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#### D.2.1 Layout of the `sysdeps` Directory Hierarchy

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.

`generic`
  As described above (see Porting), this is the subdirectory that every configuration implicitly uses after all others.
`ieee754`
  This directory is for code using the IEEE 754 floating-point format, where the C type `float` is IEEE 754 single-precision format, and `double` is 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`.
`libm-ieee754`
  This directory contains an implementation of a mathematical library usable on platforms which use IEEE 754 conformant floating-point arithmetic.
`libm-i387`
  This is a special case. Ideally the code should be in `sysdeps/i386/fpu` but for various reasons it is kept aside.
`posix`
  This directory contains implementations of things in the library in terms of POSIX.1 functions. This includes some of the POSIX.1 functions themselves. Of course, POSIX.1 cannot be completely implemented in terms of itself, so a configuration using just `posix` cannot be complete.
`unix`
  This is the directory for Unix-like things. See Porting to Unix. `unix` implies `posix`. There are some special-purpose subdirectories of `unix`:
  `unix/common`
    This directory is for things common to both BSD and System V release 4. Both `unix/bsd` and `unix/sysv/sysv4` imply `unix/common`.
  `unix/inet`
    This directory is for `socket` and related functions on Unix systems. `unix/inet/Subdirs` enables the `inet` top-level subdirectory. `unix/common` implies `unix/inet`.
`mach`
  This is the directory for things based on the Mach microkernel from CMU (including GNU/Hurd systems). Other basic operating systems (VMS, for example) would have their own directories at the top level of the `sysdeps` hierarchy, parallel to `unix` and `mach`.