I am trying to learn how to use multiple modules and hearder files. I have tried a little experiment but cannot get it to work. Here is my code and compilation attempt. Any help with finding my problems appreciated.

The main function (main01.c) calls a function located in another file (flasher01.c) that adds two integers and a constant which is defined in the header file (flasher01.h) and then outputs the result.

[me@myplace]$ cat main01.c
/* main01.c
testing multi modules */

#include <stdio.h>
#include "flasher01.h"

int main (void)
{
int a = 3, b=4, c;
c = adder(a,b);
printf ("%d%c", c, '\n');
}

[me@myplace]$ cat flasher01.c
/* flasher01.c
muulti module test */

#include "flasher01.h"

int adder (int int1, int int2)
{
return int1 + int2 + CONSTANT;
}

[me@myplace]$ cat flasher01.h
/* flasher01.h
testing multi modules */

#define CONSTANT 22
int adder (int int1, int int2)

[me@myplace]$ gcc main01.c flasher01.c flasher01.h -o mainApp
main01.c: In function `adder':
main01.c:11: error: parse error before '{' token
main01.c:10: error: parm types given both in parmlist and separately
flasher01.c: In function `adder':
flasher01.c:10: error: parse error before '{' token
flasher01.c:9: error: parm types given both in parmlist and separately
gcc: compilation of header file requested
[me@myplace]$

main01.c:11: error: parse error before '{' token
main01.c:10: error: parm types given both in parmlist and separately

main() {}

main(void) {}

/* main01.c
testing multi modules */

#include "flasher01.h"

int main()
{
     int a = 3, b = 4;
     printf ("%d\n", adder(a, b));
}

/* flasher01.h
testing multi modules */

#include <stdio.h>

#define CONSTANT 22
int adder (int int1, int int2); /* you forgot a semi-colon */

adder(3, 4);

Your are missing a trailing semicolon in the adder prototype.

Thanks very much. One question, does main() have to have a return type?
I now have
int main()
could this just have easily been
void main()
?
I think this is what I originally intended.

Yes. void means doesn't have a return value. But you don't need int in front of main(), because functions in C (and main() is a function), are expected to return int by defualt.

main()
{
}

is the same as

int main()
{
}

main does return an int as used directly in a call to "exit". This is the return value that a program returns to the parent program via wait/waitpid.

If you are using GCC try with -Wall -Werror, it will alert you to many common errors.

Thanks again. Further to this, why is "void" used as a parameter in a function that takes no arguments, i.e.
int main (void)

I see this a bit in C (I started out learning C++) , why not just use

int main()
?

Is main a special case, or do you always use void in this way in a function that takes no arguments?

Which of the following is correct in C?
myFunction(void)
OR
myFunction()
?