Tip 2: Use libraries

03 October 11. [link] PDF version

Part of a series of tips on POSIX and C. Start from the tip intro page, or get 21st Century C, the book based on this series.

level: You have the syntax down and want to get real work done
purpose: Use 40 years of prior scholarship to your advantage

Twenty years ago, it was evidently pretty difficult to pull down a good library of functions and make use of them in your current project. I say this because I couldn't find any C tutorials from the period that show you how to use a non-standard library to do real work. Which is why you can find C detractors who will say self-dissonant things like C is forty years old, so you have to write every procedure from scratch in it.

Now, it's easy. We have the GNU to thank for much of this, because free libraries now outnumber for-pay libraries, and so there are package managers and other such systems to let you pull down a library with a few mouse clicks. If you have to create windows for your program, deal with XML, encode audio streams to MP3, or manipulate DNA sequences, ask your package manager for a library before you start from zero.

Besides the politics of free/open source/libre/whatever, the GNU also has a set of tools that will prepare a library for use on any machine, by testing for every known quirk and implementing the appropriate workaround, collectively known as the autotools.

Writing a package to work under autotools is, um, hellish, but the user's life is much easier as a result. Also, it's a logical extension to Tip #1, because now that you know that having a makefile will simplify compilation, it's only logical that you'd use a tool to generate a makefile for you.

To do:
Let's try a sample package, shall we? The GNU Scientific Library includes a host of numeric computation routines. If you ever read somebody asking a question that starts I'm trying to implement something from Numeric Recipes in C..., the correct response is download the GSL, because they already did it for you.

One of the things I ♥ about using POSIX1 is that I can give people unambiguous and quick tech support over IM. No click here, then look for this button stuff, just paste this onto the command line (you have root privileges on your computer): 

wget ftp://ftp.gnu.org/gnu/gsl/gsl-1.15.tar.gz #download
tar xvzf gsl-*gz    #unzip
cd gsl-1.15
./configure         #determine the quirks of your machine
make                #compile
make install        #install to the right location---if you have permissions

If you get an error about a missing program, then use your package manager to obtain it and start over. Package managers are one of those places where I can't just tell you what to type, which is one solid reason why I'm not using one here.

If you are reading this on your laptop, then you probably have root privileges, and this will work fine. If you are at work and using a shared server, the odds are low that you have superuser rights. If you don't have superuser rights, then hold your breath for two days until Tip #3.

Now you'll need to indicate in your makefile that you will be linking programs you write to the library you just installed. The makefile from Tip #1 had a blank LIBS line; this is where you start filling it in.

If you have pkg-config on hand, then use it like so: 

LIBS=`pkg-config --libs gsl`
When you add new libraries, add them to the list like so: 
LIBS=`pkg-config --libs gsl sqlite3 apophenia`

If you're on a system without pkg-config, you'll need to explicitly specify which libraries you need: 

LIBS=-lgsl -lgslcblas -lm
Every time you install a new library, you will always need to add at least one item to the LIBS, like the -lgsl part. The GSL has a quirk that it requires a BLAS (basic linear algebra system), and -lm is the standard math library.

Did it install? The numeric integration documentation has a sample program that integrates the function specified at the top of the file. It's a good example because I get the impression that numeric integration is the sort of thing that I feel people often re-implement in C (and I already have you reading the manual--there's a lot there). Paste it into a file and use your library-improved makefile to test and install it.

Footnotes

... POSIX1
UNIX is a trademark of AT&T; POSIX (Portable Operating System Interface (the X goes uneXplained)) is a more general descriptor for things that are UNIX-like, including Linux, BSD, Mac OS X, &c.

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