sum = 0;
    for(i = 0;i<MAX_SIZE;i++){
        for(k= 0;k<MAX_SIZE;k++){
            printf("%d",m[k][i]);
        }
        sum++;
        printf("[i=]%d[k=]%d\n",i,k);
    }
    printf("%d",sum);

}

void terminate() {
  sem_close(empty);
  sem_close(full);
  sem_close(mutex);
  sem_unlink("EMPTY");
  sem_unlink("FULL");
  sem_unlink("MUTEX");
  shmctl(shmid, IPC_RMID, NULL);
}

int main(int argc, char **argv)
{
    int i,sum;
    init();

    for(i = 0;i<MAXCHILDS;i++){
        if((child[i]= fork()) < 0) // error occured
        {
            perror("Fork Failed");
            exit(1);
        }
        if((child[i] =fork())==0){
            writer(sh_mem->m,i);
            exit(0);
        }
    }

    /*father*/
    sleep(10);
    sum++;
    printf("%d\n",sum); 
    reader(sh_mem->m);
    wait(NULL);

    terminate();


    return 0;
}

I have two problems right now … I need to save the array to a memory mapped file which i dont seem to understand and the output of the father is throwing is very strange …His output is

0000100000[i=]3[k=]10
0000010000[i=]4[k=]10
0000001000[i=]5[k=]10
0100000000[i=]0[k=]10
0000000100[i=]6[k=]10
0010000000[i=]1[k=]10
0000000010[i=]7[k=]10
0001000000[i=]2[k=]10
0000000001[i=]8[k=]10
0000100000[i=]3[k=]10
0000000000[i=]9[k=]10
0000010000[i=]4[k=]10
0000001000[i=]5[k=]10
0000000100[i=]6[k=]10
0000000010[i=]7[k=]10
0000000001[i=]8[k=]10
0000000000[i=]9[k=]10

And it should be something like this:

0000100000[i=]0[k=]10
0000010000[i=]1[k=]10
0000001000[i=]2[k=]10
0100000000[i=]3[k=]10
0000000100[i=]4[k=]10
0010000000[i=]5[k=]10
0000000010[i=]6[k=]10
0001000000[i=]7[k=]10
0000000001[i=]8[k=]10
0000100000[i=]9[k=]10

Answer 1

The code calls fork() in two places. Remove the second call.

Change:

    if((child[i] =fork())==0){

to:

    if(child[i] == 0){