Question

(Sorry for bad English.)

Question 1.

void foo(void)
{
    goto inside;
    for (;;) {
        int stack_var = 42;
inside:
        ...
    }
}

Will be a place in stack allocated for the stack_var when I goto the inside label? I.e. can I correctly use the stack_var variable within ...?

Question 2.

void foo(void)
{
    for (;;) {
        int stack_var = 42;
        ...
        goto outside;
    }
outside:
    ...
}

Will be a place in stack of the stack_var deallocated when I goto the outside label? E.g. is it correct to do return within ...?

In other words, is goto smart for correct working with stack variables (automatic (de)allocation when I walk through blocks), or it’s just a stupid jump?

Answer 1

Question 1:

can I correctly use the stack_var variable within …?

The code in … can write to stack_var. However, this variable is uninitialized because the execution flow jumped over the initialization, so the code should not read from it without having written to it first.

From the C99 standard, 6.8:3

The initializers of objects that have automatic storage duration […] are evaluated and the values are stored in the objects (including storing an indeterminate value in objects without an initializer) each time the declaration is reached in the order of execution

My compiler compiles the function below to a piece of assembly that sometimes returns the uninitialized contents of x:

int f(int c){
  if (c) goto L;
  int x = 42;
 L:
  return x;
}

    cmpl    $0, %eax
    jne LBB1_2
    movl    $42, -16(%rbp)
LBB1_2:
    movl    -16(%rbp), %eax
...
    popq    %rbp
    ret

Question 2:

Will be a place in stack of the stack_var deallocated when I goto the outside label?

Yes, you can expect the memory reserved for stack_var to be reclaimed as soon as the variable goes out of scope.