A GNU configuration name has three parts: the CPU type, the manufacturer's name, and the operating system. configure uses these to pick the list of system-dependent directories to look for. If the `--nfp' option is not passed to configure, the directory machine/fpu is also used. The operating system often has a base operating system; for example, if the operating system is `Linux', the base operating system is `unix/sysv'. The algorithm used to pick the list of directories is simple: configure makes a list of the base operating system, manufacturer, CPU type, and operating system, in that order. It then concatenates all these together with slashes in between, to produce a directory name; for example, the configuration `i686-linux-gnu' results in unix/sysv/linux/i386/i686. configure then tries removing each element of the list in turn, so unix/sysv/linux and unix/sysv are also tried, among others. Since the precise version number of the operating system is often not important, and it would be very inconvenient, for example, to have identical irix6.2 and irix6.3 directories, configure tries successively less specific operating system names by removing trailing suffixes starting with a period.
As an example, here is the complete list of directories that would be tried for the configuration `i686-linux-gnu' (with the crypt and linuxthreads add-on):
sysdeps/i386/elf crypt/sysdeps/unix linuxthreads/sysdeps/unix/sysv/linux linuxthreads/sysdeps/pthread linuxthreads/sysdeps/unix/sysv linuxthreads/sysdeps/unix linuxthreads/sysdeps/i386/i686 linuxthreads/sysdeps/i386 linuxthreads/sysdeps/pthread/no-cmpxchg sysdeps/unix/sysv/linux/i386 sysdeps/unix/sysv/linux sysdeps/gnu sysdeps/unix/common sysdeps/unix/mman sysdeps/unix/inet sysdeps/unix/sysv/i386/i686 sysdeps/unix/sysv/i386 sysdeps/unix/sysv sysdeps/unix/i386 sysdeps/unix sysdeps/posix sysdeps/i386/i686 sysdeps/i386/i486 sysdeps/libm-i387/i686 sysdeps/i386/fpu sysdeps/libm-i387 sysdeps/i386 sysdeps/wordsize-32 sysdeps/ieee754 sysdeps/libm-ieee754 sysdeps/generic
Different machine architectures are conventionally subdirectories at the top level of the sysdeps directory tree. For example, sysdeps/sparc and sysdeps/m68k. These contain files specific to those machine architectures, but not specific to any particular operating system. There might be subdirectories for specializations of those architectures, such as sysdeps/m68k/68020. Code which is specific to the floating-point coprocessor used with a particular machine should go in sysdeps/machine/fpu.
There are a few directories at the top level of the sysdeps hierarchy that are not for particular machine architectures.
floatis IEEE 754 single-precision format, and
doubleis IEEE 754 double-precision format. Usually this directory is referred to in the Implies file in a machine architecture-specific directory, such as m68k/Implies.
socketand related functions on Unix systems. unix/inet/Subdirs enables the inet top-level subdirectory. unix/common implies unix/inet.