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I am working on a comprehensive long-term C programming project that will require a modular programming approach. As part of the design, libraries will be created, so I wanted to confirm a true/false interpretation of header file organization:

Problem

Suppose that you are creating a library. After thinking it over, you have decided that the ultimate library you wish to conceive, aka "godzilla", should consist of two individual C files that bind to a header file. In this header file there will exist the function declarations; for example:

// offense.c
void attack_city(uint32_t force);    // (in Newtons) - capable of a LOT of force!
...

// measure.c
void measure_effect(void);
...

Here is a basic network diagram you drew:

diagram

Because both force.c and measure.c includes godzilla.h, everything will be referenced from this header file and included in the compilation process, correct?

EDIT

Application: ARM microprocessor

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    Nope. You still have to list each C file on the compiler command line or in your make file. Commented Jan 29, 2013 at 21:14
  • What if it is in an IDE? How would this change? Commented Jan 29, 2013 at 21:15
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    IDE is just an IDE, it's not the compiler. So there would be an equivalent mechanism for makefile/full-compiler-command in IDE too. How would this change? It would depend on the IDE you use :) Commented Jan 29, 2013 at 21:21
  • I should have clarified - this is for an embedded application, so this is not "regular C". Commented Jan 29, 2013 at 22:56
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    Typically, the IDE will automatically add every C file you add to a project, to the "make file". Some IDEs are dumb and add every kind of file you add the project, even header files, that the linker shouldn't need to concern themselves about. In such dumb IDEs you will be forced to leave the h files outside the project. In sane IDEs, you add every file to the project and you won't have to concern yourself much with the linking. Some things you do need to do as a programmer however, is to add header guards to every h file. Commented Jan 30, 2013 at 10:01

3 Answers 3

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You should become familiar with how the linking works.

Normally when you compile you create object files with some unresolved references, that will be resolved by the linker, that will create the binary. Some of these references can point to external libraries, and will be left "unresolved" and will be resolved at run-time.

An header file just gives the compiler the signature of the function (gives no information on where to find the actual implementation) so it will know how to pass the arguments to the function. If the function is not defined in the same module the linker will need to find it. I have no clue of which system you are using so I can't help you.

And in general before starting to write a C library you should understand these concepts very well, or you will make a crappy library that will change ABI at every minor release and will have a bad karma for that.

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4 Comments

I think you confuse ABI with API
If you retain the API but change ABI, all the programs using the library would need to be recompiled anyway, while if you don't change it, there is no need to recompile.
Give me an example that changes the ABI but not the API please. I can't imagine one.
They use this word in a kind of wrong way. AFAIK an ABI is the interface between programs on machine code level while an API is the interface on source code level. An ABI usually only changes together with the API. Exceptions are if the compiler changes its calling convention or something like that. Speaking of an ABI when you write a library (that is called from source) is kind of misleading.
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All the header file does is tell the compiler what functions are available and how they should be called (how many parameters of what types, and the return values).

It is the linker (or librarian, when creating a library rather than an executable) which brings the multiple object files which result from compiling each of the .c files. So you need to tell the compiler (for example on the command-line, through an IDE or Makefile) all the c files that you want it to compile.

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There is no implicit "binding" between header(s) and a .lib-file

In order to create a library you need to explicit tell what compiled files (.obj/.o) you wish to put in, the linker creates the library regardless of header.

Headers do come in play when linking if the different .obj-files share data or call one another but there is nothing implicit about that.

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