Function macro is a C preprocessor macro that looks like a function call when you use it.

This is a basic example with a single expression. sqr(x) looks and feels like a function even though it's a macro. A good habit to get into is to always enclose arguments into parenthesis when you define function macros. For example, #define sqr(x) x * x would not behave like you would expect: sqr(2 + 3) would expand into 2 + 3 * 2 + 3 which results in 11, not 25. By enclosing the arguments in parenthesis you guarantee precedence ((2 + 3) * (2 + 3)).

// Compile: gcc main.c -o program
// Run:     ./program
// Output:  Square of five is 25
#include <stdio.h>
#define sqr(x) ((x) * (x))
 
int main()
{
    printf("Square of five is %d\n",  sqr(2 + 3));
}

If want to make multiple statements in a function macro, a good way is to use do { ... } while (0). This way, you gotta use ';' like you do when you call an actual function. Just remember to use '\' to extend the macro across multiple lines!

// Compile: gcc main.c -o program
// Run:     ./program
// Output:  Square of '2 + 3' is 25!
#include <stdio.h>
#define sqr_calc(expr) do {                               \
        int result = ((expr) * (expr));                   \
        printf("Square of '" #expr "' is %d! ", result);  \
        if (result % 2 == 0) {                            \
            printf("%d is an odd number", result);        \
        } else {                                          \
            printf("%d is an even number", result);       \
        }                                                 \
    } while (0)
 
int main()
{
    do_calc_sqr(2 + 3);
}

• https://stackoverflow.com/questions/15575485/function-like-macros-in-c
• https://www.geeksforgeeks.org/cpp/macros-vs-functions/