2.6.1 Support for ISO C or POSIX functions.

When a function is not implemented by a system, the Gnulib module provides an implementation under the same name. Examples are the ‘snprintf’ and ‘readlink’ modules.

Similarly, when a function is not correctly implemented by a system, Gnulib provides a replacement. For functions, we use the pattern

#if !HAVE_WORKING_FOO
# define foo rpl_foo
#endif

and implement the foo function under the name rpl_foo. This renaming is needed to avoid conflicts at compile time (in case the system header files declare foo) and at link/run time (because the code making use of foo could end up residing in a shared library, and the executable program using this library could be defining foo itself).

For header files, such as stdint.h, we provide the substitute only if the system doesn’t provide a correct one. The template of this replacement is distributed in a slightly different name, with ‘.in’ inserted before the ‘.h’ extension, so that on systems which do provide a correct header file the system’s one is used.

The modules in this category are supported in C++ mode as well. This means, while the autoconfiguration uses the C compiler, the resulting header files and function substitutes can be used with a matching C++ compiler as well.

2.6.2 Enhancements of ISO C or POSIX functions

These are sometimes POSIX functions with GNU extensions also found in glibc—examples: ‘getopt’, ‘fnmatch’—and often new APIs—for example, for all functions that allocate memory in one way or the other, we have variants which also include the error checking against the out-of-memory condition.

2.6.3 Portable general use facilities

Examples are a module for copying a file—the portability problems relate to the copying of the file’s modification time, access rights, and extended attributes—or a module for extracting the tail component of a file name—here the portability to native Windows requires a different API than the classical POSIX basename function.

2.6.4 Reusable application code

Examples are an error reporting function, a module that allows output of numbers with K/M/G suffixes, or cryptographic facilities.

2.6.5 Object oriented classes

Examples are data structures like ‘list’, or abstract output stream classes that work around the fact that an application cannot implement an stdio FILE with its logic. Here, while staying in C, we use implementation techniques like tables of function pointers, known from the C++ language or from the Linux kernel.

2.6.6 Interfaces to external libraries

Examples are the ‘iconv’ module, which interfaces to the iconv facility, regardless whether it is contained in libc or in an external libiconv. Or the ‘readline’ module, which interfaces to the GNU readline library.

2.6.7 Build / maintenance infrastructure

An example is the ‘maintainer-makefile’ module, which provides extra Makefile tags for maintaining a package.