linuxpimp20

hi. i was wondering if you had to do anything besides #include "foo.h" to include a header file. Does the compiler automatically link the two or do you need to explicitly link them? Im using gcc on a linux box. I didn't think you had too but i am trying to use a header file and it didn't work the way i expected it to - pretty sure because i didn't write it correctly. Also if you used a header file with function foo would foo's prototype be in the header file or in your source file? thanks for any replies in advance.

Polyphemus_

Let me explain. Say, you have to files:

foo.c
bar.c

where foo is the file which needs functions from bar, so you write a bar.h file, with only the definitions, not the functions itself, and include it in foo.c. When you want to compile it, you use this command:

GCC foo.c bar.c -o outputfile

L7Sqr

Header files generally contain function declarations (not definitions), structures, enums, and defined variables (i.e. #define ...)
They can also include other header files.

C files contain the definitions of the functions contained in the header as well as other functions (possibly static) declared and defined within the file.

When you compile with gcc, by default everything is linked for you, which is why you will sometimes get linker errors regarding unfound functions...

You can stop before the link stage as well as at other points.
man gcc will give you all the details.

__________________
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linuxpimp20

ok just so everyone is on the same page what is the definition of declaration and definitions? The declaration is something like

int add(int num, int num2)?

and the definition would be

int add(int num, int num2){
int num;
int num2;
int sum;
sum = num1 + num2;
return sum;
}
?

linuxpimp20

ok thinking about this has made me think about teh subject a little more. is a header and a library the same thing? Here is what i thought a .h file was for - i thought it was to keep similar functions together so that when your writing the main program it isn't cluttered with functions and you can just call the functions to get the work done. And that way you could use the .h for different projects that needed the same functions. However when i look at the .h files in /usr/include on my system a lot of them are a bunch of preprocessor stuff. so am i wrong?

aznluvsmc

Function declaration is another term for prototyping. Function definition is the actual coded function so you are correct.

Polyphemus_

damn, i forgot again it are called declarations, not definitions :mad:

linuxpimp20

yeah it is confusing - i just knew because i had been looking at my C book today. A header full of prototypes didn't sound useful to me.

linuxpimp20

that is what i thought but this site has me confused.

"What is a header file? They contain prototypes and other compiler/pre-processor directives. Prototypes are basic abstract function definitions. More on these later..."

from site: http://www.programmingforums.org/for...wreply&p=50212

isn't a prototype just int prototype(int num, char letter) so when you go to define prototype as

prototype(char letter, char letter2)
{
function code
}
you'd get a compiler error because they don't match up? I think i read for C++ before that defining a function with same name but different arguments was a way of being able to have two functions with teh same name. Is that correct? Is that site linking a prototype with definition too much? Kind of hazy area i think.

linuxpimp20

http://www.google.com/search?hl=en&l...le&btnG=Search

i think the more i look into it the more i get confused. lol. Here is what i was going to do. For a first practice program i was going to write a calculator. I was going to have a file calc.h or similar that would have all the functions for multipling, addition, etc. something like

int add(int num1, num2)
{
int num1, num2, sum;
sum = num1 + num2;

return sum;
}

so then i'd have the main program as calc.c and have something like case switches so when the user selected + i would just have to put add(input#, input#) to call the function in calc.h. So that would make the main program more concise and readable. Is that the correct use of header files?