Why is the use of alloca() not considered good practice?

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 :)

Why is the use of alloca() not considered good practice?

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