A local variable normally exists while execution is inside its enclosing block. Enter the block again through a recursive call, and a separate instance exists for that call. TAKEAWAY: For an ordinary automatic object, lifetime follows execution of its block—not the point at which its definition is first reached. section 13.2.2 section 13.3
Block-scope variables declared without static have automatic storage duration; auto is the default, and register is another declaration option. Block-scope compound literals and some temporary results of function calls also qualify. Variable-length arrays (VLAs) and temporary objects need separate lifetime rules. section 13.2.2
A jump past a compound literal
In fgoto(2), a goto returns to a label above the compound literal. The second printed line reports that p and q are equal and that *p is 1. Reaching the label before reaching the literal’s expression again does not, by itself, end the block-scope object’s lifetime. The example separates an object’s lifetime from the execution that supplies its value. section 13.3
Listing 13.3. A contrived example for the use of a compound literal
3 void fgoto(unsigned n) {
4 unsigned j = 0;
5 unsigned* p = 0;
6 unsigned* q;
7 AGAIN:
8 if (p) printf("%u: p and q are %s, *p is %u\n",
9 j,
10 (q == p) ? "equal" : "unequal",
11 *p);
12 q = p;
13 p = &((unsigned){ j, });
14 ++j;
15 if (j <= n) goto AGAIN;
16 }
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Follow the label, pointer assignments, and compound literal to see why the jump makes lifetime—not just statement order—the question. section 13.3
Storage and initialization are different concerns. In one unoptimized machine-code example, function entry reserves space for automatic objects with a few instructions; some are then initialized, while others remain uninitialized until later. That implementation illustrates the rule, not a required machine layout. For a non-array local in performance-critical code, register also prevents taking its address with &, helping expose potential aliasing. section 13.5 section 13.2.2
