I am trying to understand how fork in c work. The problem that I am trying to solve is; given f(upper), I am trying to find f(1) + f(2) + .. f(upper). I wanted to do multi process programming to have fork each child process and have each child process calculate f(x).
So f(1) , f(2) ... f(upper) is calculated by each child process.
The parent process should calculate following f(1) + .. + f(upper). Here is my code
#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
#include<sys/shm.h>
#include<sys/ipc.h>
int upper = 0;
int n = 0;
int main(int argc, char*argv[]){
pid_t pid;
if(argc != 2){
printf("Input one argument");
return -1;
}
upper = atoi(argv[1]);
int segment_id;
int *s;
pid_t *pids;
pids = (pid_t *) malloc(sizeof(int) * upper);
s = (int *) malloc(sizeof(int) * upper);
key_t key = 4141;
if((segment_id = shmget(key, upper * sizeof(int), IPC_CREAT | 0667))< 0) perror("shmget: failure");
if((s = shmat(segment_id, NULL, 0)) == (char *) -1){
perror("shmat : failure");
exit(1);
}
for(int i = 1; i <= upper; i++){
pid = fork();
if(pid == 0) {
n = i;
break;
}
pids[i] = pid;
}
if(pid > 0){
wait(1 * upper);
int totalSum;
for(int i = 0; i < upper; i++){
totalSum += s[i];
}
printf("Total sum = %d", totalSum);
} else {
sleep(2);
int sum = 0;
for(int i = 0; i <= n; i++){
sum += i;
}
s[n - 1] = sum;
printf("n => %d : sum %d\n", n, sum);
}
}
However whenever I try to run this program with argument more than 6. I get Invalid argument error.
You are writing outside of the bounds of pids
pids = (pid_t *) malloc(sizeof(int) * upper);
...
for(int i = 1; i <= upper; i++){
pid = fork();
if(pid == 0) {
n = i;
break;
}
pids[i] = pid; /* Here */
}
Change to
for(int i = 1; i < upper; i++){
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