() function allocates
bytes of space in the stack frame of the
caller. This temporary space is automatically freed on return.
() function returns a pointer to the
beginning of the allocated space.
() 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, and alloca
determine such an error. Avoid alloca
large unbounded allocations.