How to test if a target has two's complement integers with the C preprocessor? [duplicate]

Question

Sometimes it does matter if the compiler target for a C compiler has two's complement representation of integers or not, and having the preprocessor making the detection can be useful.

Since the standard requires the MAX/MIN macros from limits.h and stdint.h to be expressions that can be used in preprocessor conditionals, I think that

#include <limits.h>
#if INT_MIN + INT_MAX == -1
# define HAVE_TWOS_COMPLEMENT 1
#endif

does the trick, since one's complement and sign/magnitude architectures have symmetrical value ranges for signed integers. The question is, am I missing something here or is there a better way to make such a test in a compiler-agnostic way?

Answer 1

In two’s complement, −1 is encoded as 111...111.

In one’s complement, −1 is encoded as 111...110.

In sign-and-magnitude, −1 is encoded as 100..001.

Therefore, the following detects the encoding of the int type:

#if   (-1 & 3) == 1
    //  The encoding is sign-and-magnitude.
#elif (-1 & 3) == 2
    //  The encoding is one’s complement.
#elif (-1 & 3) == 3
    //  The encoding is two’s complement.
#else
    //  Not possible in the C standard.
#endif

The test offered in the question, INT_MIN + INT_MAX == -1, is not reliable because C 2018 6.2.6.2 2 permits “the value with sign bit 1 and all value bits zero” to be a trap representation, in which case INT_MIN is −(2^M−1), where M is the number of value bits, and INT_MAX is 2^M−1, so INT_MIN + INT_MAX is zero.