Example of functions inside global structures, emulating "Serial" from Arduino language.

serial.c

#include <stdio.h>
#include <stdlib.h>

typedef void (*func) (const char *);

struct module {
    func print;
    func println;
};

static void println(const char *str)
{
    puts(str);
}

static void print(const char *str)
{
    printf("%s", str);
}

static struct module Serial = { print, println };

int main(void)
{
    Serial.print("hello, ");
    Serial.print("world!");
    Serial.println("");
    Serial.println("exiting...");
    return 0;
}