Macro substitution in C is a feature of the preprocessor that allows you to define macros that can be used to replace code in the source file before it is compiled. Macros are defined using the `#define` directive, and are typically used to define constants or to create shorthand notations for complex expressions.
The syntax for defining a macro is:
#define MACRO_NAME macro_value
Here, `MACRO_NAME` is the name of the macro, and `macro_value` is the text that the macro will be replaced with.
For example, the following code defines a macro named `MAX` that expands to the larger of two values:
#define MAX(a, b) ((a) > (b) ? (a) : (b))
This macro can be used in the source code like this:
int x = 10, y = 20, z; z = MAX(x, y);
After the preprocessor has processed this code, it will be transformed into:
int x = 10, y = 20, z; z = ((x) > (y) ? (x) : (y));
The result of `z` will be 20, since `y` is larger than `x`.
It’s important to note that macro substitution is a simple text replacement mechanism, and can lead to unexpected results if used improperly. For example, macros that use expressions with side effects may produce unexpected results if they are used multiple times in the same statement. In addition, macros can make code harder to read and debug, and can lead to naming conflicts if not carefully chosen.
Therefore, it’s important to use macros with care and to follow best practices, such as using descriptive names, avoiding side effects, and defining macros only for simple and commonly used expressions.