alloca —
memory allocator
Standard C Library (libc, -lc)
#include <stdlib.h>
void *
  
  alloca(size_t
    size);
The alloca() function allocates
  size bytes of space in the stack frame of the caller.
  This temporary space is automatically freed on return.
The alloca() function returns a pointer to the beginning
  of the allocated space. If the allocation failed, a
  NULL pointer is returned.
Few limitations can be mentioned:
  - The alloca() function is not part of any C
      standard and its use is not portable.
- The alloca() function should be supplied by the
      compiler because the compiler is allowed to make assumptions about the
      stack and frame pointers. The libcalloca()
      implementation cannot account for those assumptions. While there is a
      machine dependent implementation ofalloca() in
      libc, its use is discouraged and in most cases it will not work. Using
      this implementation will produce linker warnings.
- The alloca() function is unsafe because it cannot
      ensure that the pointer returned points to a valid and usable block of
      memory. The allocation made may exceed the bounds of the stack, or even go
      further into other objects in memory, andalloca()
      cannot determine such an error. For that allalloca() allocations should be bounded and limited
      to a small size.
- Since alloca() modifies the stack at runtime and
      the stack usage of each function frame cannot be predicted, it makes many
      compiler security features (such as
      cc(1)-fstack-protector) useless for the calling
      function. See security(7)
      for a discussion.