libweek - something like an enum, but with values not known in the source code

app.c

#include <stdio.h>
#include <libweek.h>

static char* boolstr(bool b) {
	return b ? "weekend, yay" : "a weekday";
}

int main() {
	printf("Monday (%d) is %s.\n", MONDAY, boolstr(is_weekend(MONDAY)));
	printf("Tuesday (%d) is %s.\n", TUESDAY, boolstr(is_weekend(TUESDAY)));
	printf("Wednesday (%d) is %s.\n", WEDNESDAY, boolstr(is_weekend(WEDNESDAY)));
	printf("Thursday (%d) is %s.\n", THURSDAY, boolstr(is_weekend(THURSDAY)));
	printf("Friday (%d) is %s.\n", FRIDAY, boolstr(is_weekend(FRIDAY)));
	printf("Saturday (%d) is %s.\n", SATURDAY, boolstr(is_weekend(SATURDAY)));
	printf("Sunday (%d) is %s.\n", SUNDAY, boolstr(is_weekend(SUNDAY)));
	
	return 0;
}

libweek.c

#include "libweek.h"

const day_t MONDAY = 45;
const day_t TUESDAY = 84;
const day_t WEDNESDAY = 63;
const day_t THURSDAY = 37;
const day_t FRIDAY = 99;
const day_t SATURDAY = 102;
const day_t SUNDAY = 111;

bool is_weekend(day_t day) {
	return day == SATURDAY || day == SUNDAY;
}

libweek.h

#include <stdbool.h>

typedef int day_t;

extern const day_t MONDAY;
extern const day_t TUESDAY;
extern const day_t WEDNESDAY;
extern const day_t THURSDAY;
extern const day_t FRIDAY;
extern const day_t SATURDAY;
extern const day_t SUNDAY;

bool is_weekend(day_t day);

Makefile

all:
	gcc -Wall -Wextra -Werror -std=c99 -shared libweek.c -o libweek.dll
	gcc -Wall -Wextra -Werror -std=c99 -L. -I. app.c -o app.exe -lweek