c - LONG_BIT in limits.h -

i'm writing program study , therefore have descripe few wars limits of data types. when i'm writing this:

#include <limits.h> #include <stdio.h>  int main(void) {     printf("%d\n", char_bit);     printf("%d\n", long_bit);     return 0; } 

but gives me following error:

a.c: in function ‘main’: a.c:7:17: error: ‘long_bit’ undeclared (first use in function)   printf("%d\n", long_bit);                  ^ a.c:7:17: note: each undeclared identifier reported once each function appears in 

even gcc -e gives me this

int main(void) {  printf("%d\n", 8);  printf("%d\n", long_bit);  return 0; } 

but grep in limits.h doesn't give me answer. bits/xopen_lim.h has declaration , should included when

__use_xopen 

is declared, manual declaration won't give me result.

so problem? in manpage says there long_bit macro gcc says no.

gcc version 4.8.0 (gcc) os arch

[edit] long_bit not c-standard, type

man 0 limits.h 

and search long_bit. me there 2 entries under numerical limits , therefore think long_bit exist. , no, including bits/xopen_lim.h not realy option, because should included limits.h , not manually

your issue shouldn't defining __use_xopen. if take @ /usr/include/features.h see explicitly undefines , redefines these macros based on feature test macros. want define _xopen_source instead, like:

gcc -d_xopen_source=700 -o longbit longbit.c 

from features.h:

_xopen_source includes posix , xpg things. set 500 if single unix conformance wanted, 600 sixth revision, 700 seventh revision.
_xopen_source_extended xpg things , x/open unix extensions.

also, wrt directly including xopen_lim.h:

/*  * never include file directly; use <limits.h> instead.  */ 

so, wouldn't recommend directly including it. also, see man feature_test_macros.