CODEWITHSUNDEEP
c
John
John

Reputation: 111

C: long long always 64 bit?

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?

Upvotes: 8

Views: 14952

Answers (4)

Jens Gustedt
Jens Gustedt

Reputation: 78963

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.

Upvotes: 2

Lou Franco
Lou Franco

Reputation: 89232

With 

#if CHAR_BIT * sizeof (long long) != 64
   #pragma error "long long is not 64 bits"
#endif

or some equivalent.

Based on comment: if you want to support compilers where sizeof can't be used in the pre-processor, see this thread:

http://www.daniweb.com/forums/thread13553.html

Something like this:

 char longlongcheck[(sizeof(long long) * CHAR_BIT) == 64]; // won't compile if the expression is 0.

Upvotes: 0

Jerry Coffin
Jerry Coffin

Reputation: 490593

They are guaranteed to be a minimnum of 64 bits. It's theoretically possible that they could be larger (e.g., 128 bits) though I'm reasonably they're only 64 bits on anything currently available.

Upvotes: 0

Pablo Santa Cruz
Pablo Santa Cruz

Reputation: 181430

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;

Upvotes: 15

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