c - purpose of comparing pointer with (unsigned) -4000L -

in open source component, cjson,

#define is_error(ptr) ((unsigned long)ptr > (unsigned long)-4000l)

above statement used check validity of pointer shown below

json_object* reply = json_object_new_object(); if (!reply || is_error(reply)) {  . . . //error handling } 

how comparing pointer (unsigned long)-4000l validates pointer?

the reason looks they're using pointer value contain "either pointer or error value".

look here:

struct json_object* json_tokener_parse(const char *str) {   struct json_tokener* tok;   struct json_object* obj;    tok = json_tokener_new();   obj = json_tokener_parse_ex(tok, str, -1);   if(tok->err != json_tokener_success)     obj = error_ptr(-tok->err); // <<<<<---   json_tokener_free(tok);   return obj; } 

the function returning special value pointer. err_ptr macro returns negative of error code, presumably because author assumes never valid pointer address.

here test demonstrates expected usage of macro, i.e. malformed json.

 new_obj = json_tokener_parse("{ foo }");   if(is_error(new_obj)) printf("got error expected\n"); 

so, reason using special value can hold "either pointer structure or error code". done union or struct, or other means, chose way.