How do you actually use a C library?

Question

I'm sure this question has been asked many times, but I can't figure this out. Bear with me.

So when you download a library, you get a bunch of .c and .h files, plus a lot of other stuff. Now say you want to write a program using this library.

I copy all the .h files into my project directory. It just doesn't compile.

Great, so then I get the library as a bunch of .dll's, and i copy the dlls into my project directory. Still doesn't compile.

How does this work?

What do you do, like right after creating the folder for your project? What parts of the library package do you copy/paste into the folder? How do you make it so that it can compile? Go through the steps with me please.

1. Where to put the .h files?
2. Where to put the .dll files?
3. How to compile?

Thanks.

(the library I'm trying to get working is libpng, I'm in windows with MinGW, and i'm looking to compile from command-line like usual.)

(from what i gather, you put the .h files in directory A and the .dll files in directory B and you can use -l and -L compiler options to tell the compiler where to find them, is this correct?)

Answer 1

You must link against a .lib or something equivalent i.e. add the ".lib" to the libraries read by the linker. At least that's how it works under Linux... haven't done Windows so a long while.

The ".lib" contains symbols to data/functions inside the .dll shared library.

Answer 2

Doing it under windows (supposing you user Visual Studio)

• After unpacking add the library include directories to your projects' settings (Project -> Properties -> C/C++ -> Additional Include Directories)

• Do the same thing for the Libraries Directory (Project -> Properties -> Linker -> Additional Library Directories)

• Specify the name of the library in your Linker Input: Project -> Properties -> Linker -> Input -> Additional Dependencies

After this hopefully should compile.

I don't recommend adding the directories above to the Global settings in Visual Studio (Tools -> Options -> Project and Solutions) since it will create and environment where something compiles on your computer and does NOT compile on another one.

Now, the hard way, doing it for a Makefile based build system:

• Unpack your stuff
• Specify the include directory under the -I g++ flag
• Specify the Library driectory under the -L g++ flag
• Specify the libraries to use like: -llibrary name (for example: -lxml2 for libxml2.so)
• Specify the static libraries like: library name.a

at the end you should have a command which is ugly and looks like:

g++ -I/work/my_library/include -L/work/my_library/lib -lmylib my_static.a -o appname_exe MYFILE.CPP

(the line above is not really tested just a general idea)

I reccommend go, grab a template makefile from somewhere and add in all your stuff.

Answer 3

When you compile, assuming you have the libs and the headers in the same folder as the sources you are compiling, you need to add to your compile line -L . -I . -lpng. -L tells the linker where to look for the library, -I tells the compiler where to look for the headers and -lpng tells the linker to link with the png library.

[Edit] Normal projects would have some sort of hierarchy where the headers are in an /include folder and the 3rd party libs are in a /libs folder. In this case, you'd put -I ./include and -L ./libs instead of -I . and -L.

[Edit2] Most projects make use of makefile in order to compile from the command line. You can only compile manually for a small number of files, it gets quite hectic after that

Answer 4

It depends on the library. For examples, some libraries contain precompiled binaries (e.g. dlls) and others you need to compile them yourself. You'd better see the library's documentation.

Basically, to compile you should:

(1) have the library's include (.h) file location in the compiler's include path,

(2) have the library stubs (.lib) location in the linker's library path, and have the linker reference the relevant library file.

In order to run the program you need to have the shared libraries (dlls) where the loader can see them, for example in your system32 directory.

Answer 5

There are two kinds of libraries: static and dynamic (or shared.)

Static libraries come in an object format and you link them directly into your application.

Shared or dynamic libraries reside in a seperate file (.dll or .so) which must be present at the time your application is run. They also come with object files you must link against your application, but in this case they contain nothing more than stubs that find and call the runtime binary (the .dll or the .so).

In either case, you must have some header files containing the signatures (declarations) of the library functions, else your code won't compile.

Some 'libraries' are header-only and you need do nothing more than include them. Some consist of header and source files. In that case you should compile and link the sources against your application just as you would do with a source file you wrote.

Answer 6

Here's a brief guide to what happens when you compile and build a basic C project:

• The first stage compiles all your source files - this takes the source files you've written and translates them into what are called object files. At this stage the compiler needs to know the declaration of all functions you use in your code, even in external libraries, so you need to use #include to include the header files of whatever libraries you use. This also means that you need to tell the compiler the location of those header files. With GCC you can use the -I command line to feed in directories to be searched for header files.

• The next stage is to link all the object files together into one executable. At this stage the linker needs to resolve the calls to external libraries. This means you need the library in object form. Most libraries will give you instructions on how to generate this or might supply it ready built. Under Linux the library file is often a .a or .so file, though it might just be a .o. Again you can feed the location of the library's object file to GCC with the -L option.

Thus your command line would look like this:

gcc myProg.c -I/path/to/libpng/include -L/path/to/libpng/lib -lpng -o myProg.exe

(Note that when using the -l command line GCC automatically adds lib to the start of the library, so -lpng causes libpng.a to be linked in.)

Hope that helps.

Answer 7

Also,
you may want to look over Dynamic Loading support in various languages and on various platforms.
This support is very handy in cases when you want to use a library optionally and you don't want your program to fail in case that library is not available.