for some reason, I was tempted to play with dependency injection in C

depinjex.c

#include "depinjex.h"

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

#ifdef ENABLE_DEPENDENCY_INJECTION
depinjex_dependencies_t depinjex_dependencies = { 
  ._atoi           = atoi,

  ._fprintf        = fprintf,

  ._fizzbuzz_main  = fizzbuzz_main,
  ._fizzbuzz_loop  = fizzbuzz_loop,
  ._fizzbuzz       = fizzbuzz
};
# define _(f, ...) depinjex_dependencies._ ## f __VA_ARGS__
#else
# define _(f, ...) f __VA_ARGS__
#endif // ENABLE_DEPENDENCY_INJECTION

int fizzbuzz_main(int argc, char **argv) {
  int const max = (argc == 2 ? _( atoi,(argv[1]) ) : 100);
  _( fizzbuzz_loop,(max) ) ;
  return 0;
}

void fizzbuzz_loop(int max) {
  for (int i = 1; i <= max; i++) {
    _( fizzbuzz,(i) );
  }
}

void fizzbuzz(int n) {
  static char * fmt[4] = { "%d\n", "fizz\n", "buzz\n", "fizzbuzz\n" };
  _( fprintf,(_(stdout), fmt[ 2 * (n % 5 == 0) + (n % 3 == 0) ], n) );
}

depinjex.h

#ifndef DEPINJEX_H
#define DEPINJEX_H

int fizzbuzz_main(int argc, char **argv);
void fizzbuzz_loop(int max);
void fizzbuzz(int n);

#ifdef ENABLE_DEPENDENCY_INJECTION
#include <stdio.h>
typedef struct {
  int (*_atoi)(const char *str);
  int (*_fprintf)(FILE *out, char const *fmt, ...);
  FILE *_stdout; // must be initialized at runtime
  int (*_fizzbuzz_main)(int argc, char **argv);
  void (*_fizzbuzz_loop)(int max);
  void (*_fizzbuzz)(int n);
} depinjex_dependencies_t;

depinjex_dependencies_t depinjex_dependencies;
#endif // ENABLE_DEPENDENCY_INJECTION
#endif // DEPINJEX_H

main.c

#include "depinjex.h"
int main(int argc, char **argv){
  return fizzbuzz_main(argc,argv);
}

Makefile

fizzbuzz: depinjex.c main.c
	gcc -o $@ $^

fizzbuzz-test: depinjex.c test.c
	gcc -DENABLE_DEPENDENCY_INJECTION -o $@ $^

test: fizzbuzz-test
	./$^

clean:
	rm -f *.o fizzbuzz fizzbuzz-test

test.c

#include "depinjex.h"

#include <unistd.h>
#include <assert.h>
#include <stdio.h>
int main(int argc, char**argv){
  // setup fake stdout
  int stdout_pair[2] = {};
  pipe(stdout_pair);
  FILE * stdout_read  = fdopen(stdout_pair[0], "r");
  FILE * stdout_write = fdopen(stdout_pair[1], "w");

  // use fake stdout in fizzbuzz_main
  depinjex_dependencies._stdout = stdout_write;

  // check return status of fizzbuzz_main
  char * test_argv[2] = { "fizzbuzz", "5" };
  assert( 0 == fizzbuzz_main(2, test_argv) );

  // check output from fizzbuzz_main
  char const * expected = "1\n2\nfizz\n4\nbuzz\n";
  fclose(stdout_write);
  for (;*expected;expected++) {
    assert( *expected == fgetc(stdout_read) );
  }
  assert( EOF == fgetc(stdout_read) );
  fclose(stdout_read);

  return 0;
}