A call-shaped replacement
`#define putchar(A) putc(A, stdout)` gives `putchar` a parameter, `A`, and replacement text that passes it to `putc`. A use looks like a function call, but the preprocessor replaces it with text before compilation. section 8.1.2
A function-like macro is defined with a parameter list after its name and is invoked with a parenthesized argument list. During preprocessing, commas outside nested parentheses separate arguments. Arguments are expanded, substituted for parameters in a copy of the replacement text, and the result is scanned for further macro replacements. The macro being expanded is temporarily disabled to prevent infinite recursion. section 16.1
A repeated argument is not one evaluation
Consider `#define square_macro(X) (X*X)`. Substituting `count()` for `X` produces `count()*count()`: the side-effecting function is called twice. By contrast, an inline function taking one unsigned parameter receives the result of one `count()` call. TAKEAWAY: Prefer an inline function when the number and types of arguments are known. section 16.0
If a macro is needed, make its call-like appearance trustworthy:
- Parenthesize parameters and the whole replacement expression. Without an outer pair, a macro replacing `sum(5, 2)*7` with `5+2*7` does not group the sum as intended. section 10.2.1
- Avoid repeating an argument that might have side effects; a replacement can evaluate it more than once. section 10.2.1
- Keep control flow visible and make statement-like uses fit their surrounding syntax. A `do { ... } while (false)` wrapper can prevent an `else` from attaching to an `if` hidden inside a macro. section 10.2.1
Macros remain useful when a function interface cannot express a requirement. Surrounding an argument with string literals in a replacement can require a literal rather than a character array; a tracing macro uses this to constrain its format argument. Other uses include default arguments, variable argument lists, and type-generic interfaces combined with `_Generic`. section 16.2 section 16.7
