C - Integer constant out of range warning

C doesn't define sizes exactly. C types such as int and long are defined as being "at least 16 bits'' and "at least 32 bits'' respectively.

If we want to define variables which have a specific size independent of platform, then we can use types such as int32_t, int64_t defined in sys/types.h header file.

So we would assume that the following code should work:

#include <stdio.h>
#include <sys/types.h>

int64_t num;
num = 0xabcdef1234567890;
printf("num = %llx\n", num);


Intended O/P: abcdef1234567890

BUT on some platform/compiler combination the output can come out to be 34567890.

Such as on IBM/AIX, compiler will give a warning

1506-207 (W) Integer constant <value> out of range.

Whereas GCC/LINUX will print the intended output.

The reason is that the integer literal 0xabcdef1234567890 wasn't explicitly specified to be of type int64_t.

A solution to this is to specify literal as 0xabcdef1234567890LL but a better way of doing this is to use macros in stdint.h (included in inttypes.h)

#include <stdio.h>
#include <inttypes.h>

int64_t num;
num = INT64_C(0xabcdef1234567890);
printf("num = %" PRIx64 "\n", num);

INT64_C will itself append "LL" or appropriate size specifier to literal depending on platform.