Are variables on the stack "statically allocated"?

Question

I was reading this article and saw this: "This article assumes that you already know and understand at least basically how the memory map in GNU/Linux system works, and specially the difference between memory statically allocated in the stack and memory dynamically allocated in the heap."

This confused me because I thought that stack and heap are dynamically allocated, meaning only allocated if necessary, and global variables and variables declared as "static" inside of a function are statically allocated, meaning always allocated.

For example, if I have

void f() {
    int x = 1;
    ...
}

the value 1 only gets put on the stack and the stack pointer only gets incremented if the function f() gets called. Likewise, if I have

void f() {
    int x = malloc(1 * sizeof(int));
    ...
}

that heap memory only gets allocated if f() is called. However, if I have "int x = 1;" in the global section of the program or "static int x = 1;" within a function body, any time I run this program, that memory will be allocated in the data section with the value 1.

Am I wrong about any of this?

Answer 1

The stack itself is statically allocated. Variables allocated in the stack come and go, as control flow enters and leaves their scope.

Answer 2

Static variables are initialized only once, even if the initialization statement is inside of a function body.

Check the wikipedia example:

#include <stdio.h>

void func() {
    static int x = 0; 
    /* x is initialized only once across five calls of func() and
      the variable will get incremented five 
      times after these calls. The final value of x will be 5. */
    x++;
    printf("%d\n", x); // outputs the value of x
}

int main() { //int argc, char *argv[] inside the main is optional in the particular program
    func(); // prints 1
    func(); // prints 2
    func(); // prints 3
    func(); // prints 4
    func(); // prints 5
        return 0;
}

The stack is basically a large statically allocated array with a movable pointer that starts at the beginning of it. When you call a function (starting w/ main), the pointer moves (creates a stack frame) and the space in the stack frame is sliced up and given to local variables (how many local variables you have determines how big your function's stack frame will be). So local variables are kind of dynamic (they only emerge once you enter the function), but the stack is of a static size. If you allocate a super large structure on it or use too much recursion, you'll go past the end and the OS will take you down—a phenomenon known as stackoverflow. ( The icon actually illustrates this. The gray container at the bottom represents the static array that the stack is. The orange rectangles are frames created by function calls. As in a proper stack overflow, the frames overflow the container and boom -- you're program is dead. The fact that the frames go up illustrates another, rather special thing about the stack—new frames have lower addresses than the old ones so stackoverflows really happen at the beginning of the stack array rather than the end of it (unless you think of arrays as starting at their largest index and ending at 0). )

Answer 3

Stack is allocated in unit of stack frame.

• When a function is called, a stack frame is allocated for it,
• When a function returns, its stack frame disappear,

And, stack helps to store function arguments & local variables.

After a function get its stack frame, yes, within the stack frame the function use bytes of it as need dynamic.

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Dynamic allocate stack like heap

If u want to allocate memory on stack like the way as heap, then you can use alloca() from <alloca.h>, it's quicker than heap, but in most case u don't need that, it has disadvantages, thus not suggested in general case.

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Describe stack allocation in different context might make it more clear:

• From the view of a linux thread, (by the way, each process has 1 thread on creation by default, as main thread), the stack is of fix size & allocated on thread creation, (2Mb for IA-32, 32Mb for IA-64, by default), and you can change the default size as need. So you can say this is fix, and static.
• From the view of a function within thread or process, the stack frame is allocated for it from the thread's stack memory when the function starts, and the stack frame disappear when the function finish.
• From the view of a non-static local variable inside a function, the variable is allocated from the stack frame of the function as need, dynamically.

So, should it be called static or dynamical, you decide.