Are auto variables deleted when (parent)block exits?

A common practice for compilers is to allocate memory for all auto variables of a function at the function’s entry point, simply by decrementing the stack pointer by the size of all the variables.

This is an easy way to handle all forms of block exits, especially in cases where "goto" is used.

So, in general, when control exits a block but stays within the function, the memory occupied by "auto" variables that have become inaccessible is not freed and retains the values of those variables.

Look at this example, which works with gcc and msvc. It is, we all agree, bad practice, and may not works with other compilers:

#include <stdio.h>
#include <stdlib.h>


int main(int argc, char **argv) {
    int v1;
    int *p;
    v1 = 123;
    {
        int v2;
        p = &v2; // Keeps track of v2 memory
        v2 = 321;
        printf("v1=%d v2=%d\n", v1, v2);
    }

    // Access to v2 memory, which is out of scope,
    // still prints the good value: "*p=321
    printf("*p=%d\n", *p);

    return EXIT_SUCCESS;
}

Once again, this example is ONLY for knowledge: DO NOT APPLY THIS IN SCHOOL OR PROFESSIONNAL WORK.

If I have an auto variable, it depends on the parent block if it exists or not (once "control" goes out of the block, the variable gets destroyed in RAM).

C 2024 6.2.4 says the lifetime of an automatic object without variable length array type “extends from entry into the block with which it is associated until execution of that block ends in any way.”

Calling a subroutine only suspends execution; it does not end it. Execution ends when program controls flows out of the block, goto is executed, or a call is made to one of various routines that affect program execution such as exit or longjmp.

When the parent block exits (the function or block that the variable is in), will the variable get destroyed in RAM (will it no longer occupy space in RAM)? Or is the variable present in RAM but we don't have access to it?

C does not have concepts of destroying objects (except for some thread-related functions specified in C 2024 7.28, <threads.h>). An object is said to exist when storage is reserved for it. That does not need to be anything more than an accounting procedure; memory can be reserved for an object or released by adjusting the stack pointer, by updating the internal data structures malloc and free use to track memory, or by the compiler planning the use of memory.

So, most often, the memory of an object continues to be accessible, but it may be used for other purposes.

auto doesn't change lifetime rules. When a variable is declared in a scope, its lifetime ends when it goes out of scope. End of story. Whether or not the memory still physically exists in RAM is up to the implementation.

In any case, there is no need to use auto in C at all. See: Where is the C auto keyword used?