C Language Implementation-defined behaviour Representation of signed integers
Example
Each signed integer type may be represented in any one of three formats; it is implementation-defined which one is used. The implementation in use for any given signed integer type at least as wide as int can be determined at runtime from the two lowest-order bits of the representation of value -1 in that type, like so:
enum { sign_magnitude = 1, ones_compl = 2, twos_compl = 3, };
#define SIGN_REP(T) ((T)-1 & (T)3)
switch (SIGN_REP(long)) {
case sign_magnitude: { /* do something */ break; }
case ones_compl: { /* do otherwise */ break; }
case twos_compl: { /* do yet else */ break; }
case 0: { _Static_assert(SIGN_REP(long), "bogus sign representation"); }
}
The same pattern applies to the representation of narrower types, but they cannot be tested by this technique because the operands of & are subject to "the usual arithmetic conversions" before the result is computed.
