From 242448316a5872eb33374716ac12408e9bf2f5ac Mon Sep 17 00:00:00 2001 From: Manuel Canales Esparcia Date: Mon, 20 Dec 2004 17:49:20 +0000 Subject: Removed text in chapter 05 - last round. git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@4434 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689 --- chapter05/toolchaintechnotes.xml | 193 +-------------------------------------- 1 file changed, 1 insertion(+), 192 deletions(-) (limited to 'chapter05/toolchaintechnotes.xml') diff --git a/chapter05/toolchaintechnotes.xml b/chapter05/toolchaintechnotes.xml index 666e181e2..c644ba162 100644 --- a/chapter05/toolchaintechnotes.xml +++ b/chapter05/toolchaintechnotes.xml @@ -7,197 +7,6 @@ Toolchain technical notes -This section attempts to explain some of the rationale and technical -details behind the overall build method. It's not essential that you understand -everything here immediately. Most of it will make sense once you have performed -an actual build. Feel free to refer back here at any time. - -The overall goal of is to provide a sane, -temporary environment that we can chroot into, and from which we can produce a -clean, trouble-free build of the target LFS system in -. Along the way, we attempt to divorce ourselves -from the host system as much as possible, and in so doing build a -self-contained and self-hosted toolchain. It should be noted that the -build process has been designed to minimize the risks for -new readers and provide maximum educational value at the same time. In other -words, more advanced techniques could be used to build the system. - - -Before continuing, you really should be aware of the name of your working -platform, often also referred to as the target triplet. For -many folks the target triplet will probably be -i686-pc-linux-gnu. A simple way to determine your target -triplet is to run the config.guess script that comes with -the source for many packages. Unpack the Binutils sources and run the script: -./config.guess and note the output. - -You'll also need to be aware of the name of your platform's -dynamic linker, often also referred to as the -dynamic loader, not to be confused with the standard linker -ld that is part of Binutils. The dynamic linker is provided -by Glibc and has the job of finding and loading the shared libraries needed by a -program, preparing the program to run and then running it. For most folks the -name of the dynamic linker will be ld-linux.so.2. On -platforms that are less prevalent, the name might be -ld.so.1 and newer 64 bit platforms might even have -something completely different. You should be able to determine the name -of your platform's dynamic linker by looking in the -/lib directory on your host system. A -sure-fire way is to inspect a random binary from your host system by running: -readelf -l <name of binary> | grep interpreter -and noting the output. The authoritative reference covering all platforms is in -the shlib-versions file in the root of the Glibc source -tree. - - -Some key technical points of how the build -method works: - - -Similar in principle to cross compiling whereby tools installed -into the same prefix work in cooperation and thus utilize a little GNU -magic. - -Careful manipulation of the standard linker's library search -path to ensure programs are linked only against libraries we -choose. - -Careful manipulation of gcc's -specs file to tell the compiler which target dynamic -linker will be used. - - -Binutils is installed first because the ./configure runs of both GCC and Glibc perform various -feature tests on the assembler and linker -to determine which software features to enable -or disable. This is more important than one might first realize. An incorrectly -configured GCC or Glibc can result in a subtly broken toolchain where the impact -of such breakage might not show up until near the end of the build of a whole -distribution. Thankfully, a test suite failure will usually alert us before too -much time is wasted. - -Binutils installs its assembler and linker into two locations, -/tools/bin and -/tools/$TARGET_TRIPLET/bin. In reality, -the tools in one location are hard linked to the other. An important facet of -the linker is its library search order. Detailed information can be obtained -from ld by passing it the --verbose -flag. For example: ld --verbose | grep SEARCH will -show you the current search paths and their order. You can see what files are -actually linked by ld by compiling a dummy program and -passing the --verbose switch to the linker. For example: -gcc dummy.c -Wl,--verbose 2>&1 | grep succeeded -will show you all the files successfully opened during the linking. - -The next package installed is GCC and during its run of -./configure you'll see, for example: - -
checking what assembler to use... /tools/i686-pc-linux-gnu/bin/as -checking what linker to use... /tools/i686-pc-linux-gnu/bin/ld
- -This is important for the reasons mentioned above. It also demonstrates -that GCC's configure script does not search the PATH directories to find which -tools to use. However, during the actual operation of gcc -itself, the same search paths are not necessarily used. You can find out which -standard linker gcc will use by running: -gcc -print-prog-name=ld. -Detailed information can be obtained from gcc by passing -it the -v flag while compiling a dummy program. For -example: gcc -v dummy.c will show you detailed -information about the preprocessor, compilation and assembly stages, including -gcc's include search paths and their order. - -The next package installed is Glibc. The most important considerations for -building Glibc are the compiler, binary tools and kernel headers. The compiler -is generally no problem as Glibc will always use the gcc -found in a PATH directory. The binary tools and kernel headers can be a little -more troublesome. Therefore we take no risks and use the available configure -switches to enforce the correct selections. After the run of -./configure you can check the contents of the -config.make file in the -glibc-build directory for all the -important details. You'll note some interesting items like the use of -CC="gcc -B/tools/bin/" to control which binary tools are -used, and also the use of the -nostdinc and --isystem flags to control the compiler's include search -path. These items help to highlight an important aspect of the Glibc package: -it is very self-sufficient in terms of its build machinery and generally does -not rely on toolchain defaults. - -After the Glibc installation, we make some adjustments to ensure that -searching and linking take place only within our /tools -prefix. We install an adjusted ld, which has a hard-wired -search path limited to /tools/lib. Then -we amend gcc's specs file to point to our new dynamic -linker in /tools/lib. This last step is -vital to the whole process. As mentioned above, a -hard-wired path to a dynamic linker is embedded into every ELF shared -executable. You can inspect this by running: -readelf -l <name of binary> | grep interpreter. -By amending gcc's specs file, we are ensuring that every -program compiled from here through the end of this chapter will use our new -dynamic linker in /tools/lib. - -The need to use the new dynamic linker is also the reason why we apply the -Specs patch for the second pass of GCC. Failure to do so will result in the GCC -programs themselves having the name of the dynamic linker from the host system's -/lib directory embedded into them, which -would defeat our goal of getting away from the host. - -During the second pass of Binutils, we are able to utilize the ---with-lib-path configure switch to control -ld's library search path. From this point onwards, the -core toolchain is self-contained and self-hosted. The remainder of the - packages all build against the new Glibc in -/tools and all is well. - -Upon entering the chroot environment in , the -first major package we install is Glibc, due to its self-sufficient nature that -we mentioned above. Once this Glibc is installed into -/usr, we perform a quick changeover of -the toolchain defaults, then proceed for real in building the rest of the -target LFS system. - - -Notes on static linking - -Most programs have to perform, beside their specific task, many rather -common and sometimes trivial operations. These include allocating memory, -searching directories, reading and writing files, string handling, pattern -matching, arithmetic and many other tasks. Instead of obliging each program to -reinvent the wheel, the GNU system provides all these basic functions in -ready-made libraries. The major library on any Linux system is -Glibc. - -There are two primary ways of linking the functions from a library to a -program that uses them: statically or dynamically. When a program is linked -statically, the code of the used functions is included in the executable, -resulting in a rather bulky program. When a program is dynamically linked, what -is included is a reference to the dynamic linker, the name of the library, and -the name of the function, resulting in a much smaller executable. (A third way -is to use the programming interface of the dynamic linker. See the -dlopen man page for more information.) - -Dynamic linking is the default on Linux and has three major advantages -over static linking. First, you need only one copy of the executable library -code on your hard disk, instead of having many copies of the same code included -into a whole bunch of programs -- thus saving disk space. Second, when several -programs use the same library function at the same time, only one copy of the -function's code is required in core -- thus saving memory space. Third, when a -library function gets a bug fixed or is otherwise improved, you only need to -recompile this one library, instead of having to recompile all the programs that -make use of the improved function. - -If dynamic linking has several advantages, why then do we statically link -the first two packages in this chapter? The reasons are threefold: historical, -educational, and technical. Historical, because earlier versions of LFS -statically linked every program in this chapter. Educational, because knowing -the difference is useful. Technical, because we gain an element of independence -from the host in doing so, meaning that those programs can be used -independently of the host system. However, it's worth noting that an overall -successful LFS build can still be achieved when the first two packages are -built dynamically. - - +See testing -- cgit v1.2.3-54-g00ecf