In the following example, can I define the size of a C element in bits?
#include <stdio.h>
typedef enum {
false = 0,
true = ~0
} bool;
int main(void) {
bool x;
printf("%d", sizeof x);
return 0;
}
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motoku
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Do you want to know the size, or to make it be a desired size? – ugoren Mar 24 '14 at 21:54
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@ugoren I don't need to know how to get the size; it is being printed. – motoku Mar 24 '14 at 21:55
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Do you perhaps want a [bit field](http://en.wikipedia.org/wiki/Bit_field)? – Kninnug Mar 24 '14 at 21:56
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Clang, as an extension for C++ interop with ObjC, has the ability to have a fixed underlying type for an enum, that's about as close as you can get to my knowledge. – Richard J. Ross III Mar 24 '14 at 21:59
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2An enum is int in C. There is no way around that, as far as I know. So your enum will have the same size as an int. – Gábor Buella Mar 24 '14 at 22:02
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@BuellaGábor I feared as much. – motoku Mar 24 '14 at 22:04
2 Answers
6
In general, no. The minimum addressable unit is a byte, not a bit.
You can do funny things with bitfields, such as:
struct {
unsigned a : 31;
unsigned b : 1;
};
That struct will likely have a sizeof == 4, a will use 31 bits of space, and b will use 1 bit of space.
Bill Lynch
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+1. I notice (and appreciate) you use _likely_. It does depend on things like ***[THIS](http://www.geeksforgeeks.org/structure-member-alignment-padding-and-data-packing/)***. – ryyker Mar 24 '14 at 22:05
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From what I recall with working on embedded systems (it's been a while), compilers render single bit bitfields as efficiently as you'd imagine, but multi-bit fields are a crap-shoot in terms of efficiency. Safer to just use masks. – Nick T Mar 24 '14 at 22:09
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enum is of int size. All i need to do is make the implicit cast explicit.
#include <stdio.h>
typedef enum {
true = ~(int)0,
false = (int)0
} bool;
int main(void) {
return false;
}
motoku
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