CODEWITHSUNDEEP
ccompilationlinker
mary
mary

Reputation: 919

Compiling multiple C files in a program

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.

Upvotes: 26

Views: 128124

Answers (4)

Sangeeth Saravanaraj
Sangeeth Saravanaraj

Reputation: 16597

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 
$

Upvotes: 53

Dave
Dave

Reputation: 11162

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.

Upvotes: 4

Paul R
Paul R

Reputation: 212969

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.

Upvotes: 7

chrisaycock
chrisaycock

Reputation: 37930

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;
}

Upvotes: 8

Related Questions