Should I teach my students alloca?

Question

How widely used is alloca in the real world? Should I teach my students to use alloca when it makes sense? Or should I teach them never to use it? Coming from a C++ RAII background, the idea of not having to call free manually sounds promising, especially in functions with multiple exit points.

Related: http://stackoverflow.com/questions/1018853/why-is-alloca-not-considered-good-practice — , Oct 25 '11 at 13:51
@cnicutar alloca() is implementation-dependent but at least many implementations return NULL on failure. C99 VLAs have no way to indicate failure. — Complicated see bio, Oct 25 '11 at 13:53
@sbi: For SO this is an open-ended question that doesn't really fit the format of what the closers think SO posts ought to be. It is too subjective, there is no clear cut answer, merely opinion. Which is fine but not for SO. Notice also that out of respect for the OP's rep no one is down voting, we're just closing this question as off topic. — Paul Sasik, Oct 25 '11 at 13:54
@PascalCuoq You shouldn't be allocating a lot with alloca / VLAs anyway. If unsure what "a lot" is in the current context, use `malloc`. — , Oct 25 '11 at 13:56
Oh. I only now see that you are asking this with a C tag. I guess my C++ answer is nonsense then. I'll delete it. Sorry for the noise. — sbi, Oct 25 '11 at 14:01
@cni: Because the Microsoft compiler does not support them, and that's what they use. — fredoverflow, Oct 25 '11 at 14:07
@sbi - FWIW, a thread will be closed if enough members vote to close, so a better question would be "Has anyone given meth to the programmers (again)?" — BlackJack, Oct 25 '11 at 16:14
@BlackJack: I also close questions when they are bad, unanswerable, or blatantly off-topic. But there's too many now on SO who will close questions that, IMO, are way too good and interesting to be closed. Those being more relaxed about closing questions usually call those being less relaxed that "closing police". In some cases, others will consider _me_ part of the closing squat. (And I'm not complaining, that's just the way it is.) But IMO the closing of good, interesting questions has gotten much worse in the last two years. Many good questions of a few years ago would have no chance today. — sbi, Oct 25 '11 at 18:14
The GNU man page for `alloca` says: "If the allocation causes stack overflow, program behavior is undefined" and "The inlined code often consists of a single instruction adjusting the stack pointer, and does not check for stack overflow. Thus, there is no NULL error return." — Keith Thompson, Oct 29 '13 at 19:49
How widely used is alloca in the real world? I have seen it used once in 35 years. Should I teach my students to use alloca when it makes sense? No. It plays Russian Roulette with your program's execution. Or should I teach them never to use it? Don't teach it at all, except perhaps in PhD-level CompSci courses. It's like giving a grenade to a carpenter monkey. Works fine for pounding in nails, until it doesn't; and you can never tell why after the fact. I come from a C++ background, and am teaching objects in C: Sigh. Stick with C++, that's why it was invented. structs are OK but. — DragonLord, Nov 10 '15 at 23:44

score 31 · Answer 1 · answered Oct 25 '11 at 14:02

If you are holding a course in general C programming, you shouldn't teach them a thing that is not in the standard. Beginner programmers needlessly writing non-standard and/or non-portable code because they were taught that way, has been a huge problem for the software industry during the past 20-30 years or so. The cost for not teaching them the standard and nothing but the standard is likely astronomic.

If you are holding a more advanced course in algorithms or application programming, it might be good to mention it. On the other hand, I have programmed everything from hard realtime embedded apps to Windows application fluff for 15 years without ever using that function.

The only use I've seen where it wouldn't be better done another way was doing stack smash detection on some versions of Windows, where a failure to alloca indicated insufficient space left — or maybe it just had some gnarly ASM to catch the crash; it's been a while since I looked at that code. It worked, but was a bit of a horror. — Donal Fellows, Oct 29 '13 at 09:43

score 13 · Answer 2 · answered Oct 25 '11 at 14:24

I can see two things happening:

The students understand the impact of alloca, read about the differences between stack and heap, and use alloca carefully. (unlikely)
The students think "wow, this is like malloc without worrying about the free," use it excessively, get stack overflow, and have no idea what's up.

I think it's much better if you describe alloca, then run this code:

#include <malloc.h>

int OverflowMyStack(int start) {
    if (start == 0)
        return 0;

    char * p = (char *)_alloca(4096);
    *p = '0';
    return OverflowMyStack(start - 1);
}

int main () {
    return OverflowMyStack(512);
}

Source: http://www.strchr.com/alloca

show them the dangers, and then tell them not to use it. They'll still learn about stack vs. heap, see the perils in action, and can move on with standard stuff.

I think most students still fall into the second category even after showing them the example code. — , Oct 31 '11 at 21:33
@WTP - Probably, but that's why you tell them not to use it even after showing them what can happen. — BlackJack, Oct 31 '11 at 23:02
Why does that code use `_alloca` rather than `alloca`? And why does it cast the result? — Keith Thompson, Nov 01 '11 at 02:06

score 5 · Answer 3 · answered Oct 25 '11 at 14:16

The answer to this question should be based on what your objectives are in the first place.

Do you want to teach someone who already knows how to program how to write C and work with existing C code in the wild? If so, tell about alloca and anything else you want.

On the other hand, if you are teaching an introductory course that only is using C by coincidence (and because C is a very small language and so on) you should focus on the important parts (writing modular programs, subroutines, collections, ...). From a student's perspective, alloca is a bir redundant since malloc is enough in most cases and from a good-code perspective you are better off explicitely mentioning how manual memory management is annoying and how other languages deal with this problem After all, there are more things to memory management then alloca or RAII so you really shouldn't restrict yourself to these and as you mentioned already, its much easier to understand the purpose of alloca if you compare it to other "more standard" ways to do things in other languages (or C99...)

score 1 · Answer 4 · answered Oct 25 '11 at 15:35

1

No.

The only reason a C programmer should even be aware of alloca's existence is to understand and fix legacy code that's using it.

Any use of alloca is either

Useless, i.e. it could trivially be replaced by fixed-size variables of automatic storage duration, OR
A dangerous stack overflow waiting to happen.

Aside from a few thought experiments for which I have never found any real-world examples, there is no usage case for alloca (or VLA's) that's not either useless or vulnerable (one of the above 2 cases).

answered Oct 25 '11 at 15:35

R.. GitHub STOP HELPING ICE

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Of course, absolutely nobody could ever possibly use it responsibly. No, never. – DeadMG Oct 25 '11 at 17:57
The only "responsible" use of `alloc` is 100% equivalent to fixed-size automatic arrays, and less portable. – R.. GitHub STOP HELPING ICE Oct 25 '11 at 19:52
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I guess, then, that nobody could ever want to allocate some dynamic amount, say, a few kilobytes which would never overflow. Or call some OS API function which will tell them how much is available. Or just increase the stack size to a lot. – DeadMG Oct 25 '11 at 22:50
If it's a few KB and you're confident you have a few KB, you can just use `T foo[5000];` or whatever. – R.. GitHub STOP HELPING ICE Oct 26 '11 at 01:14
Only if T has a trivial default constructor. If I was performance-needy, even simple zeroing of memory might cost me. But other types can have even more complex default-construction logic. If I wanted to, for example, dynamically create an array of `std::mutex`, I might invoke a kernel call and context switch for five thousand mutexes. Not cheap. Not to mention the additional cache cost of placing local variables after the array. – DeadMG Oct 26 '11 at 01:20
C does not have constructors. – R.. GitHub STOP HELPING ICE Oct 26 '11 at 01:31
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Good point. I concede that. However, I'm still going to say that using an OS API or other safe means to determine how much is left and then dynamically allocating could be a better idea than allocating a magic fixed number, – DeadMG Oct 26 '11 at 14:12

score 1 · Answer 5 · answered Oct 25 '11 at 17:45

1

My opinion is do not encourage using it unless you are teaching low level compiler principles used to allocate stack space for local variables. Teach it in that context.

answered Oct 25 '11 at 17:45

MartyTPS

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user117529 · Answer 6 · 2013-10-29T19:16:38.210

The GCC documentation has a couple practical pros and cons on alloca(). From a practical perspective, a decent amount of free software uses it, so it's good to be understand how it works and where it's used in existing code.

Passing -Wl,-stack=new-stack-size to gcc increases the maximum stack size; you'll need to do this if your project uses alloca(), or allocates large temporary arrays or uses recursion past a certain context-dependent depth.

Should I teach my students alloca?

6 Answers6