_Bool in C

Someone asked me why you have to include stdbool.h to have bool in C. I opened the header and saw that it does #define bool _Bool. I wondered what _Bool is, looked up, type from C99 representing booleans. Gotcha.

What makes _Bool different from char? Well, among other things, the C compiler tries to enforce it only having values 0 or 1.

At this point I opened Compiler Explorer.

void meow() {
	_Bool x = 3;
}

"meow":
	push    rbp
	mov     rbp, rsp
	mov     BYTE PTR [rbp-1], 1
	nop
	pop     rbp
	ret

_Bool x = 3; actually becomes an assignment of 1 in Assembly. Huh.

But what if I cast?

int meow(int x) {
	_Bool y = (_Bool)x;
	return y;
}

"meow":
	push    rbp
	mov     rbp, rsp
	mov     DWORD PTR [rbp-20], edi
	cmp     DWORD PTR [rbp-20], 0
	setne   al
	mov     BYTE PTR [rbp-1], al
	movzx   eax, BYTE PTR [rbp-1]
	pop     rbp
	ret

Huh! So it turns the plain assignment into x != 0. Interesting!

And then I thought: but what about reading a pointer? So I wrote the following program:

int printf(const char *, ...);

int meow(char x) {
	_Bool *y = (_Bool *)&x;
	return *y;
}

int main(void) {
	printf("%d\n", meow(3));
}

I then compiled it with gcc meow.c -o meow, I ran it, and it output 1!

At this point I was quite surprised, but when I copied the code into Compiler Explorer, I got... very confused.

int meow(char x) {
	_Bool *y = (_Bool *)&x;
	return *y;
}

"meow":
	push    rbp
	mov     rbp, rsp
	mov     BYTE PTR [rbp-17], dil
	lea     rax, [rbp-17]
	mov     QWORD PTR [rbp-8], rax
	mov     rax, QWORD PTR [rbp-8]
	movzx   eax, BYTE PTR [rax]
	movzx   eax, al
	pop     rbp
	ret

It didn't look like there's anything to convert the 3 into a 1! What's going on here?

I thought maybe there's some weird stuff I'm not understanding, so I went to OnlineGDB, pasted the code, ran it... 3. WHAT!

Well, turns out, I'm doing all of this on a Mac! And macOS hardlinks^[1] gcc to clang. If I change the compiler used on Compiler Explorer to clang, I get quite the different output!

int meow(char x) {
	_Bool *y = (_Bool *)&x;
	return *y;
}

meow:
	push    rbp
	mov     rbp, rsp
	mov     al, dil
	mov     byte ptr [rbp - 1], al
	lea     rax, [rbp - 1]
	mov     qword ptr [rbp - 16], rax
	mov     rax, qword ptr [rbp - 16]
	mov     al, byte ptr [rax]
	and     al, 1
	movzx   eax, al
	pop     rbp
	ret

So it's quite interesting that clang does make sure that reading from a pointer can only return 0 or 1, while gcc seems to not care about that!

This would be it for my first blog post. Hope you found it interesting!