From: already5chosen@yahoo.com
On Thu, 1 Jan 2026 22:54:05 +0100
highcrew wrote:
> Hello,
>
> While I consider myself reasonably good as C programmer, I still
> have difficulties in understanding undefined behavior.
> I wonder if anyone in this NG could help me.
>
> Let's take an example. There's plenty here:
> https://en.cppreference.com/w/c/language/behavior.html
> So let's focus on https://godbolt.org/z/48bn19Tsb
>
> For the lazy, I report it here:
>
> int table[4] = {0};
> int exists_in_table(int v)
> {
> // return true in one of the first 4 iterations
> // or UB due to out-of-bounds access
> for (int i = 0; i <= 4; i++) {
> if (table[i] == v) return 1;
> }
> return 0;
> }
>
> This is compiled (with no warning whatsoever) into:
>
> exists_in_table:
> mov eax, 1
> ret
> table:
> .zero 16
>
>
> Well, this is *obviously* wrong. And sure, so is the original code,
> but I find it hard to think that the compiler isn't able to notice it,
> given that it is even "exploiting" it to produce very efficient code.
>
> I understand the formalism: the resulting assembly is formally
> "correct", in that UB implies that anything can happen.
> Yet I can't think of any situation where the resulting assembly
> could be considered sensible. The compiled function will
> basically return 1 for any input, and the final program will be
> buggy.
>
> Wouldn't it be more sensible to have a compilation error, or
> at least a warning? The compiler will be happy even with -Wall
> -Wextra -Werror.
>
> There's plenty of documentation, articles and presentations that
> explain how this can make very efficient code... but nothing
> will answer this question: do I really want to be efficiently
> wrong?
>
> I mean, yes I would find the problem, thanks to my 100% coverage
> unit testing, but couldn't the compiler give me a hint?
>
> Could someone drive me into this reasoning? I know there is a lot of
> thinking behind it, yet everything seems to me very incorrect!
> I'm in deep cognitive dissonance here! :) Help!
>
On related note.
struct bar1 {
int table[4];
int other_table[4];
};
struct bar2 {
int other_table[4];
int table[4];
};
int foo1(struct bar1* p, int v)
{
for (int i = 0; i <= 4; ++i)
if (p->table[i] == v)
return 1;
return 0;
}
int foo2(struct bar2* p, int v)
{
for (int i = 0; i <= 4; ++i)
if (p->table[i] == v)
return 1;
return 0;
}
According to C Standard, access to p->table[4] in foo1() is UB.
[O.T.]
I want to use language (or, better, standardize dialect of C) in which
behavior in this case is defined, but I am bad at influencing other
people. So can not get what I want.
[/O.T.]
Now the question.
What The Standard says about foo2() ? Is there UB in foo2() as well?
gcc code generator does not think so.
.file "ub.c"
.text
.p2align 4
.globl foo1
.def foo1; .scl 2; .type
32; .endef .seh_proc foo1
foo1:
.seh_endprologue
movl $1, %eax
ret
.seh_endproc
.p2align 4
.globl foo2
.def foo2; .scl 2; .type
32; .endef .seh_proc foo2
foo2:
.seh_endprologue
leaq 16(%rcx), %rax
addq $36, %rcx
.L5:
cmpl %edx, (%rax)
je .L6
addq $4, %rax
cmpq %rcx, %rax
jne .L5
xorl %eax, %eax
ret
.p2align 4,,10
.p2align 3
.L6:
movl $1, %eax
ret
.seh_endproc
.ident "GCC: (Rev8, Built by MSYS2 project) 15.2.0"
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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