Is there a portable way to define INT_MAX?

Question

I found the following definitions in /usr/include/limits.h:

# define INT_MIN (-INT_MAX - 1)

# define INT_MAX 2147483647

Also, it seems that all XXX_MAX's in this header file are explicitly defined from a numerical constant.

I wonder if there is a portable way (against different word sizes across platforms) to define a INT_MAX ?

I tried the following:

~((int)-1)

But this seems incorrect.

A short explanation is also highly regarded.

Answer 1

For the INT_MAX in the standard header limits.h, the implementor's hands are tied by the fact that it's required to be usable in preprocessor #if directives. This rules out anything involving sizeof or casts.

If you just want a version that works in actual C expressions, perhaps this would work:

(int)-1U/2 == (int)(-1U/2) ? (int)-1U : (int)(-1U/2)

The concept here is that int may have the same number of value bits as unsigned, or one fewer value bit; the C standard allows either. In order to test which it is, check the result of the conversion (int)-1U. If -1U fits in int, its value must be unchanged by the cast, so the equality will be true. If -1U does not fit in int, then the cast results in an implementation-defined result of type int. No matter what the value is, though, the equality will be false merely by the range of possible values.

Note that, technically, the conversion to int could result in an implementation-defined signal being raised, rather than an implementation-defined value being obtained, but this is not going to happen when you're dealing with a constant expression which will be evaluated at compile-time.

Answer 2

I like the defintions:

#define INT_MIN (1 << (sizeof(int)*CHAR_BIT-1))
#define INT_MAX (-(INT_MIN+1))