Why do most C developers use define instead of const? [duplicate]

Answer 1

Expanding on R's answer a little bit: fieldWidth is not a constant expression; it's a const-qualified variable. Its value is not established until run-time, so it cannot be used where a compile-time constant expression is required (such as in an array declaration, or a case label in a switch statement, etc.).

Compare with the macro FIELD_WIDTH, which after preprocessing expands to the constant expression 10; this value is known at compile time, so it can be used for array dimensions, case labels, etc.

Answer 2

The best way to define numeric constants in C is using enum. Read the corresponding chapter of K&R's The C Programming Language, page 39.

Answer 3

Although a const int will not always be appropriate, an enum will usually work as a substitute for the #define if you are defining something to be an integral value. This is actually my preference in such a case.

enum { FIELD_WIDTH = 16384 };
char buf[FIELD_WIDTH];

In C++ this is a huge advantage as you can scope your enum in a class or namespace, whereas you cannot scope a #define.

In C you don't have namespaces and cannot scope an enum inside a struct, and am not even sure you get the type-safety, so I cannot actually see any major advantage, although maybe some C programmer there will point it out to me.