If I were the author, I would skip the part about using the compiler explorer and reading the assembly. When I write C code, I need to satisfy the rules of the C language. Unless I’m debugging the compiler or dealing with performance issues, my experience is that reading the generated assembler and understanding it is usually a slow way of debugging. The author eventually did compile with -fsanitize=undefined but I would honestly do that first. It prints a nice error message about the exact issue as the author has shown.
I have to disagree. If you merely want to fix the problem, you can stop as soon as you find something that's awry and whose alteration removes the problem. But don't you want to understand the problem? Don't you want to see how the compiler can reasonably generate code that says a bool variable is true and false at the same time?
It’s about abstraction layers. Most of the time, understanding the generated assembler code isn’t useful when it comes to understanding the problem. It satisfies my curiosity sure, but the problem is at the C level, an undefined behavior.
Understanding what the C compiler generates is interesting, but without a corresponding intuition about the optimizer passes, such understanding is shallow and unlikely to be generalized to other problems in the future. You probably won’t even remember this the next time you debug another undefined behavior. On the other hand, if you were to know the optimizer passes employed by the compiler and could deduce this code from that, then it is a useful exercise to enhance your intuition about them.
> 1. Explicitly set the C standard to C17 or older, so the code is built using the custom boolean type.
> Option 1) seemed like the easiest one, but it also felt a bit like kicking the can down the road – plus, it introduced the question of which standard to use.
Arguably, that's the sanest one: you can't expect the old C code to follow the rules of the new versions of the language. In a better world, each source file would start with something like
#pragma lang_ver stdc89
and it would automatically kick off the compatibility mode in the newer compilers, but oh well. Even modern languages such as Go miss this obvious solution.
On the topic of the article, yeah, sticking anything other than 0 or 1 into C99 bool type is UB. Use ints.
Yikes. I think this article undersells the point somewhat. This line of code undermines the type system by spraying -1's into an array of structs, so the only surprise to me is that it took this long to break.
> Ah-ha! The generated instructions were ever so slightly different. This would be great news, if it wasn't for me forgetting about one little detail: I have zero knowledge of x86 assembly.
Lol'd at this. I've been there: "ah hah! I found you. hrm, now what does this mean..."
TFA makes me thankful my work doesn't involve C / C++. Learning it earlier in life was enough.
This brings memories - back when I was a student programming in Turbo Pascal 6, I got the same invalid bool (due to array range overflow) which was both true and false at the same time.
That is a fucking travesty. If there’s one thing we should be able to rely on C for it’s that it works with assembly, and it’s always been the case that 0 is false and any other value is true. That’s a compiler bug as far as I’m concerned. I don’t use C++ because it’s gone in a ludicrous unhelpful direction since 2000 or so, but it’s sad to learn that C of all languages decided to favor pedantry over working code.
Note that this is explicitly comparing two values, which is very different from checking whether a single value is true. Surely you wouldn't expect -1 == 0 to evaluate to true.
Excellent debugging journey.
If I were the author, I would skip the part about using the compiler explorer and reading the assembly. When I write C code, I need to satisfy the rules of the C language. Unless I’m debugging the compiler or dealing with performance issues, my experience is that reading the generated assembler and understanding it is usually a slow way of debugging. The author eventually did compile with -fsanitize=undefined but I would honestly do that first. It prints a nice error message about the exact issue as the author has shown.
I have to disagree. If you merely want to fix the problem, you can stop as soon as you find something that's awry and whose alteration removes the problem. But don't you want to understand the problem? Don't you want to see how the compiler can reasonably generate code that says a bool variable is true and false at the same time?
It’s about abstraction layers. Most of the time, understanding the generated assembler code isn’t useful when it comes to understanding the problem. It satisfies my curiosity sure, but the problem is at the C level, an undefined behavior.
Understanding what the C compiler generates is interesting, but without a corresponding intuition about the optimizer passes, such understanding is shallow and unlikely to be generalized to other problems in the future. You probably won’t even remember this the next time you debug another undefined behavior. On the other hand, if you were to know the optimizer passes employed by the compiler and could deduce this code from that, then it is a useful exercise to enhance your intuition about them.
> 1. Explicitly set the C standard to C17 or older, so the code is built using the custom boolean type.
> Option 1) seemed like the easiest one, but it also felt a bit like kicking the can down the road – plus, it introduced the question of which standard to use.
Arguably, that's the sanest one: you can't expect the old C code to follow the rules of the new versions of the language. In a better world, each source file would start with something like
and it would automatically kick off the compatibility mode in the newer compilers, but oh well. Even modern languages such as Go miss this obvious solution.On the topic of the article, yeah, sticking anything other than 0 or 1 into C99 bool type is UB. Use ints.
Rust does the right thing, with the per-crate
statement.> memset (sprtemp,-1, sizeof(sprtemp));
Yikes. I think this article undersells the point somewhat. This line of code undermines the type system by spraying -1's into an array of structs, so the only surprise to me is that it took this long to break.
> For this, I used the Godbolt compiler explorer.
> Ah-ha! The generated instructions were ever so slightly different. This would be great news, if it wasn't for me forgetting about one little detail: I have zero knowledge of x86 assembly.
Lol'd at this. I've been there: "ah hah! I found you. hrm, now what does this mean..."
TFA makes me thankful my work doesn't involve C / C++. Learning it earlier in life was enough.
This brings memories - back when I was a student programming in Turbo Pascal 6, I got the same invalid bool (due to array range overflow) which was both true and false at the same time.
That is a fucking travesty. If there’s one thing we should be able to rely on C for it’s that it works with assembly, and it’s always been the case that 0 is false and any other value is true. That’s a compiler bug as far as I’m concerned. I don’t use C++ because it’s gone in a ludicrous unhelpful direction since 2000 or so, but it’s sad to learn that C of all languages decided to favor pedantry over working code.
The code in question is:
Note that this is explicitly comparing two values, which is very different from checking whether a single value is true. Surely you wouldn't expect -1 == 0 to evaluate to true.