Compiling multiple C files in a program

Question

I have the following two files:

file1.c

int main(){
  foo();
  return 0;
}

file2.c

void foo(){

 }

Can I compile and link the two files together so the file1.c will recognize the foo function without adding extern?

Updated the prototype.

gcc file1.c file2.c throws: warning: implicit declaration of function foo.

gcc file1.c file2.c, also I don't know C's exact rules for function calls when it's not seen a prototype but you might have to add int foo(); above main — Seth Carnegie
– Seth Carnegie, Commented Jan 4, 2012 at 14:48
@Seth: please always -include -Wall when giving gcc examples - it helps to get noobs into good habits. — Paul R
– Paul R, Commented Jan 4, 2012 at 14:54

Sangeeth Saravanaraj · Accepted Answer · 2012-01-04 15:29:15Z

54

The correct way is as follows:

file1.c

#include <stdio.h>
#include "file2.h"

int main(void){
    printf("%s:%s:%d \n", __FILE__, __FUNCTION__, __LINE__);
    foo();
    return 0;
}

file2.h

void foo(void);

file2.c

#include <stdio.h>
#include "file2.h"

void foo(void) {
    printf("%s:%s:%d \n", __FILE__, __func__, __LINE__);
    return;
}

output

$
$ gcc file1.c file2.c -o file -Wall
$
$ ./file 
file1.c:main:6 
file2.c:foo:6 
$

edited Jan 4, 2012 at 15:29

answered Jan 4, 2012 at 15:04

Sangeeth Saravanaraj

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6 Comments

pmg Over a year ago

The C99 pre-defined identifier is __func__. Prefer this to the gcc specific __FUNCTION__.

Timothy Jones Over a year ago

I don't think it's good practice to pass the header to the compiler like that - if it's needed, the preprocessor will bring it in.

Paul R Over a year ago

Also please always include -Wall in any gcc command line example.

Minuet Over a year ago

@PaulR What's the -Wall for?

Paul R Over a year ago

@Minuet: it tells the compiler to enable all warnings - compiler warnings are really helpful and important, particularly for people who are still learning the language, but unfortunately they are not enabled by default, Always compile with warnings enabled and pay attention to any warnings that the compiler omits.

|

chrisaycock · Accepted Answer · 2012-01-04 15:03:27Z

8

You don't need an extern, but file1.c must see a declaration that foo() exists. Usually this declaration is in a header file.

To add a forward declaration without using a header file, simply modify file1.c to:

int foo();  // add this declaration

int main(){
  foo();
  return 0;
}

edited Jan 4, 2012 at 15:03

answered Jan 4, 2012 at 14:49

chrisaycock

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3 Comments

mary Over a year ago

can I do it without any header files just by compiling the two together?

chrisaycock Over a year ago

You still need a forward declaration of foo(), which you could do in file1.c if you want.

Seth Carnegie Over a year ago

@mary yes, header files are for the compiler but this is done by the linker

Paul R · Accepted Answer · 2012-01-04 15:10:13Z

7

You can, but you shouldn't.

Use a header file, file2.h:

// file2.h

void foo(); // prototype for function foo()

Then add:

#include "file2.h"

in file1.c

To compile:

$ gcc -Wall file1.c file2.c -o foo

As a general rule it's better (more robust) to use a header file to define the interface of each module rather than ad hoc prototypes within dependent modules. This is sometimes known as the SPOT (Single Point Of Truth) principle.

edited Jan 4, 2012 at 15:10

answered Jan 4, 2012 at 14:48

Paul R

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11 Comments

Seth Carnegie Over a year ago

You don't need extern for calling a function, you only need the prototype, like int foo(); in the header and the body in file2.c

Paul R Over a year ago

@Seth: true - you don't need it, but there is nothing wrong with making it explicit.

Seth Carnegie Over a year ago

I've never seen anyone make a function extern just because it's in another file. Saying you can but shouldn't write a function in another file without using extern on the prototype is completely baseless, unless you know something I don't (which is likely)

Paul R Over a year ago

@Seth: not at all - the prototype is extern whether you explicitly declare it extern or use C's implicit rules. I put it there to make it clear for noobs that it's a prototype, but mostly people don't use the explicit extern declaration form these days.

Jonathan Leffler Over a year ago

Note that until you are using C23, the line void foo(); // prototype for function foo() in file2.h provides a declaration of fucntion foo() but does NOT provide a prototype for the function. The empty parentheses say "takes an undefined list of arguments, but is not a variadic function (like printf() — there is no , ... at the end of the prototype)". Providing a declaration is sufficient to allow the function to be called in C99, C11, C18.

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Dave · Accepted Answer · 2012-01-04 15:13:29Z

4

It's ugly, but using gcc, you could:

gcc -include file2.c file1.c

-include is a flag to the preprocessor which will include the contents of file2.c at the very top of file1.c. Having said that, it's a poor choice, and breaks down for all but the simplest of programs.

answered Jan 4, 2012 at 15:13

Dave

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