Anything at the marker or above the marker is not on the stack. The call stack segment holds the memory used for the call stack. When the application starts, the main function is pushed on the call stack by the operating system.
Then the program begins executing. When a function call is encountered, the function is pushed onto the call stack. When the current function ends, that function is popped off the call stack.
Thus, by looking at the functions pushed on the call stack, we can see all of the functions that were called to get to the current point of execution. Our mailbox analogy above is fairly analogous to how the call stack works.
The stack itself is a fixed-size chunk of memory addresses. A stack frame keeps track of all of the data associated with one function call.
The stack pointer keeps track of where the top of the call stack currently is. If we later push a new stack frame to this same memory, it will overwrite the old value we never cleaned up.
Here is the sequence of steps that takes place when a function is called:. Some architectures include the return value as part of the stack frame.
Others use CPU registers. Typically, it is not important to know all the details about how the call stack works. However, understanding that functions are effectively pushed on the stack when they are called and popped off when they return gives you the fundamentals needed to understand recursion, as well as some other concepts that are useful when debugging.
A technical note: on some architectures, the call stack grows away from memory address 0. On others, it grows towards memory address 0. As a consequence, newly pushed stack frames may have a higher or a lower memory address than the previous ones. The stack has a limited size, and consequently can only hold a limited amount of information. On Windows, the default stack size is 1MB. This means calling free to free up the space for future use. For instance:.
However, after the function, memory is still reserved for it. To counter memory leaks such as these, you can do a few things:. Since this is a small program that ends after main statements are executed, it does not matter much. However, programs with a main loop that require a lot of work can also contain memory leaks.
If that is the case, leak upon leak will cause the program to take op too much memory and simply freeze. Can you see this in increase in your task manager? The above task outputs:. A second mistake could be that things are indeed being freed, but pointers still refer to the freed up space, which is now being rendered invalid.
This is called a dangling pointer , and can happen both on the heap while dereferencing an invalid pointer after freeing up space :. This cleaning process, that automatically frees up space in multiple parts of the allocated memory space, is called garbage collecting. And it is completely absent in C, so beware! That is platform-dependent and will hopefully crash instead of cause all forms of pain.
There are a few possibilities:. Write a program with an infinite loop that puts stuff on the stack. What is the program output? What happens now? The stack is a limited, but fast piece of program memory, made available for your program by the OS.
The keyword here is limited. Unlike the heap, it will not dynamically grow, and it is typically hard-wired in the OS. Simply keeping on adding stuff to the stack, such as calling methods within methods without a stop condition infinite recursion , will cause a stack overflow exception, signaling that the OS prevented your program from taking over everything:.
This causes a segmentation fault , signaling that it was killed by the OS. Add printf statements to your liking. How do I know how big the stack can be on my OS?
Use ulimit -a to find out: Executed on a MacBbook Air. Although the stack size stays more or less the same, the max processes number dramatically increased. Improve Article.
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