Consider three C declarations: `int main(void);`, `double A[5];`, and `size_t i;`. The names `main`, `A`, and `i` are identifiers. The surrounding syntax says what each name represents: a function, an array, and an object of type `size_t`, respectively. section 2.2
int main(void);
double A[5];
size_t i;
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Compare the parentheses, brackets, and plain object declaration. Each declaration gives the compiler properties of a name; these lines alone do not define the function or initialize the objects. section 2.2
An identifier is a name given to a program entity. Identifiers can name data objects, type aliases, functions, and constants. `A` and `i` name objects; `size_t` names a type alias; `main` and `printf` name functions; and `EXIT_SUCCESS` is a named constant. Unlike a literal such as `0`, an identifier does not itself spell out a fixed value. section 2.1
TAKEAWAY: A name is not the thing it names. Keep these distinctions in view:
- Declare an identifier before using it so the compiler knows its properties. Keywords are predefined by the language and cannot be declared or redefined as identifiers. section 2.2
- Distinguish an object's identifier from its type and value. An object definition specifies where that object comes to be; a declaration specifies properties associated with its name. section 2.2 section 2.5
- An identifier is visible only within its scope, starting at its declaration. A declaration of the same name in a subordinate scope can shadow it without making the earlier object unusable. section 13.2
- In a preprocessor condition, an unknown identifier evaluates to `0`. This is a special rule for such conditions, not a substitute for declaring identifiers used in C code. section 8.1.5 section 2.2
One spelling, two objects
In the `extern` example, file-scope `i` is initialized to `1`, while a local `i` is initialized to `2`. The local definition creates a distinct object. Inside the `if`, `extern unsigned i;` refers back to the file-scope object, so the program prints `1`. Reading the spelling alone is not enough: check the declaration and scope that govern each use. section 13.2
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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Trace each occurrence of `i` through the file-scope definition, local definition, and inner `extern` declaration to check which object is printed. section 13.2
Choose a consistent naming policy, but check technical restrictions before debating style. C reserves some identifiers, including names with certain leading-underscore patterns; standard-library and file-scope names carry further restrictions. Header-visible names deserve particular care because macros and later platform changes can expose collisions. Expose as few names globally as an interface needs. section 9.2
