URL decode in C with only stdlib as dependency, written for use on an embedded device.

url_decode.c

#include <stdio.h>
#include <stdlib.h>

void urldecode(char* pch_url) {
  int i = 0;
  char* ptr = pch_url;

  while (*ptr != '\0') {
    if (*ptr != '%') {
      pch_url[i++] = *(ptr++);
    } else {
      char val[3] = {0,0,0};

      if ((val[0] = *(ptr+1)) != '\0' &&
          (val[1] = *(ptr+2)) != '\0') {
        char* end = NULL;
        long int x = strtol(val, &end, 16);
        if (end != val+2) {
          // invalid hex value
          break;
        }
        pch_url[i++] = x & 0xFF;
        ptr += 3;
      } else {
        // malformed
        break;
      }
    }
  }
  pch_url[i] = '\0';
}

char str1[] = "kk%40kk.com%0a%7e";

int main(int argc, char* argv) {
  printf("before %s\n", str1);
  urldecode(str1);
  printf("after %s\n", str1);
}