Sizeof() doesn't work when applied to bitfields:
# cat p.c
#include<stdio.h>
int main( int argc, char **argv )
{
struct { unsigned int bitfield : 3; } s;
fprintf( stdout, "size=%d\n", sizeof(s.bitfield) );
}
# gcc p.c -o p
p.c: In function ‘main’:
p.c:5: error: ‘sizeof’ applied to a bit-field
...obviously, since it can't return a floating point partial size or something. However, it brought up an interesting question. Is there an equivalent, in C, that will tell you the number of bits in a variable/type? Ideally, it would also work for regular types as well, like char and int, in addition to bitfields.
Update:
If there's no language equivalent of sizeof() for bitfields, what is the most efficient way of calculating it - at runtime! Imagine you have loops that depend on this, and you don't want them to break if you change the size of the bitfield - and no fair cheating and making the bitfield size and the loop length a macro. ;-)