A local object
Consider `int main(void) { int x = 2; { int y = x + 1; x = y; } return x; }`. The inner block can use `x`, which belongs to the enclosing block. Its own `y` exists while that inner block executes. After the closing brace, `y` can no longer be used. section 13.2.2
The ordinary local variables `x` and `y` have automatic storage duration: their lifetimes are determined at runtime by execution of their defining blocks. Block-scope variables have this duration by default unless declared `static`; `auto` and `register` also designate it. Block-scope compound literals and some temporary objects returned by function calls are automatic too. section 13.2.2
TAKEAWAY: Except for variable-length arrays (VLAs) and temporary objects, an automatic object's lifetime corresponds to execution of its defining block. Each recursive call creates a separate local instance. An object cannot be accessed outside its lifetime; attempting to do so has undefined behavior. section 13.2.2 section 13.2
Listing 13.1. An example of shadowing with local variables
1 void squareIt(double* p) {
2 *p *= *p;
3 }
4 int main(void) {
5 double x = 35.0;
6 double* xp = &x;
7 {
8 squareIt(&x); /* Refers to double x */
9 ...
10 int x = 0; /* Shadow double x */
11 ...
12 squareIt(xp); /* Valid use of double x */
13 ...
14 }
15 ...
16 squareIt(&x); /* Refers to double x */
17 ...
18 }
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The inner `int x` hides the name of the outer `double x`, but a pointer saved before the inner declaration can still refer to the outer object. Shadowing a name does not end that object's lifetime. section 13.2
Lifetime and initialization are separate questions. Automatic objects must be initialized explicitly to put them in a well-defined state; unlike objects with static or thread storage duration, they receive no default initialization. section 13.4
A `register` declaration does not permit applying `&` to the variable. For a non-array local variable, this can expose address-taking that may limit optimization; it does not promise placement in a hardware register. Arrays need separate care because they commonly decay to an address. section 13.2.2
