#include <stdlib.h>
#include <string.h>

size_t replace(const char *src, char *dst, const char *pattern, const char *subs)
{
  const char *tail, *head;
  int hits = 0;
  size_t pattern_len, subs_len;

  if (!dst) {
    head = src;
    pattern_len = strlen(pattern);
    while(head = strstr(head, pattern)) {
      hits++;
      head += pattern_len;
    }
    return (hits ? strlen(src) + hits * (strlen(subs) - pattern_len) : 0);
  }

  tail = head = src;
  pattern_len = strlen(pattern);
  subs_len = strlen(subs);
  while (head = strstr(head, pattern)) {
    strncpy(dst, tail, head - tail);
    dst = dst + (head - tail);
    strcpy(dst, subs);
    dst = dst + subs_len;
    head += pattern_len;
    tail = head;
  }
  strcpy(dst, tail);
  return 0;
}

void main()
{
  const char *s = "one two three foo two five two";
  const char *p = "two", *subs = "zu";
  char *d = NULL;
  int d_l = 0;
  d_l = replace(s, d, p, subs);
  d = (char*)malloc(sizeof(char) * d_l + 1);
  replace(s, d, p, subs);
}