From: David Laight > Sent: 06 December 2024 02:26 (now it is no longer 2am...) Linus suggested this a while back: > in https://lore.kernel.org/all/CAHk-=wiq=GUNWJwWh1CRAYchW73UmOaSkaCovLatfDKeveZctA@xxxxxxxxxxxxxx/ > > /* > * iff 'x' is a non-zero constant integer expression, > * then '!(x)' will be a zero constant integer expression, > * and casting that to 'void *' will result in a NULL pointer. > * Otherwise casting it to 'void *' will be just a regular 'void *'. > * > * The type of '0 ? NULL : (char *)' is 'char *' > * The type of '0 ? (void *) : (char *) is 'void *' > */ > #define const_true(x) \ > _Generic(0 ? (void *)((long)!(x)) : (char *)0, char *: 1, void *: 0) But some of the things built on it has issues with compiler warnings. I think there have several variations before and since with subtle differences. Probably const_zero() const_true() const_false() and const_expr(). While the 'base' define is really const_zero() with just (long)(x) that will mask high bits off 'long long'. A const_false() could 'fix' that using (long)!!(x) but even !(x) starts giving compile errors. If called for pointers (long)((x) == 0) is also problematic. (Perhaps this is less likely now that min/max don't use it.) So we may end up with (long)((x) ? 0 : 1) which really might as well be written ((x) ? 0L : 1L). The use is likely to be const_true(x > y) so perhaps there isn't a reason to have const_false() since the boolean operator can be inverted. const_expr() has relied on '* 0' to make all constant expressions zero. But it has to handle pointers - so needs a conditional. Since it is only one line, maybe just replicate the whole thing as: #define const_true(x) _Generic(0 ? (void *)((x) ? 0L : 1L) : (char *)0, char *: 1, void *: 0) #define const_expr(x) _Generic(0 ? (void *)((x) ? 0L : 0L) : (char *)0, char *: 1, void *: 0) Or maybe (mostly because the lines are shorter): #define const_NULL(x) _Generic(0 ? (x) : (char *)0, char *: 1, void *: 0) #define const_true(x) const_NULL((void *)(x) ? 0L : 1L)) #define const_expr(x) const_NULL((void *)(x) ? 0L : 0L)) Or even: #define const_true(x) const_NULL((x) ? NULL : (void *)1L)) #define const_expr(x) const_NULL((x) ? NULL : NULL)) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
