并行程序中的GCC分段错误

Question

This a simple parallel program in c.

I am using ubuntu and gcc for compiling.

The program takes input for number of processes, and creates and ask for same number of numbers from the user. Then each process is used to calculate the factorial of each number.

#include<stdlib.h>
#include<stdio.h>
#include<sys/types.h>

int factorial(int n)
{
    int fact=1;
    int i;
    for(i=1 ; i<=n ; i++)
    {
        fact=fact*i;
    }
    return fact;
}

int main()
{
    int process_id;
    int num_process;
    int pid;
    int i;

    printf("\nEnter No. of Process : ");
    scanf("%d",&num_process);

    int num[num_process];

    for ( i=1; i < num_process; i++ )
    {
        printf("\nEnter %d Number : ",i);
        scanf("%d", &num[i]);
    }

    for ( i=0; i < num_process; i++ )
    {
        pid=fork();
        if (pid == -1)
        {
            perror("Error forking");
            return -1;
        }
        else if(pid > 0)
        {
            waitpid(-1, NULL, 0);
        }
        else
        {
            printf("\nFactorial of %d is : %d", num[i], factorial(num[i]));
            exit(0);
        }

    }
    return 0;
}

Never heard of segmentation fault, can someone explain it what does it mean ?

Answer 1

This:

for ( i=1; i <= num_process; i++ )
{
    printf("\nEnter %d Number : ",i);
    scanf("%d", num[num_process]);
}

is problematic. The valid indices for num are 0 to num_process - 1. Change the loop to:

for ( i=0; i < num_process; i++ )

Answer 2

In your factorial function, fact is not initialized. Also

 scanf("%d", num[num_process]);

should be

 scanf("%d", &num[num_process]);

Answer 3

A description of segmentation fault can be read here .

A fault is here:

   scanf("%d", num[num_process]);

Because you are counting from 1 - Arrays start at zero

This line for ( i=0; i <= num_process; i++ ) will give you too many processes.

ALSO fork creates another process - so therefore the program is not parallel - you need to use threads. Please google it.