int main(void);
double A[5];
size_t i;
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Start with three declarations: int main(void);, double A[5];, and size_t i;. Each gives the compiler a name and properties of what that name denotes. section 2.2
A declaration specifies what an identifier represents before the identifier is used. The type comes first in these examples: main declares a function returning int, A declares an array of five double items, and i declares a variable of type size_t. Parentheses mark the function declaration; brackets mark the array declaration. Keywords differ from identifiers: they are predefined by C and are not declared. section 2.2
TAKEAWAY: All identifiers must be declared. For a variable, keep four things separate: its identifier (name), type (specification), object (the item that holds a value), and value (its contents). A declaration alone need not supply that value or say where the object can be found. section 2.2 section 2.3
Adding a definition
Compare size_t i; with size_t i = 0;. The initializer gives i an initial value and, in this case, the declaration also defines its object: storage is provided for that value. Likewise, int main(void); declares a function, while int main(void) followed by a body in braces defines it. TAKEAWAY: A declaration specifies an identifier; a definition specifies the associated object or function. section 2.3 section 2.1
Listing 13.2. An example of shadowing with an extern variable
1 #include <stdio.h>
2
3 unsigned i = 1;
4
5 int main(void) {
6 unsigned i = 2; /* A new object */
7 if (i) {
8 extern unsigned i; /* An existing object */
9 printf("%u\n", i);
10 } else {
11 printf("%u\n", i);
12 }
13 }
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Follow the three declarations of i. The file-scope unsigned i = 1; defines one object, and the local unsigned i = 2; defines another. Inside the if block, extern unsigned i; declares the existing file-scope object rather than creating a third; the printf there therefore prints 1. section 13.2
A name is visible within its scope starting at its declaration, but a declaration in a subordinate scope can shadow the same name. Do not equate a newly visible declaration with a new object: the extern declaration above names an existing one. Conversely, each variable definition creates a distinct object. section 13.2
