strToInt.c

#include <stdio.h>  // printf
#include <string.h> // strlen
#include <math.h>   // pow

int intFromAscii(char ascii) {
  switch(ascii) {
    case '1': return 1;
    case '2': return 2;
    case '3': return 3;
    case '4': return 4;
    case '5': return 5;
    case '6': return 6;
    case '7': return 7;
    case '8': return 8;
    case '9': return 9;
    default:  return 0;
  }
}

int main(int argc, const char** args) {
  if(argc <= 1) return 0;

  const char* input = args[1];
  int total = 0;
  int len = strlen(input);

  for(int i = len-1; i+1 > 0; i--) {
    int power = len - i - 1;
    total += pow(10, power) * intFromAscii(input[i]);
  }

  if(input[0] == '-') total = total * -1;

  return total;
}