How do I force 64 bit integer arithmetic on OS X?

Question

I am trying to force 64 bit long integers on OS X 10.5.6. running on an Apple MacBook Intel Core 2 Duo. Here is my c code:

#include<stdio.h>

int main()
{
    long a = 2147483647; /*== 2^32 - 1*/
    long aplus1;

    printf("a== %d. sizeof(a) == %d  \n", a, sizeof(a));

    aplus1 = a+1;

    printf("aplus1 = %d \n", aplus1);
}

Compiling without any switches yields the following:

$ gcc testlong.c -o testlong ;./testlong

a== 2147483647. sizeof(a) == 4  
aplus1 = -2147483648

Compiling with the -m64 switch yields:

$ gcc testlong.c -o testlong -m64; ./testlong

a== 2147483647. sizeof(a) == 8  
aplus1 = -2147483648

So the second version is apparently using 64 bit storage, but still generates the overflow error, although 2^32 should be well within the range of a 64 bit integer. Any ideas?

I would prefer a solution that can be forced from a gcc option rather than requiring me to change multiple lines of source code (my actual problem is not the above example specifically, rather I need to force long integer arithmetic in a more general situation).

Answer 1

Have you tried:

long long a = 2147483647;

Answer 2

If you are using C99, include stdint.h and use uint64_t and int64_t. Other than that, unsigned long long a = 0x100000000ull; should work too.

Answer 3

Not only do you have to use long long, but you must also change the printf() statements accordingly.

#include<stdio.h>

int main()
{
    long long a = 2147483647; /*== 2^32 - 1*/
    long long aplus1;

    printf("a== %lld. sizeof(a) == %d  \n", a, sizeof(a));

    aplus1 = a+1;

    printf("aplus1 = %lld \n", aplus1);
}

%lld is the code for long longs.

Apparently true 64-bit programs can use %d for 64-bit integers - I don't know if it's possible to configure it to compile in this mode.

Answer 4

Use the C99 standard:

#include <stdint.h>

uint64_t a = 2147483647ULL;

There's an excellent C99 library overview at Dinkumware.

Answer 5

The literal 2147483647 in C code is of type int. If you want it to be a long, it doesn't help to have a long on the left-hand side, it's still an int on the right.

Make it a long literal, by appending a 'L': 2147483647L (upper-case is recommended, lower-case 'l' works too but can be very confusing depending on the font).