char *curl_easy_unescape( CURL *curl, const char *url , int inlength, int *outlength );
If the length argument is set to 0 (zero), curl_easy_unescape(3) will use strlen() on the input url string to find out the size.
If outlength is non-NULL, the function will write the length of the returned string in the integer it points to. This allows an escaped string containing %00 to still get used properly after unescaping. Since this is a pointer to an int type, it can only return a value up to INT_MAX so no longer string can be unescaped if the string length is returned in this parameter.
You must curl_free(3) the returned string when you're done with it.