Linux Programming Hints

In this column, I'll explore the GNU C Library. The Free Software Foundation (FSF) has written an excellent reference manual, available in an electronic form that can be printed or read on-line, but I think that an introduction will help some people get started.

The GNU C Library is more than a re-implementation of the Standard C Library; while it has all the features of the Standard C Library, it has far more interesting and useful features as well. Unfortunately, it is not necessarily a good idea to use all those features in your programs.

One method that the FSF has used to avoid copyright infringement lawsuits from unhappy commercial vendors has been to remove restrictions and arbitrary limits from the GNU versions of programs. For example, where the standard version of a program might be limited to handling lines less than 4096 characters long, the GNU version is likely to handle lines of any length that memory can hold.

They have followed the same philosophy in their version of the C Library: why not make improvements, so long as the library is still compatible? So where most standard C libraries contain a printf() which causes a segmentation violation when something like printf("%s", NULL) is called, the GNU C library prints (null). This is not a feature used to print (null), but a debugging aid which allows the programmer to find and correct buggy code more easily, without having to inspect core files caused by segmentation violations.

While maintaining POSIX compatibility, the FSF has significantly extended the C library, making it far more useful in the process. Unfortunately, when you use these extensions, your program becomes less portable to other platforms. To make a program generally useful, the GNU C library should be ported to any platform where your program might be useful.

On the other hand, writing good software that requires the GNU C library may encourage the further spread of the GNU C library. It may also make your programs work better, since the better the library the program is built on, the better the program may be; and some of the higher-level functions may allow you to write simpler, more maintainable code. You can spend less effort getting around library limitations. Buggy libraries can waste a lot of a programmer's time, as veteran programmers know. Since the GNU C library has a reputation as a good implementation of the Standard C library, with useful extensions, you may be doing all your fellow programmers a favor by encouraging the spread of the GNU C library.

Another reason to encourage the spread of the GNU C Library is the very fact that it is free software. It can be a tremendous help to be able to read library source when you don't understand what a library function call is doing.

The linux C library is based almost completely on the GNU C library and will probably be merged with the GNU C library eventually. This does not imply that writing programs under linux requires or encourages writing non-portable programs. The -ansi switch for GCC enforces fairly strict ANSI compliance(1), and by default masks references to all the GNU extensions from the header files, so that you can be sure your program is completely portable. Section 1.3.4, Feature Test Macros, in the GNU C Library Reference Manual, explains how to choose which features you want included while using the GNU C Library.

If you write programs based on books like W. Richard Stevens'—Advanced Programming in the Unix Environment, Kernighan and Ritchie's—The C Programming Language, Donald Lewine's—POSIX Programmer's Guide and other such standard references, your code should be portable to many operating systems as well as to linux. However, with linux, you have the choice of using GNU-specific library routines, and of promoting the use of the GNU C library on other platforms as well.

For the rest of the column, we will leave such philosophical ramblings behind and assume that you have chosen to use the GNU C library in all its glory, above and beyond the ANSI standard and that you want an introduction to its extensions so that you know what features are there to be used. I will go through the reference manual, pointing out and briefly explaining many of the useful enhancements of the GNU C library. This is not a coherent discussion of the GNU C library, but a list of extensions that people intending to use the GNU C library for serious programming should know about. This way they can decide whether or not to use the features, rather than being condemned by ignorance to ignore them....

If you find these functions worth using, please look them up in the—GNU C Library Reference Manual. Don't try to use them just from my descriptions here - these descriptions are just to catch your interest. Follow the references instead.