malloced array VS. variable-length-array

Question

There are two ways to allocate memory to an array, of which the size is unknown at the beginning. The most common way is using malloc like this

int * array;
... // when we know the size
array = malloc(size*sizeof(int));

But it's valid too in C99 to define the array after we know the size.

... // when we know the size
int array[size];

Are they absolutely the same?

Answer 1

No they're not absolutely the same. While both let you store the same number and type of objects, keep in mind that:

• You can free() a malloced array, but you can't free() a variable length array (although it goes out of scope and ceases to exist once the enclosing block is left). In technical jargon, they have different storage duration: allocated for malloc versus automatic for variable length arrays.
• Although C has no concept of a stack, many implementation allocate a variable length array from the stack, while malloc allocates from the heap. This is an issue on stack-limited systems, e.g. many embedded operating systems, where the stack size is on the order of kB, while the heap is much larger.
• It is also easier to test for a failed allocation with malloc than with a variable length array.
• malloced memory can be changed in size with realloc(), while VLAs can't (more precisely only by executing the block again with a different array dimension--which loses the previous contents).
• A hosted C89 implementation only supports malloc().
• A hosted C11 implementation may not support variable length arrays (it then must define __STDC_NO_VLA__ as the integer 1 according to C11 6.10.8.3).
• Everything else I have missed :-)