Variable Size Automatic (The GNU C Library)

#### 3.2.7 Automatic Storage with Variable Size ¶

The function `alloca` supports a kind of half-dynamic allocation in which blocks are allocated dynamically but freed automatically.

Allocating a block with `alloca` is an explicit action; you can allocate as many blocks as you wish, and compute the size at run time. But all the blocks are freed when you exit the function that `alloca` was called from, just as if they were automatic variables declared in that function. There is no way to free the space explicitly.

The prototype for `alloca` is in `stdlib.h`. This function is a BSD extension.

Function: `void *` alloca `(size_t size)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The return value of `alloca` is the address of a block of size bytes of memory, allocated in the stack frame of the calling function.

Do not use `alloca` inside the arguments of a function call—you will get unpredictable results, because the stack space for the `alloca` would appear on the stack in the middle of the space for the function arguments. An example of what to avoid is `foo (x, alloca (4), y)`.