A STACK ILLUSTRATION
Most microprocessors have an internal pointer (the stack pointer) which references
memory so that the micro can keep track of the point of execution as it varies because
of interrupts, function calls, and so forth. The stack (memory to which it points)
is also used by many systems (sometimes unfortunately) as a storage place for local
values, saved registers, and so forth.
Just prior to a call, the stack pointer, which is much like any pointer one defines, is
pointing to some place in memory (designated by the programmer or the operating
system) for its use. When you call a function, it works something like this (its usage varies
somewhat from language to language).
stack pointer -->| orig position | In most systems, the stack pointer moves toward
| | lower addresses as you use it.
At call: | orig position |
| arguments |
stack pointer -->| last argument |
There may be zero or more arguments. They are pushed onto the stack in a predetermined
order. For C/C++, it is right-to-left. The stack pointer moves with each push.
After call: | orig position |
| argument here |
| (maybe more)
stack pointer -->| ret addr here |
Into procedure | orig position |
Arguments avail | arguments... | When you modify the argument(s), you modify
for use | ret addr here | the value(s) stored here. If an argument
| saved regs, | is a reference or pointer you may use it to
| locals, etc. | modify the value pointed to elsewhere
| in this area | (in the calling procedure, say). If you write
stack pointer -->| | more data to one of the local variables than it
~ ~ can store, guess where the excess winds up.
The function does its work and unwinds the stack (locals, etc.)
Before return | orig position | Immediately before the return, after storage
| arguments... | for saved registers, locals, etc. has already
stack pointer -->| ret addr here | been recovered (and disappeared). The arguments
| | are still on the stack.
After return: | orig position | Immediately after the return. The very first
stack pointer -->| arguments... | thing the machine is going to do next is destroy
| | the arguments, whether you've modified them or not.
| | Sometimes the arguments are removed by the called
| | function and the return address position adjusted
| | appropriately.
stack pointer -->| orig position | And it's done; you are right back where you started,
| | bookkeeping wise, when you made the call. Any
| | changes you made to the arguments are history, for
~ ~ all practical purposes (they may persist until the
next stack operation).
If you passed an argument as a reference, any modifications you made to the value it
referred to are, of course, in force. If you modified the reference, itself, to point
to something else, you could modify that something else, also (for example, subsequent
bytes pointed to by a char *). The reference itself disappears. If you pass an argument
by value, that value is perfectly usable to the called procedure; it could specify a length to
use for some operation, for example. If you modify the value, such modifications disappear
when the arguments disappear, immediately after the called procedure returns to the
caller. If you want lasting changes in the caller, you need to make them by reference or
RETURN a value from the called procedure.