Comment by jstimpfle
8 hours ago
I would agree that C is "really flexible", but I would say it's primarily flexible because it lets you cast say from a void pointer to a typed pointer without requiring much boilerplate. It's also flexible because it lets you control memory layout and resource management patterns quite closely.
If you want to be standards correct, yes you have to know the standard well. True. And you can always slip, and learn another gotcha. Also true. But it's still extremely flexible.
The problem is that a lot of the flexibility introduced by UB doesn't serve the developer.
Take signed integer overflow, for example. Making it UB might've made sense in the 1970s when PDP-1 owners would've started a fight over having to do an expensive check on every single addition. But it's 2026 now. Everyone settled on two's complement, and with speculative execution the check is basically free anyways. Leaving it UB serves no practical purpose, other than letting the compiler developer skip having to add a check for obscure weird legacy architectures. Literally all it does is serve as a footgun allowing over-eager optimizations to blow up your program.
Although often a source of bugs, C's low-level memory management is indeed a great source of flexibility with lots of useful applications. It's all the other weird little UB things which are the problem. As the article title already states: writing C means you are constantly making use of UB without even realizing it - and that's a problem.
If we're talking two's complement it's not undefined that is right. Having to emit checks though, that is where I beg to differ. A check is only useful if you want to actually change the behavior when it happens, otherwise it is useless. Furthermore, it might be "essentially free" from a branch prediction point, but low and behold caches exist. You would pollute both the instruction cache with those instructions _and_ the branch prediction cache. From this it doesn't follow at all, that there is no cost.
In the end small things do add up, and if you're adding many little things "because it doesn't cost much nowadays" you will end up with slow software and not have one specific bottleneck to look at. I do agree that having the option for checked operations is nice (see C#), but I have needed this behavior (branching on overflow) exactly once so far.
> A check is only useful if you want to actually change the behavior when it happens, otherwise it is useless.
You almost always want to change the behavior to erroring out on overflow. The few cases where overflow really is intended and fine can be handled by explicit opt-out.
And I refuse to buy the argument that "small things add up" in the world where we do string building and parsing every few microseconds. Checked math will have unnoticable impact compared to all the other things we do, in almost every type of program.
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Signed overflow checks are typically not free unfortunately they have a cost of about 5% or thereabouts
In hot paths it can be even more. This is why even Rust defines it as wrapping but elides the overflow panic in release builds.
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You can run your code under ASAN and UBSAN nowadays, it will catch many or most of issues as they happen.
But that's completely besides the point. UB on signed overflow, or really most of UB, is not unrelated to C flexibility. It is a detail of the spec related to portability and performance. IIRC it is even required to make such trivial optimizations as turning
into
saving arithmetics and saving a register, on architectures where `int` is smaller than pointers. But there is also options like -fwrapv on GCC for example, allowing you to actually use signed overflow.
How is undefined behavior necessary for this transformation?
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*is not related to C flexibility
It's not flexible in practice, because knowing the standard isn't optional. If you make the choice to not follow the standard, you're making the choice to write fundamentally broken software. Sometimes with catastrophic consequences.
I'm making the choice to pass pointers as void to get low-friction polymorphism. I'm making the choice to control the memory layout of my data structures, including of levels and type of indirection. I'm making the choice to control my own memory allocators and closely control lifetimes, closely control (almost) everything that happens in the system.
That has nothing to do with not following the standard.
But be as you may you’re not following the standard.
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