7.2. Allocating Memory on the Stack: alloca()

Like the functions in the malloc package, alloca() allocates memory dynamically. However, instead of obtaining memory from the heap, alloca() obtains memory from the stack by increasing the size of the stack frame. This is possible because the calling function is the one whose stack frame is, by definition, on the top of the stack. Therefore, there is space above the frame for expansion, which can be accomplished by simply modifying the value of the stack pointer.

#include <alloca.h>

void *alloca(size_t size);