the main use for this macro seems to be knowing the correct allocation
granularity for dynamic-sized fd_set objects. such usage is
non-conforming and results in undefined behavior, but it is widespread
in applications.
to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.
casting to int would not be correct because high bits could be lost.
mapping the high bits down onto low bits would be costlier in the
common case where the result is just used in a conditional. changing
the type of the bit array elements to int would permute the order of
the bit array on 64-bit big endian systems, so that's not an option
either.
Rich Felker