[jabl]

If you want to make an argument for something else being the representations of boolean variables than 0 for false and 1 for true, one could make the case for true being all bits set.

That would make it slightly easier to do things like memset()'ing a vector of boolean, or a struct containing a boolean like in this case. Backwards compatibility with pre-_Bool boolean expressions in C99 probably made that a non starter in any case.

[adrian_b]

A 1-bit integer can be interpreted as either a signed integer or as an unsigned integer, exactly like an integer number of any other size.

Converting a 1-bit integer to a byte-sized or word-sized integer, by using the same extension rules as for any other size (i.e. by using either sign extension or zero extension), yields as the converted value for "true" either "1" for the unsigned integer interpretation or the value with all ones (i.e. "-1") for the signed integer interpretation.

So you could have "unsigned bool" and "signed bool", exactly like you have "unsigned char" and "signed char", to choose between the 2 possible representations.

[alberto-m]

> one could make the case for true being all bits set

Historical note: this was the case in QBasic, where true was defined as -1.

[Joker_vD]

There, apparently, were quite a number of ISAs where checking the sign bit was more convenient (performant?) than checking (in)equality with zero.

[jabl]

Some Fortran compilers also did this. MS Powerstation Fortran at least, IIRC.