Atmel CAVR-4 Manuale Utente
CAVR-4
24
The stack and auto variables
AVR® IAR C/C++ Compiler
Reference Guide
The stack is a fixed block of memory, divided into two parts. The first part contains
allocated memory used by the function that called the current function, and the function
that called it, etc. The second part contains free memory that can be allocated. The
borderline between the two areas is called the top of stack and is represented by the stack
pointer, which is a dedicated processor register. Memory is allocated on the stack by
moving the stack pointer.
allocated memory used by the function that called the current function, and the function
that called it, etc. The second part contains free memory that can be allocated. The
borderline between the two areas is called the top of stack and is represented by the stack
pointer, which is a dedicated processor register. Memory is allocated on the stack by
moving the stack pointer.
A function should never refer to the memory in the area of the stack that contains free
memory. The reason is that if an interrupt occurs, the called interrupt function can
allocate, modify, and—of course—deallocate memory on the stack.
memory. The reason is that if an interrupt occurs, the called interrupt function can
allocate, modify, and—of course—deallocate memory on the stack.
Advantages
The main advantage of the stack is that functions in different parts of the program can
use the same memory space to store their data. Unlike a heap, a stack will never become
fragmented or suffer from memory leaks.
use the same memory space to store their data. Unlike a heap, a stack will never become
fragmented or suffer from memory leaks.
It is possible for a function to call itself—a so-called a recursive function—and each
invocation can store its own data on the stack.
invocation can store its own data on the stack.
Potential problems
The way the stack works makes it impossible to store data that is supposed to live after
the function has returned. The following function demonstrates a common
programming mistake. It returns a pointer to the variable
the function has returned. The following function demonstrates a common
programming mistake. It returns a pointer to the variable
x
, a variable that ceases to exist
when the function returns.
int * MyFunction()
{
int x;
... do something ...
return &x;
}
Another problem is the risk of running out of stack. This will happen when one function
calls another, which in turn calls a third, etc., and the sum of the stack usage of each
function is larger than the size of the stack. The risk is higher if large data objects are
stored on the stack, or when recursive functions—functions that call themselves either
directly or indirectly—are used.
calls another, which in turn calls a third, etc., and the sum of the stack usage of each
function is larger than the size of the stack. The risk is higher if large data objects are
stored on the stack, or when recursive functions—functions that call themselves either
directly or indirectly—are used.