Programming

Hi,

Please let me know if the extern keyword is necessary for using functions which is defined and declared in another file and and used in a different file where both these files are linked together.

thanks

No. You don't have to do this.

Code:

# cat test.c
#include<stdio.h>

int main(int argc, char *argv[]){
        adder(5,6);
        exit(0);
}
# cat test1.c
#include<stdio.h>
void adder(int a,int b) {
        fprintf(stdout,"%d\n",a+b);
}
# cc test.c test1.c
# ./a.out
11

A rudimentary example, but works.

extern is not needed for functions, but is needed when refering to external data to avoid duplicate declaration of statics which can be very hard to track down.

Prototypes however are a good idea and required even by K&R if the function does not return an int.

It is good practice and warms you up for when you start mixing C with C++.

Oh God ! I always thought that if a function in a file file1.c calls another function func2 in file file2.c, then we must include prototype of func2 in file2.h and include it in file1.c

file1.c

Code:

#include<stdio.h>
#include "file2.h"
main()
{
   func2();
}

file2.c

Code:

#include<stdio.h>
func2()
{

}

file2.h

Code:

void func2();

It is good practice to do so.

Also, file2.c should include file2.h to confirm the actual function agrees with your public interface.

I also recommend compiling with maximum warnings on to help catch errors, with gcc this would be "-Wall -Werror". While some may claim you don't technically need to in many cases, the errors that it catches outweigh any time saved not using prototypes.

Also, don't put #include <stdio.h> in file1. and file2.c. Just make it so they both include file2.h, and #include <stdio.h> in file2.h.

You could do, but you don't need to put every include file under the sun in file2.h, only put include files required to support the types used in file2.h.

Typically you have an include file common to the project which includes all the include files required by most files and common types.

Typically with UNIX you try and include the minimum number of include files you need to support your project, where as on Windows, windows.h includes almost everything you could think of.