Method in header files

Question

Why do some people write methods in header file ?

And what's the difference between writing a method in a procedure file and a header file?

Here's the code :

Work.h

#include <time.h>

void DoWork(int n)
{
    clock_t t = clock() + n * CLOCKS_PER_SEC / 1000;
    while(clock() < t);
}

Program.c

#include <stdio.h>
#include "work.h"
int main(void)
{
     printf("Starting work\n");
     DoWork(100);
     printf("Work has finished\n");
}

Is there any difference between writing a method in a procedure file and header file or are they same?

Edit 1 : The only difference I know is if I write DoWork() in procedure file, then I have to compile the procedure file and then pass the object code, while compiling the main program.

Thanks.

That's a bad thing to do in `C`. I believe it's common in `C++` though. — pmg, Sep 16 '10 at 10:18
@pmg And can you tell me, why is it bad? Is there any performance issue or any other issues? — Searock, Sep 16 '10 at 10:19
@pmg: It's common to put `inline` functions into headers in C++. With non-`inline` functions it's just as bad as in C. (Note, however, that class member functions which are not only _declared_, but also _defined_ within their class' definition are implicitly inlined.) — sbi, Sep 16 '10 at 10:20
also, putting only the function prototype allows the developer who's looking into the lib or application functions to understand better what they do, rather than being bombarded with the whole function code. It also protects the code if you're distributing a closed source library, and the only documentation is in the headers. — Ricardo Ferreira, Sep 16 '10 at 10:26
In C there are no methods. There are only functions/procedures (and this one is a procedure) :P (just wanted to point it out, don't take it too seriously) — George, Sep 16 '10 at 10:52
@George +1 I love people who point out my mistakes and point me to the right direction : ) — Searock, Sep 16 '10 at 11:40

score 3 · Accepted Answer · answered Sep 16 '10 at 10:22

3

The preprocessor will - among other things - recursively replace all #include directives by the code of these headers included. (Basically, it's a dumb text replacement machine.) The result, a source file with all included headers recursively copied into it, is called a translation unit. Effectively, this is what your compiler sees (although modern compilers often mesh together different stages of the translation process in order to be faster).
You can have multiple translation units contribute to a single resulting program. (In fact, with anything above 50 lines of code, that's pretty much the standard.)

When you are defining functions in headers (as opposed to only declaring them), and the headers with these definitions are then included in multiple source files that are to be linked to some executable, then the linker will find multiple definitions of the same functions, and give up emitting a nasty error message.

answered Sep 16 '10 at 10:22

sbi

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Do you mean to say that DoWork will be added to the main program and then compiled? – Searock Sep 16 '10 at 10:27
@Searock: No, I mean exactly what I said: When the header is included in several translation units (among them probably the one created from your `Program.c`), they will all end up with their own definition of `DoWork()`. However, definitions must only ever appear once in a program, and programs can consist of more than one translation unit. – sbi Sep 16 '10 at 10:32
@Searock, look at [this answer](http://stackoverflow.com/questions/1410563/what-is-the-difference-between-a-definition-and-a-declaration/1410632#1410632), which explains the differences between declaration and definition. That answer is for C++ and there are a few subtle differences between C and C++ (besides C++ having classes etc.), but the underlying principle is the same. – sbi Sep 16 '10 at 10:34

score 1 · Answer 2 · answered Sep 16 '10 at 10:38

It all boils down to "project management".

In every project all functions must be named uniquely (exceptions notwithstanding) so that the linker can complete the program by generating calls to the right functions.

In a small project (like your "work" project above) it's easy to see there's no duplication of code, but as time goes by and your project grows things get harder to manage. If several of your '.c' files #include "work.h", each of them will get a copy of the function definition DoWork. If you put the function definition in a '.c' file ot it's own and a declaration in a '.h' file, there will be no misshaps.

function declaration: tell the compiler how to use the function
function definiton: tell the compiler what the function does

Method in header files

2 Answers2