C - replace substring

char *replace_str(char *str, char *orig, char *rep)
{
  static char buffer[4096];
  char *p;

  if(!(p = strstr(str, orig)))  // Is 'orig' even in 'str'?
    return str;

  strncpy(buffer, str, p-str); // Copy characters from 'str' start to 'orig' st$
  buffer[p-str] = '\0';

 sprintf(buffer+(p-str), "%s%s", rep, p+strlen(orig));

 return buffer;
}

Example:
replace_str("Hello, world!", "world", "Miami@ #")