Segmentation fault without altering my program - c

Question

I was trying to write a program that calculates prime numbers using threads and the program was working but not giving the desired results (it was telling all numbers were prime numbers). Today I tried to run the program again and I'm getting a segmentation fault even though I didn't alter my program. I tried using gdb to find when it was happening and I think it's happening on the pthread_join function.
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
int n_threads; // number of threads
int* numbers; // array of numbers
int elements_per_thread;
int isPrime(int n){
int i = 0;
for(i=0; i < n; i++)
if(n%i == 0)
return 0;
return 1;
}
void * threadPrime(void * arg){
int* idxp = (int*)arg;
int idx = *idxp;
int start = idx* elements_per_thread;
int finish = (idx+1)*elements_per_thread-1;
int i;
for(i=start; i<finish; i++)
if(!isPrime(i))
numbers[i]=0;
pthread_exit(NULL);
}
int main(int argc, char* argv[]) {
if (argc != 3) {
printf("usage: %s largest_number number_threads\n", argv[0]);
return 1;
}
int largest_number = atoi(argv[1]); // value of the largest number to test for primality
int n_numbers = largest_number-1; // number of numbers to test
n_threads = atoi(argv[2]);
// create and fill vector of numbers to test
numbers = (int *)malloc(n_numbers*sizeof(int)) ; // allocate a vector for n_numbers integers
int i;
for (i = 2; i <= largest_number; i++)
numbers[i-2] = i;
int* id = (int *)malloc(n_threads*sizeof(int));
// compute primes
pthread_t* thid = (pthread_t *)malloc(n_threads*sizeof(int));
for(i=0;i<n_threads;i++){
id[i] = i;
if(pthread_create(&thid[i],NULL,threadPrime,(void*)(id+i)) != 0){
printf("Erro\n");
exit(0);
}
}
for(i=0;i<n_threads;i++){
if(pthread_join(thid[i],NULL) != 0){
printf("Erro\n");
exit(0);
}
}
// print result
printf("Primes:");
int n_primes = 0;
for (i = 0; i < n_numbers; i++)
if (numbers[i]) {
n_primes++;
printf (" %d", numbers[i]);
}
printf("\nTotal: %d primes\n", n_primes);
return 0;
}
Problem solved. Correct code below.
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
int n_threads; // number of threads
int* numbers; // array of numbers
int elements_per_thread;
int isPrime(int n){
int i = 0;
for(i=2; i < n; i++)
if(n%i == 0)
return 0;
return 1;
}
void * threadPrime(void * arg){
int* idxp = (int*) arg;
int idx = *idxp;
int start = idx*elements_per_thread;
int finish = (idx+1)*elements_per_thread;
int i;
for(i=start; i<=finish; i++)
if(!isPrime(numbers[i]))
numbers[i]=0;
pthread_exit(NULL);
}
int main(int argc, char* argv[]) {
if (argc != 3) {
printf("usage: %s largest_number number_threads\n", argv[0]);
return 1;
}
int largest_number = atoi(argv[1]); // value of the largest number to test for primality
int n_numbers = largest_number-1; // number of numbers to test
n_threads = atoi(argv[2]);
// create and fill vector of numbers to test
numbers = (int *)malloc(n_numbers*sizeof(int)) ; // allocate a vector for n_numbers integers
int i;
for (i = 2; i <= largest_number; i++)
numbers[i-2] = i;
int* id;
id = (int *)malloc(n_threads*sizeof(int));
// compute primeselements_per_thread = n_numbers/n_threads;
elements_per_thread = (n_numbers/n_threads)+1;
pthread_t* thid = malloc(n_threads*sizeof(*thid));
for(i=0;i<n_threads;i++){
id[i] = i;
if(pthread_create(&thid[i],NULL,threadPrime,(void*)(id+i)) != 0){
printf("Erro\n");
exit(0);
}
}
for(i=0;i<n_threads;i++){
if(pthread_join(thid[i],NULL) != 0){
printf("Erro\n");
exit(0);
}
}
// print result
printf("Primes:");
int n_primes = 0;
for (i = 0; i < n_numbers; i++)
if (numbers[i]) {
n_primes++;
printf (" %d", numbers[i]);
}
printf("\nTotal: %d primes\n", n_primes);
return 0;
}

Answer 1

There are a number of issues:
1) The starting index for the loop should be 2. Otherwise, you are going to have divide by zero error here:
for(i=0; i < n; i++) // should be i=2
if(n%i == 0)
return 0;
2) elements_per_thread is not set at all. So it's going to be 0 (since it's a global variable) and the loops in thread function will never be called. Set it in main():
elements_per_thread = n_numbers/n_threads;
3) When you call isPrime() you are passing i. But you really wanted to pass numbers[i]. You also want to include the finish in the primality test. So it should be
for(i=start; i<=finish; i++)
if(!isPrime(numbers[i]))
numbers[i]=0;
4) The allocation for thread array is wrong. It should be
pthread_t* thid = malloc(n_threads * sizeof *thid);
There are more efficient ways to test primes (e.g. you only need to check upto n/2 to see if it's prime). But once you fix the above issues, you'll have a working code and think about improving it later.

Answer 2

You didn't allocate the right amount of memory for thid. This is the main reason for your segmentation fault.
pthread_t* thid = (pthread_t *)malloc(n_threads*sizeof(int));
should be
pthread_t* thid = malloc(n_threads*sizeof(p_thread));
(you don't need to cast malloc in C)
This is why I don't usually use an explicit type as the operand of sizeof, and instead just use the variable name so that the compiler can deduce the type itself.
pthread_t* thid = malloc(n_threads*sizeof(*thid));