What do i64 and i32 at the end of the values in limits.h mean?

Question

I was looking at limits.h on windows and found this:

#define LLONG_MAX     9223372036854775807i64       // maximum signed long long int value
#define LLONG_MIN   (-9223372036854775807i64 - 1)  // minimum signed long long int value
#define ULLONG_MAX    0xffffffffffffffffui64       // maximum unsigned long long int value

What does the i64 at the end of the value mean?

Answer 1

The i64 suffix is a Microsoft extension to specify 64-bit integer constants.

A portable alternative is (int64_t)9223372036854775807, but older versions of Microsoft C did not support C99 <stdint.h> types.

You can also use the standard suffix LL to specify a constant of of type long long, which has at least 63 value bits, but may have more on some platforms. The case of L is not significant, so 1ll is equivalent to 1LL, but significantly more confusing because l looks a lot like 1, especially with some fixed type fonts.

Note that 9223372036854775807 as an integer constant has the smallest type with enough range to express it in the list int, long int, long long int. Given the common sizes for these types on Microsoft platforms, the type long long int is probably the only one with 64-bits.

The suffix is more useful for smaller constants such as 1 as show below:

uint64_t x = 1 << 32;     // undefined behavior
uint64_t x = 1ULL << 32;  // fully defined, x is 0x800000000
uint64_t x = 1ui64 << 32; // Microsoft specific, non portable