qsort in c library

qsort shouldn't use a compare function like

int cmp(const void* a, const void* b){
    return *(int*)a - *(int*)b;
}

it would easily cause a overflow, like 2000000000-(-2000000000), it would  overflow and become a negative number.

instead, we should use a compare function like

int cmp(const void* a, const void* b){
    if(*(int*)a > *(int*)b)return 1;
    else if(*(int*)a < *(int*)b)return -1;
    return 0;
}

An easy way to parallel your c code by using openmp

if your code has low dependencies between each others, you can just use openmp to parallelize your code.
In ubuntu, openmp is installed defaultly, so we don't need to install anything.
In your code, add

#include <omp.h>

in headers.
And add

#pragma omp parallel for

before your for loop.
like

And compile with flag -fopenmp.

gcc ctest.c -fopenmp .....

Then it would run like this.