Stack and Function Return in C: Explained

The Stack and Function Return in C Programming

In C programming language, the return value of a function is usually stored in a register or in memory, depending on the architecture and calling convention used. However, the location where the return value is stored does not necessarily have to be on the stack section of memory.

In C programming, when a function is called, a new section of memory called the "stack" is used to manage local variables, function parameters, return addresses, and other information related to the function's execution. The stack follows the Last-In-First-Out (LIFO) principle, meaning that the last item pushed onto the stack is the first one to be popped off.

When a function returns in C, several things happen in the stack section:

1. Return Value: If the function has a return value, it is typically stored in a CPU register like EAX (on x86 architecture) or RAX (on x86-64 architecture) for efficient access. For more complex data types, a pointer to the return value might be stored.

2. Local Variables: Local variables declared within the function are deallocated from the stack. This means that the memory occupied by these variables is reclaimed and can be used for other purposes.

3. Stack Pointer Adjustment: The stack pointer is adjusted to "pop" the function's stack frame. The stack pointer is moved back to where it was before the function was called. This effectively removes the function's stack frame from the stack.

4. Return Address: The return address, which is the memory location in the calling function where execution should continue after the called function completes, is retrieved from the stack. This allows the program to continue executing from where it left off.

5. Function Parameters: If the called function had any parameters, they are usually removed from the stack as well, although this might depend on the calling convention being used.

Here's a simplified example to illustrate the process:

#include <stdio.h>
int add(int a, int b) {
    return a + b;
}

int main() {
    int result = add(3, 5);
    printf("Result: %d\n", result);
    return 0;
}

Remember that the exact behavior can vary depending on the compiler and the architecture being used. Different calling conventions might influence how parameters are passed and how the stack is managed during function calls and returns.