Thread: Makefile help

Hello,

I am trying to use make to compile and link two simple C programs in linux. Here is the two programs.
combine1.c

Code:

//combine1.c
#include <stdio.h>

int main(void)
{
	int n;

	printf("This is a test\n");
	
	printf("n=%d\n", input(n));
	printf("Done");
	return 0;
}

combine2.c

Code:

//combine2.c
input(int n)
{
	n = 10;
	return n;
}

I can compile and link these two using gcc without any trouble:

Code:

 gcc -o combine combine1.c combine2.c

I want to compile and link the two using make but I don't think I am doing my Makefile correctly. Can someone explain how to do this? Here is what I have for my Makefile:

Code:

project1:	combine1.o
	gcc -o project1 combine1.o

combine1.o:		combine1.c combine2.c
	gcc -c combine1.c
		
clean:
	rm project1 combine1.o

When I type $make
I get the following:

Code:

combine1.o: In function `main':
combine1.c:(.text+0x1b): undefined reference to `input'
collect2: ld returned 1 exit status
make: *** [project1] Error 1

Thanks for any help.

This should work

Code:

project1: combine1.o combine2.o
	gcc -o project1 combine1.o combine2.o

combine1.o: combine1.c
	gcc -c combine1.c

combine2.o: combine2.c
	gcc -c combine2.c
		
clean:
	rm project1 combine1.o

combine1.c doesn't include a header file for combine2.h, but you're trying to call a function that combine1.c doesn't know anything about.

Yes, that's true, the declaration is missing, but gcc can cope with that (assuming warnings are not enabled, which is really bad, as always); in our case, the error occurs at link time (gcc -o project1 combine1.o, combine2.o is missing, so input() is missing too).

okay, C_ntua's suggestion woked well. What would be the proper way to call a function from main using a separate program? Would I declare the function before main in combine1.c:

Code:

//combine1.c
#include <stdio.h>

input(int n);

int main(void)
{
	int n;

	printf("This is a test\n");
	
	printf("n=%d\n", input(n));
	printf("Done");
	return 0;
}

Or would I just save combine2.h as a header file and include combine2.h in combine1.c

Code:

//combine1.c
#include <stdio.h>
#include <combine2.h>

int main(void)
{
	int n;

	printf("This is a test\n");
	
	printf("n=%d\n", input(n));
	printf("Done");
	return 0;
}

You just rename combine2.c to a combine2.h file. Then you include that in combine1 as:

Code:

include "combine2.h"

even if you include combine2.c to your combine1.c file it will work. The .h is just better to make clear it is a header file.

Now, you will ask why do we have .o files then? Because then every time you compiled a program ALL files included would have to be compiled. To avoid this, you make an object file .o which isn't compiled every time.

So in your case you should have a combine2.c. You will create an object file with gcc -c combine2.c. This will create combine2.o. Now, you will just have to include "combine2.c" to your combine1.c file and the linker will finde the combine2.o file and will do what you want, without re-compiling the .o file.

A better way is to have also combine2.h file. In this file you will just add your functions using the extern keyword. So you will have:

Code:

//combine2.h
extern int input(int n);

//combine2.c
int input(int n)
{
...
}

//combine1.c
include "combine1.h"

That is what is commonly used to separate the code into files for better management.

okay, so my first example, the declared function in the main program, worked. Provided I put the type int in for the function. I am not sure how to do this using a header file combine2.h though.

See if I'm on the right track. I would need three files:
combine1.c
combine2.c
combine2,h
I would create a Makefile using these three files. If I am correct so far I will try Makefile for the three programs.

okay, I edited my files and I am still having trouble. Here is what I have.

Code:

//combine1.c
#include <stdio.h>
#include "combine2.h"



int main(void)
{
	int n;
	
	printf("This is a test\n");
	
	printf("n=%d\n", input(n));
	printf("Done");
	return 0;
}

//combine2.c

#include "combine2.h"

int input(int n)
{
	n = 10;
	return n;
}



//combine2.h
extern input(int n)
{
	n = 10;
	return n;
}

#Makefile
project1:	combine2.o combine1.o
	gcc -o project1 combine2.o combine1.o

combine2.o:		combine2.c combine2.h
	gcc -c combine2.c
	
combine1.o:		combine2.h combine1.c
	gcc -c combine1.c
	
clean:
	rm project1 combine1.o

Here is the errors I am getting when I run make:

Code:

scott@scott-laptop:~$ make
gcc -c combine2.c
combine2.c:6: error: redefinition of ‘input’
combine2.h:3: error: previous definition of ‘input’ was here
make: *** [combine2.o] Error 1
scott@scott-laptop:~$

I tried playing around with combine2.c and combine2.h but I can't get rid of the errors. Any ideas?

I remove the prototype in combine2.c:

Code:

//combine2.c

#include "combine2.h"


{
	n = 10;
	return n;
}

I declare the return type of input in combine.h and I still get an error.

Code:

scott@scott-laptop:~$ make
gcc -c combine2.c
combine2.c:6: error: expected identifier or ‘(’ before ‘{’ token
make: *** [combine2.o] Error 1
scott@scott-laptop:~$

okay, I edited my last post to reflect combine2.c.

Even though it works, is it correct?
Am I understanding this correctly? If I have a file called main.c that has a function call in it. I would put the function definition in a second .c file example function.c and I would make a third file example function.h and put only the declarization for the function definition in it?

Am I correct in this statement? If so, I think I now have a handle on things.