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.
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'.
socket
and related functions on Unix systems.
`unix/inet/Subdirs' enables the `inet' top-level subdirectory.
`unix/common' implies `unix/inet'.
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