C: long long always 64 bit?

Question

If i'm using long longs in my code, can i absolutely 100% guarantee that they will have 64 bits no matter what machine the code is run on?

Answer 1

No, C99 standard says that it will have at least 64 bits. So it could be more than that at some point I guess. You could use int64_t type if you need 64bits always assuming you have stdint.h available (standard in C99).

#include <stdint.h>
int64_t your_i64;

Answer 2

You can test if your compiler is C99 complying with respect to numbers in the preprocessor with this

# if (~0U < 18446744073709551615U)
#  error "this should be a large positive value, at least ULLONG_MAX >= 2^{64} - 1"
# endif

This works since all unsigned values (in the preprocessor) are required to be the same type as uintmax_t and so 0U is of type uintmax_t and ~0U, 0U-1 and -1U all are the maximum representable number.

If this test works, chances are high that unsigned long long is in fact uintmax_t.

For a valid expression after the preprocessing phase to test this with the real types do

unsigned long long has_ullong_max[-1 + 2*((0ULL - 1) >= 18446744073709551615ULL)];

This does the same sort of trick but uses the postfix ULL to be sure to have constants of type unsigned long long.