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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?
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The alloca() function allocates size bytes of space in the stack frame of the caller. This temporary space is automatically freed when the function that called alloca() returns to its caller.
Using alloca wastes very little space and is very fast. (It is open-coded by the GNU C compiler.) Since alloca does not have separate pools for different sizes of blocks, space used for any size block can be reused for any other size. alloca does not cause memory fragmentation.
Since every compiler does alloca() differently, you'd have to rewrite your freea() for each compiler. But I find it hard to believe this would be worth the trouble. Just use malloc/free if you need to explicitly free it - those functions are usually heavily optimized.
Stack is a linear data structure whereas Heap is a hierarchical data structure. Stack memory will never become fragmented whereas Heap memory can become fragmented as blocks of memory are first allocated and then freed. Stack accesses local variables only while Heap allows you to access variables globally.
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.
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