alloca() allocates memory on the stack rather than on the heap, as in the case of malloc(). So, when I return from the routine the memory is freed. So, actually this solves my problem of freeing up dynamically allocated memory. Freeing of memory allocated through malloc() is a major headache and if somehow missed leads to all sorts of memory problems.
Why is the use of alloca() discouraged in spite of the above features?
The answer is right there in the man page (at least on Linux):
RETURN VALUE
The alloca() function returns a pointer to the beginning of the
allocated space. If the
allocation causes
stack overflow, program behaviour is undefined.
Which isn't to say it should never be used. One of the OSS projects I work on uses it extensively, and as long as you're not abusing it (alloca'ing huge values), it's fine. Once you go past the "few hundred bytes" mark, it's time to use malloc and friends, instead. You may still get allocation failures, but at least you'll have some indication of the failure instead of just blowing out the stack.
One of the most memorable bugs I had was to do with an inline function that used alloca. It manifested itself as a stack overflow (because it allocates on the stack) at random points of the program's execution.
In the header file:
void DoSomething() {
wchar_t* pStr = alloca(100);
//......
}
In the implementation file:
void Process() {
for (i = 0; i < 1000000; i++) {
DoSomething();
}
}
So what happened was the compiler inlined DoSomething function and all the stack allocations were happening inside Process() function and thus blowing the stack up. In my defence (and I wasn't the one who found the issue; I had to go and cry to one of the senior developers when I couldn't fix it), it wasn't straight alloca, it was one of ATL string conversion macros.
So the lesson is - do not use alloca in functions that you think might be inlined.
Old question but nobody mentioned that it should be replaced by variable length arrays.
char arr[size];
instead of
char *arr=alloca(size);
It's in the standard C99 and existed as compiler extension in many compilers.
alloca() is very useful if you can't use a standard local variable because its size would need to be determined at runtime and you can
absolutely guarantee that the pointer you get from alloca() will NEVER be used after this function returns.
You can be fairly safe if you
do not return the pointer, or anything that contains it.
do not store the pointer in any structure allocated on the heap
do not let any other thread use the pointer
The real danger comes from the chance that someone else will violate these conditions sometime later. With that in mind it's great for passing buffers to functions that format text into them :)
As noted in this newsgroup posting, there are a few reasons why using alloca can be considered difficult and dangerous:
Not all compilers support alloca.
Some compilers interpret the intended behaviour of alloca differently, so portability is not guaranteed even between compilers that support it.
Some implementations are buggy.
One issue is that it isn't standard, although it's widely supported. Other things being equal, I'd always use a standard function rather than a common compiler extension.
still alloca use is discouraged, why?
I don't perceive such a consensus. Lots of strong pros; a few cons:
C99 provides variable length arrays, which would often be used preferentially as the notation's more consistent with fixed-length arrays and intuitive overall
many systems have less overall memory/address-space available for the stack than they do for the heap, which makes the program slightly more susceptible to memory exhaustion (through stack overflow): this may be seen as a good or a bad thing - one of the reasons the stack doesn't automatically grow the way heap does is to prevent out-of-control programs from having as much adverse impact on the entire machine
when used in a more local scope (such as a while or for loop) or in several scopes, the memory accumulates per iteration/scope and is not released until the function exits: this contrasts with normal variables defined in the scope of a control structure (e.g. for {int i = 0; i < 2; ++i) { X } would accumulate alloca-ed memory requested at X, but memory for a fixed-sized array would be recycled per iteration).
modern compilers typically do not inline functions that call alloca, but if you force them then the alloca will happen in the callers' context (i.e. the stack won't be released until the caller returns)
a long time ago alloca transitioned from a non-portable feature/hack to a Standardised extension, but some negative perception may persist
the lifetime is bound to the function scope, which may or may not suit the programmer better than malloc's explicit control
having to use malloc encourages thinking about the deallocation - if that's managed through a wrapper function (e.g. WonderfulObject_DestructorFree(ptr)), then the function provides a point for implementation clean up operations (like closing file descriptors, freeing internal pointers or doing some logging) without explicit changes to client code: sometimes it's a nice model to adopt consistently
in this pseudo-OO style of programming, it's natural to want something like WonderfulObject* p = WonderfulObject_AllocConstructor(); - that's possible when the "constructor" is a function returning malloc-ed memory (as the memory remains allocated after the function returns the value to be stored in p), but not if the "constructor" uses alloca
a macro version of WonderfulObject_AllocConstructor could achieve this, but "macros are evil" in that they can conflict with each other and non-macro code and create unintended substitutions and consequent difficult-to-diagnose problems
missing free operations can be detected by ValGrind, Purify etc. but missing "destructor" calls can't always be detected at all - one very tenuous benefit in terms of enforcement of intended usage; some alloca() implementations (such as GCC's) use an inlined macro for alloca(), so runtime substitution of a memory-usage diagnostic library isn't possible the way it is for malloc/realloc/free (e.g. electric fence)
some implementations have subtle issues: for example, from the Linux manpage:
On many systems alloca() cannot be used inside the list of arguments of a function call, because the stack space reserved by alloca() would appear on the stack in the middle of the space for the function arguments.
I know this question is tagged C, but as a C++ programmer I thought I'd use C++ to illustrate the potential utility of alloca: the code below (and here at ideone) creates a vector tracking differently sized polymorphic types that are stack allocated (with lifetime tied to function return) rather than heap allocated.
#include <alloca.h>
#include <iostream>
#include <vector>
struct Base
{
virtual ~Base() { }
virtual int to_int() const = 0;
};
struct Integer : Base
{
Integer(int n) : n_(n) { }
int to_int() const { return n_; }
int n_;
};
struct Double : Base
{
Double(double n) : n_(n) { }
int to_int() const { return -n_; }
double n_;
};
inline Base* factory(double d) __attribute__((always_inline));
inline Base* factory(double d)
{
if ((double)(int)d != d)
return new (alloca(sizeof(Double))) Double(d);
else
return new (alloca(sizeof(Integer))) Integer(d);
}
int main()
{
std::vector<Base*> numbers;
numbers.push_back(factory(29.3));
numbers.push_back(factory(29));
numbers.push_back(factory(7.1));
numbers.push_back(factory(2));
numbers.push_back(factory(231.0));
for (std::vector<Base*>::const_iterator i = numbers.begin();
i != numbers.end(); ++i)
{
std::cout << *i << ' ' << (*i)->to_int() << '\n';
(*i)->~Base(); // optionally / else Undefined Behaviour iff the
// program depends on side effects of destructor
}
}
Lots of interesting answers to this "old" question, even some relatively new answers, but I didn't find any that mention this....
When used properly and with care, consistent use of alloca()
(perhaps application-wide) to handle small variable-length allocations
(or C99 VLAs, where available) can lead to lower overall stack
growth than an otherwise equivalent implementation using oversized
local arrays of fixed length. So alloca() may be good for your stack if you use it carefully.
I found that quote in.... OK, I made that quote up. But really, think about it....
#j_random_hacker is very right in his comments under other answers: Avoiding the use of alloca() in favor of oversized local arrays does not make your program safer from stack overflows (unless your compiler is old enough to allow inlining of functions that use alloca() in which case you should upgrade, or unless you use alloca() inside loops, in which case you should... not use alloca() inside loops).
I've worked on desktop/server environments and embedded systems. A lot of embedded systems don't use a heap at all (they don't even link in support for it), for reasons that include the perception that dynamically allocated memory is evil due to the risks of memory leaks on an application that never ever reboots for years at a time, or the more reasonable justification that dynamic memory is dangerous because it can't be known for certain that an application will never fragment its heap to the point of false memory exhaustion. So embedded programmers are left with few alternatives.
alloca() (or VLAs) may be just the right tool for the job.
I've seen time & time again where a programmer makes a stack-allocated buffer "big enough to handle any possible case". In a deeply nested call tree, repeated use of that (anti-?)pattern leads to exaggerated stack use. (Imagine a call tree 20 levels deep, where at each level for different reasons, the function blindly over-allocates a buffer of 1024 bytes "just to be safe" when generally it will only use 16 or less of them, and only in very rare cases may use more.) An alternative is to use alloca() or VLAs and allocate only as much stack space as your function needs, to avoid unnecessarily burdening the stack. Hopefully when one function in the call tree needs a larger-than-normal allocation, others in the call tree are still using their normal small allocations, and the overall application stack usage is significantly less than if every function blindly over-allocated a local buffer.
But if you choose to use alloca()...
Based on other answers on this page, it seems that VLAs should be safe (they don't compound stack allocations if called from within a loop), but if you're using alloca(), be careful not to use it inside a loop, and make sure your function can't be inlined if there's any chance it might be called within another function's loop.
All of the other answers are correct. However, if the thing you want to alloc using alloca() is reasonably small, I think that it's a good technique that's faster and more convenient than using malloc() or otherwise.
In other words, alloca( 0x00ffffff ) is dangerous and likely to cause overflow, exactly as much as char hugeArray[ 0x00ffffff ]; is. Be cautious and reasonable and you'll be fine.
I don't think anyone has mentioned this: Use of alloca in a function will hinder or disable some optimizations that could otherwise be applied in the function, since the compiler cannot know the size of the function's stack frame.
For instance, a common optimization by C compilers is to eliminate use of the frame pointer within a function, frame accesses are made relative to the stack pointer instead; so there's one more register for general use. But if alloca is called within the function, the difference between sp and fp will be unknown for part of the function, so this optimization cannot be done.
Given the rarity of its use, and its shady status as a standard function, compiler designers quite possibly disable any optimization that might cause trouble with alloca, if would take more than a little effort to make it work with alloca.
UPDATE:
Since variable-length local arrays have been added to C, and since these present very similar code-generation issues to the compiler as alloca, I see that 'rarity of use and shady status' does not apply to the underlying mechanism; but I would still suspect that use of either alloca or VLA tends to compromise code generation within a function that uses them. I would welcome any feedback from compiler designers.
Everyone has already pointed out the big thing which is potential undefined behavior from a stack overflow but I should mention that the Windows environment has a great mechanism to catch this using structured exceptions (SEH) and guard pages. Since the stack only grows as needed, these guard pages reside in areas that are unallocated. If you allocate into them (by overflowing the stack) an exception is thrown.
You can catch this SEH exception and call _resetstkoflw to reset the stack and continue on your merry way. Its not ideal but it's another mechanism to at least know something has gone wrong when the stuff hits the fan. *nix might have something similar that I'm not aware of.
I recommend capping your max allocation size by wrapping alloca and tracking it internally. If you were really hardcore about it you could throw some scope sentries at the top of your function to track any alloca allocations in the function scope and sanity check this against the max amount allowed for your project.
Also, in addition to not allowing for memory leaks alloca does not cause memory fragmentation which is pretty important. I don't think alloca is bad practice if you use it intelligently, which is basically true for everything. :-)
One pitfall with alloca is that longjmp rewinds it.
That is to say, if you save a context with setjmp, then alloca some memory, then longjmp to the context, you may lose the alloca memory. The stack pointer is back where it was and so the memory is no longer reserved; if you call a function or do another alloca, you will clobber the original alloca.
To clarify, what I'm specifically referring to here is a situation whereby longjmp does not return out of the function where the alloca took place! Rather, a function saves context with setjmp; then allocates memory with alloca and finally a longjmp takes place to that context. That function's alloca memory is not all freed; just all the memory that it allocated since the setjmp. Of course, I'm speaking about an observed behavior; no such requirement is documented of any alloca that I know.
The focus in the documentation is usually on the concept that alloca memory is associated with a function activation, not with any block; that multiple invocations of alloca just grab more stack memory which is all released when the function terminates. Not so; the memory is actually associated with the procedure context. When the context is restored with longjmp, so is the prior alloca state. It's a consequence of the stack pointer register itself being used for allocation, and also (necessarily) saved and restored in the jmp_buf.
Incidentally, this, if it works that way, provides a plausible mechanism for deliberately freeing memory that was allocated with alloca.
I have run into this as the root cause of a bug.
Here's why:
char x;
char *y=malloc(1);
char *z=alloca(&x-y);
*z = 1;
Not that anyone would write this code, but the size argument you're passing to alloca almost certainly comes from some sort of input, which could maliciously aim to get your program to alloca something huge like that. After all, if the size isn't based on input or doesn't have the possibility to be large, why didn't you just declare a small, fixed-size local buffer?
Virtually all code using alloca and/or C99 vlas has serious bugs which will lead to crashes (if you're lucky) or privilege compromise (if you're not so lucky).
alloca () is nice and efficient... but it is also deeply broken.
broken scope behavior (function scope instead of block scope)
use inconsistant with malloc (alloca()-ted pointer shouldn't be freed, henceforth you have to track where you pointers are coming from to free() only those you got with malloc())
bad behavior when you also use inlining (scope sometimes goes to the caller function depending if callee is inlined or not).
no stack boundary check
undefined behavior in case of failure (does not return NULL like malloc... and what does failure means as it does not check stack boundaries anyway...)
not ansi standard
In most cases you can replace it using local variables and majorant size. If it's used for large objects, putting them on the heap is usually a safer idea.
If you really need it C you can use VLA (no vla in C++, too bad). They are much better than alloca() regarding scope behavior and consistency. As I see it VLA are a kind of alloca() made right.
Of course a local structure or array using a majorant of the needed space is still better, and if you don't have such majorant heap allocation using plain malloc() is probably sane.
I see no sane use case where you really really need either alloca() or VLA.
Processes only have a limited amount of stack space available - far less than the amount of memory available to malloc().
By using alloca() you dramatically increase your chances of getting a Stack Overflow error (if you're lucky, or an inexplicable crash if you're not).
A place where alloca() is especially dangerous than malloc() is the kernel - kernel of a typical operating system has a fixed sized stack space hard-coded into one of its header; it is not as flexible as the stack of an application. Making a call to alloca() with an unwarranted size may cause the kernel to crash.
Certain compilers warn usage of alloca() (and even VLAs for that matter) under certain options that ought to be turned on while compiling a kernel code - here, it is better to allocate memory in the heap that is not fixed by a hard-coded limit.
alloca is not worse than a variable-length array (VLA), but it's riskier than allocating on the heap.
On x86 (and most often on ARM), the stack grows downwards, and that brings with it a certain amount of risk: if you accidentally write beyond the block allocated with alloca (due to a buffer overflow for example), then you will overwrite the return address of your function, because that one is located "above" on the stack, i.e. after your allocated block.
The consequence of this is two-fold:
The program will crash spectacularly and it will be impossible to tell why or where it crashed (stack will most likely unwind to a random address due to the overwritten frame pointer).
It makes buffer overflow many times more dangerous, since a malicious user can craft a special payload which would be put on the stack and can therefore end up executed.
In contrast, if you write beyond a block on the heap you "just" get heap corruption. The program will probably terminate unexpectedly but will unwind the stack properly, thereby reducing the chance of malicious code execution.
Sadly the truly awesome alloca() is missing from the almost awesome tcc. Gcc does have alloca().
It sows the seed of its own destruction. With return as the destructor.
Like malloc() it returns an invalid pointer on fail which will segfault on modern systems with a MMU (and hopefully restart those without).
Unlike auto variables you can specify the size at run time.
It works well with recursion. You can use static variables to achieve something similar to tail recursion and use just a few others pass info to each iteration.
If you push too deep you are assured of a segfault (if you have an MMU).
Note that malloc() offers no more as it returns NULL (which will also segfault if assigned) when the system is out of memory. I.e. all you can do is bail or just try to assign it any way.
To use malloc() I use globals and assign them NULL. If the pointer is not NULL I free it before I use malloc().
You can also use realloc() as general case if want copy any existing data. You need to check pointer before to work out if you are going to copy or concatenate after the realloc().
3.2.5.2 Advantages of alloca
Actually, alloca is not guaranteed to use the stack.
Indeed, the gcc-2.95 implementation of alloca allocates memory from the heap using malloc itself. Also that implementation is buggy, it may lead to a memory leak and to some unexpected behavior if you call it inside a block with a further use of goto. Not, to say that you should never use it, but some times alloca leads to more overhead than it releaves frome.
In my opinion, alloca(), where available, should be used only in a constrained manner. Very much like the use of "goto", quite a large number of otherwise reasonable people have strong aversion not just to the use of, but also the existence of, alloca().
For embedded use, where the stack size is known and limits can be imposed via convention and analysis on the size of the allocation, and where the compiler cannot be upgraded to support C99+, use of alloca() is fine, and I've been known to use it.
When available, VLAs may have some advantages over alloca(): The compiler can generate stack limit checks that will catch out-of-bounds access when array style access is used (I don't know if any compilers do this, but it can be done), and analysis of the code can determine whether the array access expressions are properly bounded. Note that, in some programming environments, such as automotive, medical equipment, and avionics, this analysis has to be done even for fixed size arrays, both automatic (on the stack) and static allocation (global or local).
On architectures that store both data and return addresses/frame pointers on the stack (from what I know, that's all of them), any stack allocated variable can be dangerous because the address of the variable can be taken, and unchecked input values might permit all sorts of mischief.
Portability is less of a concern in the embedded space, however it is a good argument against use of alloca() outside of carefully controlled circumstances.
Outside of the embedded space, I've used alloca() mostly inside logging and formatting functions for efficiency, and in a non-recursive lexical scanner, where temporary structures (allocated using alloca() are created during tokenization and classification, then a persistent object (allocated via malloc()) is populated before the function returns. The use of alloca() for the smaller temporary structures greatly reduces fragmentation when the persistent object is allocated.
Why no one mentions this example introduced by GNU documention?
https://www.gnu.org/software/libc/manual/html_node/Advantages-of-Alloca.html
Nonlocal exits done with longjmp (see Non-Local Exits) automatically
free the space allocated with alloca when they exit through the
function that called alloca. This is the most important reason to use
alloca
Suggest reading order 1->2->3->1:
https://www.gnu.org/software/libc/manual/html_node/Advantages-of-Alloca.html
Intro and Details from Non-Local Exits
Alloca Example
I don't think that anybody has mentioned this, but alloca also has some serious security issues not necessarily present with malloc (though these issues also arise with any stack based arrays, dynamic or not). Since the memory is allocated on the stack, buffer overflows/underflows have much more serious consequences than with just malloc.
In particular, the return address for a function is stored on the stack. If this value gets corrupted, your code could be made to go to any executable region of memory. Compilers go to great lengths to make this difficult (in particular by randomizing address layout). However, this is clearly worse than just a stack overflow since the best case is a SEGFAULT if the return value is corrupted, but it could also start executing a random piece of memory or in the worst case some region of memory which compromises your program's security.
IMO the biggest risk with alloca and variable length arrays is it can fail in a very dangerous manner if the allocation size is unexpectedly large.
Allocations on the stack typically have no checking in user code.
Modern operating systems will generally put a guard page in place below* to detect stack overflow. When the stack overflows the kernel may either expand the stack or kill the process. Linux expanded this guard region in 2017 to be significantly large than a page, but it's still finite in size.
So as a rule it's best to avoid allocating more than a page on the stack before making use of the previous allocations. With alloca or variable length arrays it's easy to end up allowing an attacker to make arbitrary size allocations on the stack and hence skip over any guard page and access arbitrary memory.
* on most widespread systems today the stack grows downwards.
Most answers here largely miss the point: there's a reason why using _alloca() is potentially worse than merely storing large objects in the stack.
The main difference between automatic storage and _alloca() is that the latter suffers from an additional (serious) problem: the allocated block is not controlled by the compiler, so there's no way for the compiler to optimize or recycle it.
Compare:
while (condition) {
char buffer[0x100]; // Chill.
/* ... */
}
with:
while (condition) {
char* buffer = _alloca(0x100); // Bad!
/* ... */
}
The problem with the latter should be obvious.
Related
I have a main function for my app, and I allocate, for example, paths to configuration files, etc. Currently I use malloc for them, but they are never freed and always available for use throughout the lifetime of the app. I never even free them because the OS already automatically reclaims allocated memory when an application terminates. At this point, is there any reason not to use alloca instead of malloc, because the program ends when main returns and alloca memory is only deleted once the function it was allocated in is freed. So based on this logic, memory allocated in the main function with alloca is only deallocated once the program ends which is desired. Are these statements correct, and is there any reason not to use alloca (alloca is bad practice so when I said alloca meant alloca or making a VLA in main) in main for a 'global VLA' like object that lasts until the program terminates?
You can use alloca/VLA in main, but why?
The typical reason to use them is if you have some performance sensitive part that is called a lot, and you don't want the overhead of malloc/free. For main, your data is allocated once at the beginning of the program, so the overhead of a few malloc calls is negligible.
Another reason to not use alloca/VLA's in main is that they consume stack space, which is a very limited resource compared to heap space.
Depends on how much memory you need. If it is small enough (say a few hundred bytes or so), you can safely do alloca in main() or use VLAs.
But then, if the sizes of these arrays have a known upper-limit which is not very large, it would be even better and safer to declare them globally with that upper-limit as the size. That way you don't consume stack space and you don't have to malloc and then ensure the allocation succeeded. It is also then clear to whoever is reading that this piece of memory lives as long as the program does.
If the sizes can be arbitrarily large then the best thing to do is to continue using malloc() like you are already. Btw even if you are calling malloc() in main() and use it for the lifetime of the program, it is still considered good practice to free it before exit.
Technically no, because any variable declared in a function will not be global. But you can do something like this:
char *buffer;
int main(void) {
char buf[size];
buffer = buf;
That would give you an interface to access the buffer globally.
At this point, is there any reason not to use alloca instead of malloc
This is one question that typically should be asked the other way around. Is there any reason to use alloca instead of malloc? Consider changing if you have performance issues, but if you just want to avoid using free, I'd say that's a bad reason.
But I don't really see the point here. If you have an allocated buffer that you want to live from when the program starts to when it ends, then just free it in the end of the main function.
int main(void) {
char *buf = malloc(size);
// Do work
free(buf);
}
I wrote a long answer about alloca and VLA:s that you might find useful. Do I really need malloc?
VLA (as defined by the standard) and non-standard alloca are both meant to be used for allocating temporary, small arrays at local scope. Nothing else.
Allocating large objects on the stack is a well-known source for subtle & severe stack overflow bugs. This is the reason you should avoid large VLA and alloca objects. Whenever you need large objects at file scope, they should either be static arrays or dynamically allocated with malloc.
It should be noted that stack allocation is usually faster than heap allocation, because stack allocation doesn't need to concern itself with look-ups, fragmentation and other heap implementation-specific concerns. Stack allocation just says "these 100 bytes are mine" and then you are ready to go.
Regarding general confusion about "stack vs heap" please see What gets allocated on the stack and the heap?
You can't even place a standard VLA at file scope, because the array size needs to be an integer constant expression there. Plus the standard (C17 6.7.6) explicitly says that you aren't allowed to:
If an identifier is declared to be an object with static or thread storage
duration, it shall not have a variable length array type.
As for alloca it isn't standard C and bad for that reason. But it's also bad because it doesn't have any type safety, so VLA is preferred over alloca - it is safer and more portable.
It should be noted that the main purpose of VLA in modern programming is however to enable pointers to VLA, rather than allocating array objects of VLA type, which is a feature of limited use.
I never even free them because the OS already automatically reclaims allocated memory when an application terminates.
While that is correct, it is still considered good practice to call free() manually. Because if you have any heap corruption or pointer-related bugs somewhere in the program, you'll get a crash upon calling free(). Which is a good thing, since it allows you to catch such (common) bugs early on during development.
(If you are concerned about the performance of free(), you can exclude the free() calls from the release build and only use them in debug build. Though performance is rarely an issue when closing down the program - usually you can just shut down the GUI if any then let the program chew away on clean-up code in the background.)
Generic theory question.
I'm trying to build a library from source (FFTW if anyone cares, but it really doesn't matter), and I noticed there is an option to disable the use of alloca.
I'm aware of the dangers of using alloca, but I'm assessing the performance of FFTW with and without alloca.
Does alloca have known issues with thread safety? I'm seeing an extreme performance hit when I use a certain number of threads with FFTW (which is obviously calling alloca in the background). I'm sticking to using a number of threads equal to powers of 2, if that matters.
Is it possible that FFTW is sharing objects on a thread-local stack via alloca? I'm just trying to figure out why I see such extreme performance hits with certain numbers of threads. However, I don't fully understand the theory behind what alloca is really doing w/ threads.
Short answer: It doesn't. alloca() is guaranteed to be MT-safe.
Longer answer: alloca() isn't complicated function. By specification, it returns pointer to location that can be automatically freed. Please note that it's not a good practice anymore:
The alloca() function returns a pointer to the beginning of the allocated space. If the allocation causes stack overflow, program behaviour is undefined.
As you can see, allocation can cause stackoverflow, so the space is allocated on the stack by bumping SP. Threads share heap, but not stack, so there is no way you will run into trouble with multi-threaded use of alloca().
Safer way would be using VLA rather than alloca(), because both do the same and (as I suspect), VLA is faster and lighter.
This is my problem in essence. In the life of a function, I generate some integers, then use the array of integers in an algorithm that is also part of the same function. The array of integers will only be used within the function, so naturally it makes sense to store the array on the stack.
The problem is I don't know the size of the array until I'm finished generating all the integers.
I know how to allocate a fixed size and variable sized array on the stack. However, I do not know how to grow an array on the stack, and that seems like the best way to solve my problem. I'm fairly certain this is possible to do in assembly, you just increment stack pointer and store an int for each int generated, so the array of ints would be at the end of the stack frame. Is this possible to do in C though?
I would disagree with your assertion that "so naturally it makes sense to store the array on the stack". Stack memory is really designed for when you know the size at compile time. I would argue that dynamic memory is the way to go here
C doesn't define what the "stack" is. It only has static, automatic and dynamic allocations. Static and automatic allocations are handled by the compiler, and only dynamic allocation puts the controls in your hands. Thus, if you want to manually deallocate an object and allocate a bigger one, you must use dynamic allocation.
Don't use dynamic arrays on the stack (compare Why is the use of alloca() not considered good practice?), better allocate memory from the heap using malloc and resize it using realloc.
Never Use alloca()
IMHO this point hasn't been made well enough in the standard references.
One rule of thumb is:
If you're not prepared to statically allocate the maximum possible size as a
fixed length C array then you shouldn't do it dynamically with alloca() either.
Why? The reason you're trying to avoid malloc() is performance.
alloca() will be slower and won't work in any circumstance static allocation will fail. It's generally less likely to succeed than malloc() too.
One thing is sure. Statically allocating the maximum will outdo both malloc() and alloca().
Static allocation is typically damn near a no-op. Most systems will advance the stack pointer for the function call anyway. There's no appreciable difference for how far.
So what you're telling me is you care about performance but want to hold back on a no-op solution? Think about why you feel like that.
The overwhelming likelihood is you're concerned about the size allocated.
But as explained it's free and it gets taken back. What's the worry?
If the worry is "I don't have a maximum or don't know if it will overflow the stack" then you shouldn't be using alloca() because you don't have a maximum and know it if it will overflow the stack.
If you do have a maximum and know it isn't going to blow the stack then statically allocate the maximum and go home. It's a free lunch - remember?
That makes alloca() either wrong or sub-optimal.
Every time you use alloca() you're either wasting your time or coding in one of the difficult-to-test-for arbitrary scaling ceilings that sleep quietly until things really matter then f**k up someone's day.
Don't.
PS: If you need a big 'workspace' but the malloc()/free() overhead is a bottle-neck for example called repeatedly in a big loop, then consider allocating the workspace outside the loop and carrying it from iteration to iteration. You may need to reallocate the workspace if you find a 'big' case but it's often possible to divide the number of allocations by 100 or even 1000.
Footnote:
There must be some theoretical algorithm where a() calls b() and if a() requires a massive environment b() doesn't and vice versa.
In that event there could be some kind of freaky play-off where the stack overflow is prevented by alloca(). I have never heard of or seen such an algorithm. Plausible specimens will be gratefully received!
The innards of the C compiler requires stack sizes to be fixed or calculable at compile time. It's been a while since I used C (now a C++ convert) and I don't know exactly why this is. http://gribblelab.org/CBootcamp/7_Memory_Stack_vs_Heap.html provides a useful comparison of the pros and cons of the two approaches.
I appreciate your assembly code analogy but C is largely managed, if that makes any sense, by the Operating System, which imposes/provides the task, process and stack notations.
In order to address your issue dynamic memory allocation looks ideal.
int *a = malloc(sizeof(int));
and dereference it to store the value .
Each time a new integer needs to be added to the existing list of integers
int *temp = realloc(a,sizeof(int) * (n+1)); /* n = number of new elements */
if(temp != NULL)
a = temp;
Once done using this memory free() it.
Is there an upper limit on the size? If you can impose one, so the size is at most a few tens of KiB, then yes alloca is appropriate (especially if this is a leaf function, not one calling other functions that might also allocate non-tiny arrays this way).
Or since this is C, not C++, use a variable-length array like int foo[n];.
But always sanity-check your size, otherwise it's a stack-clash vulnerability waiting to happen. (Where a huge allocation moves the stack pointer so far that it ends up in the middle of another memory region, where other things get overwritten by local variables and return addresses.) Some distros enable hardening options that make GCC generate code to touch every page in between when moving the stack pointer by more than a page.
It's usually not worth it to check the size and use alloc for small, malloc for large, since you also need another check at the end of your function to call free if the size was large. It might give a speedup, but this makes your code more complicated and more likely to get broken during maintenance if future editors don't notice that the memory is only sometimes malloced. So only consider a dual strategy if profiling shows this is actually important, and you care about performance more than simplicity / human-readability / maintainability for this particular project.
A size check for an upper limit (else log an error and exit) is more reasonable, but then you have to choose an upper limit beyond which your program will intentionally bail out, even though there's plenty of RAM you're choosing not to use. If there is a reasonable limit where you can be pretty sure something's gone wrong, like the input being intentionally malicious from an exploit, then great, if(size>limit) error(); int arr[size];.
If neither of those conditions can be satisfied, your use case is not appropriate for C automatic storage (stack memory) because it might need to be large. Just use dynamic allocation autom don't want malloc.
Windows x86/x64 the default user-space stack size is 1MiB, I think. On x86-64 Linux it's 8MiB. (ulimit -s). Thread stacks are allocated with the same size. But remember, your function will be part of a chain of function calls (so if every function used a large fraction of the total size, you'd have a problem if they called each other). And any stack memory you dirty won't get handed back to the OS even after the function returns, unlike malloc/free where a large allocation can give back the memory instead of leaving it on the free list.
Kernel thread stack are much smaller, like 16 KiB total for x86-64 Linux, so you never want VLAs or alloca in kernel code, except maybe for a tiny max size, like up to 16 or maybe 32 bytes, not large compared to the size of a pointer that would be needed to store a kmalloc return value.
If I have to write some code like below:
int a[10000000];
I know that the code might fail sometimes due to stack overflows. The question is how to handle such errors at runtime, and avoid the segfault?
In general, stack overflow exceptions are very difficult to handle in a graceful way. This is because the stack is already overflowed, and in order for more code (even exception handling code) to run there needs to be stack space available.
In general, programmers design programs so that they cannot overflow the stack. This involves:
keeping the size of automatic variables allocated on the stack to a minimum (and using other types of allocation if large data structures are needed)
avoiding unnecessary recursion, and if recursion is used, ensuring that there are reasonable constraints on the maximum depth
If you need space for ten million integers inside a function, don't allocate it on the stack - allocate it using malloc() or new (depending on whether you are actually using C or C++). Of course it is also your responsibility to free() or delete it when you are done with it.
If you are really using C++[1], then you should probably be using std::vector instead:
std::vector a(10000000);
The underlying standard library implementation will allocate the space on the free store, and will automatically deallocate it for you when your function returns.
[1] I wish people wouldn't tag questions with both c and c++ just because they are spelled similarly.
There's no way to handle this at runtime. The only sane, safe way to use objects of automatic storage duration in C is to keep them small enough that you can be sure they'll never exceed the amount of stack you know you'll have (e.g. never use more than 10% or so of what you expect to have).
in an effort to only ask what I'm really looking for here... I'm really only concerned if it's considered bad practice or not to declare an array like below where the size could vary. If it is... I would generally malloc() instead.
void MyFunction()
{
int size;
//do a bunch of stuff
size = 10; //but could have been something else
int array[size];
//do more stuff...
}
Generally yes, this is bad practice, although new standards allow you to use this syntax. In my opinion you must allocate (on the heap) the memory you want to use and release it once you're done with it. Since there is no portable way of checking if the stack is enough to hold that array you should use some methods that can really be checked - like malloc/calloc & free. In the embedded world stack size can be an issue.
If you are worried about fragmentation you can create your own memory allocator, but this is a totally different story.
That depends. The first clearly isn't what I'd call "proper", and the second is only under rather limited circumstances.
In the first, you shouldn't cast the return from malloc in C -- doing so can cover up the bug of accidentally omitting inclusion of the correct header (<stdlib.h>).
In the second, you're restricting the code to C99 or a gcc extension. As long as you're aware of that, and it works for your purposes, it's all right, but hardly what I'd call an ideal of portability.
As far as what you're really asking: with the minor bug mentioned above fixed, the first is portable, but may be slower than you'd like. If the second is portable enough for your purposes, it'll normally be faster.
For your question, I think each has its advantages and disadvantages.
Dynamic Allocation:
Slow, but you can detect when there is no memory to be given to your programmer by checking the pointer.
Stack Allocation:
Only in C99 and it is blazingly fast but in case of stackoverflow you are out of luck.
In summary, when you need a small array, reserve it on the stack. Otherwise, use dynamic memory wisely.
The argument against VLAs runs that because of the absolute badness of overflowing the stack, by the time you've done enough thinking/checking to make them safe, you've done enough thinking/checking to use a fixed-size array:
1) In order to safely use VLAs, you must know that there is enough stack available.
2) In the vast majority of cases, the way that you know there's enough stack is that you know an upper bound on the size required, and you know (or at least are willing to guess or require) a lower bound on the stack available, and the one is smaller than the other. So just use a fixed-size array.
3) In the vast majority of the few cases that aren't that simple, you're using multiple VLAs (perhaps one in each call to a recursive function), and you know an upper bound on their total size, which is less than a lower bound on available stack. So you could use a fixed-size array and divide it into pieces as required.
4) If you ever encounter one of the remaining cases, in a situation where the performance of malloc is unacceptable, do let me know...
It may be more convenient, from the POV of the source code, to use VLAs. For instance you can use sizeof (in the defining scope) instead of maintaining the size in a variable, and that business with dividing an array into chunks might require passing an extra parameter around. So there's some small gain in convenience, sometimes.
It's also easier to miss that you're using a humongous amount of stack, yielding undefined behavior, if instead of a rather scary-looking int buf[1920*1024] or int buf[MAX_IMG_SIZE] you have an int buf[img->size]. That works fine right up to the first time you actually handle a big image. That's broadly an issue of proper testing, but if you miss some possible difficult inputs, then it won't be the first or last test suite to do so. I find that a fixed-size array reminds me either to put in fixed-size checks of the input, or to replace it with a dynamic allocation and stop worrying whether it fits on the stack or not. There is no valid option to put it on the stack and not worry whether it fits...
two points from a UNIX/C perspective -
malloc is only slow when you force it to call brk(). Meaning for reasonable arrays it is the same as allocating stack space for a variable. By the way when you use method #2 (via alloca and in the code libc I have seen) also invokes brk() for huge objects. So it is a wash. Note: with #2 & #1 you still have to invoke directly or indirectly a memset-type call to zero the bytes in the array. This is just a side note to the real issue (IMO):
The real issue is memory leaks. alloca cleans up after itself when the function retruns so #2 is less likely to cause a problem. With malloc/calloc you have to call free() or start a leak.