RosettaCodeData/Task/URL-decoding/C/url-decoding.c

#include <stdio.h>
#include <string.h>

inline int ishex(int x)
{
  return  (x >= '0' && x <= '9')  ||
    (x >= 'a' && x <= 'f')  ||
    (x >= 'A' && x <= 'F');
}

int decode(const char *s, char *dec)
{
  char *o;
  const char *end = s + strlen(s);
  int c;

  for (o = dec; s <= end; o++) {
    c = *s++;
    if (c == '+') c = ' ';
    else if (c == '%' && (  !ishex(*s++)  ||
          !ishex(*s++)  ||
          !sscanf(s - 2, "%2x", &c)))
      return -1;

    if (dec) *o = c;
  }

  return o - dec;
}

int main()
{
  const char *url = "http%3A%2F%2ffoo+bar%2fabcd";
  char out[strlen(url) + 1];

  printf("length: %d\n", decode(url, 0));
  puts(decode(url, out) < 0 ? "bad string" : out);

  return 0;
}