simple calculator

makefile：

main:main.o add.o sub.o mul.o
        gcc -o $@ $^
add:add.c
        gcc -o $@ $^
sub:sub.c
        gcc -o $@ $^
mul: mul.c
        gcc -o $@ $^

.PHONY:clean
clean:
        rm -f add sub mul

add.h:

#ifndef __ADD_H__
#define __ADD_H__

#include<stdio.h>

int add(int a,int b);

#endif

add.c:

#include"add.h"

int add(int a,int b)
{
  return a+b;
}

sub.h:

#ifndef __SUB_H__
#define __SUB_H__

#include<stdio.h>

int sub(int a,int b);

#endif

sub.c:

#include"sub.h"

int sub(int a,int b)
{
  return a-b;
}

mul.h:

#ifndef __MUL_H__
#define __MUL_H__

#include<stdio.h>

int mul (int a, int b);

#endif

mul.c:

#include"mul.h"

int mul (int a, int b)
{
  return a*b;
}

main.c

#include"add.h"
#include"sub.h"
#include"mul.h"

#include<stdio.h>

intmain()
{
  int a=0;
  int b=0;
  printf("please enter:");
  scanf("%d %d\n",&a,&b);
  printf("%d + %d = %d\n",a,b,add(a,b));
  printf("%d - %d = %d\n",a,b,sub(a,b));
  printf("%d * %d = %d\n",a,b,mul(a,b));
  return 0;
}

operation result: