Splitting a program into files

/* tax.h: the interface */
#ifndef TAX_H
#define TAX_H

double gst_amount(double price, int rate_percent);
extern int default_gst_rate;           /* declared here, defined in tax.c */

#endif

/* tax.c: the implementation */
#include "tax.h"

int default_gst_rate = 18;             /* the one definition */

static double round2(double x) {       /* private to tax.c (static) */
    return (long)(x * 100 + 0.5) / 100.0;
}

double gst_amount(double price, int rate_percent) {
    return round2(price * rate_percent / 100.0);
}

/* main.c: a user of the module */
#include <stdio.h>
#include "tax.h"                       /* "" searches the project folder first; <> the system folders */

int main(void) {
    printf("GST: %.2f\n", gst_amount(1499.0, default_gst_rate));   /* GST: 269.82 */
    return 0;
}

gcc -std=c17 -Wall -c tax.c          # → tax.o
gcc -std=c17 -Wall -c main.c         # → main.o
gcc tax.o main.o -o billing          # link
# or in one step: gcc -std=c17 -Wall tax.c main.c -o billing

/usr/bin/ld: main.o: in function 'main':
main.c:(.text+0x1a): undefined reference to 'gst_amount'
collect2: error: ld returned 1 exit status

/usr/bin/ld: tax.o:(.data+0x0): multiple definition of 'default_gst_rate'; main.o:(.data+0x0): first defined here