From 98c95f5392702d4174ba3f833c8e5dde0535c1b8 Mon Sep 17 00:00:00 2001 From: Greg Schafer Date: Mon, 6 Oct 2003 04:00:40 +0000 Subject: Chapter 5: Add new section "Toolchain technical notes". Integrate and scale back the old "Why we use static linking" section. Closes Bug 658. git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@2927 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689 --- chapter01/changelog.xml | 4 + chapter05/chapter05.xml | 2 +- chapter05/toolchaintechnotes.xml | 192 +++++++++++++++++++++++++++++++++++++++ chapter05/whystatic.xml | 61 ------------- entities/chapter05.ent | 2 +- index.xml | 4 +- 6 files changed, 200 insertions(+), 65 deletions(-) create mode 100644 chapter05/toolchaintechnotes.xml delete mode 100644 chapter05/whystatic.xml diff --git a/chapter01/changelog.xml b/chapter01/changelog.xml index b37d1aac4..050dc0acf 100644 --- a/chapter01/changelog.xml +++ b/chapter01/changelog.xml @@ -95,6 +95,10 @@ +October 5th, 2003 [greg]: Chapter 5: Add new section "Toolchain +technical notes". Integrate and scale back the old "Why we use static linking" +section. Closes Bug 658. + October 4th, 2003 [greg]: Chapter 5 - Binutils Pass 1: Add extra LDFLAGS to ensure static rebuild of ld. diff --git a/chapter05/chapter05.xml b/chapter05/chapter05.xml index b5e9a89f2..4f1bc31b1 100644 --- a/chapter05/chapter05.xml +++ b/chapter05/chapter05.xml @@ -3,7 +3,7 @@ &c5-introduction; -&c5-whystatic; +&c5-toolchaintechnotes; &c5-creatingtoolsdir; &c5-addinguser; &c5-settingenviron; diff --git a/chapter05/toolchaintechnotes.xml b/chapter05/toolchaintechnotes.xml new file mode 100644 index 000000000..1308e9f80 --- /dev/null +++ b/chapter05/toolchaintechnotes.xml @@ -0,0 +1,192 @@ + +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 Chapter 5 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 Chapter 6. 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 in such a way so as to minimize the risks for +new readers and also provide maximum educational value at the same time. In +other words, more advanced techniques could be used to achieve the same +goals. + + +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 be, for example: +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 +surefire 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 Chapter 5 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 GCC which target dynamic linker will be used. + + +Binutils is installed first because both GCC and Glibc perform various +feature tests on the assembler and linker during their respective runs of +./configure 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 a build of a whole +distribution. Thankfully, a test suite failure will usually alert us before too +much harm is done. + +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 ld +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 order. You can see what files are actually linked by ld by +compiling a dummy program and passing the --verbose switch. For example: +`gcc dummy.c -Wl,--verbose 2>&1 | grep succeeded` +will show you all the files successfully opened during the link. + +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 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 it 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 we 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 GCC 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 executable binary. +You can inspect this by running: +`readelf -l <name of binary> | grep interpreter`. +By amending the GCC specs file, we are ensuring that every program compiled from +here through the end of Chapter 5 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 dynamic linker from the host system's +/lib directory embedded into them, which +would defeat our goal of getting away from the host system. + +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 Chapter 5 packages all build against the new +Glibc in /tools and all is well. + +Upon entering the chroot environment in Chapter 6, 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 Chapter 6 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. + +Why do we statically link the first two packages in Chapter 5? The reasons +are threefold: historical, educational and technical. Historical because earlier +versions of LFS statically linked every program in Chapter 5. Educational +because knowing the difference is useful. Technical because we gain an element +of independence from the host in doing so, i.e. 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. + + + + + diff --git a/chapter05/whystatic.xml b/chapter05/whystatic.xml deleted file mode 100644 index 1b07b0b2e..000000000 --- a/chapter05/whystatic.xml +++ /dev/null @@ -1,61 +0,0 @@ - -Why we use static linking - - -Most programs have to perform, beside their specific task, many rather -common and trivial operations, such as allocating memory, searching -directories, opening and closing files, reading and writing them, string -handling, pattern matching, arithmetic, and so on. Instead of obliging each -program to reinvent the wheel, the GNU system provides all these basic -functions ready-made in libraries. The major library on any Linux system is -glibc. To get an idea of what it contains, have a look at -glibc/index.html somewhere on your host system. - -There are two 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 linker, the name of the library, and -the name of the function, resulting in a much smaller executable. Under -certain circumstances, this executable can have the disadvantage of being -somewhat slower than a statically linked one, as the linking at run time takes -a few moments. It should be noted, however, that under normal circumstances on -today's hardware, a dynamically linked executable will be faster than a -statically linked one as the library function being called by the dynamically -linked executable has a good chance of already being loaded in your system's -RAM. - -Aside from this small drawback, dynamic linking has two 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. - -Nowadays saving a few megabytes of space may not seem like much, but -many moons ago, when disks were measured in megabytes and core in kilobytes, -such savings were essential. It meant being able to keep several programs in -core at the same time and to contain an entire Unix system on just a few disk -volumes. - -A third but minor advantage of dynamic linking is that 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. - -In summary we can say that dynamic linking trades run time against -memory space, disk space, and recompile time. - -But if dynamic linking saves so much space, why then are we linking -the first two packages in this chapter statically? The reason is to make them -independent from the libraries on your host system. The advantage is that, if -you are pressed for time, you could skip the second passes over GCC and -Binutils, and just use the static versions to compile the rest of this chapter -and the first few packages in the next. In the next chapter we will be -chrooted to the LFS partition and once inside the chroot environment, the host -system's Glibc won't be available, thus the programs from GCC and Binutils -will need to be self-contained, i.e. statically linked. However, we strongly -advise against skipping the second passes. - - - diff --git a/entities/chapter05.ent b/entities/chapter05.ent index e0d0dfc24..0593644be 100644 --- a/entities/chapter05.ent +++ b/entities/chapter05.ent @@ -1,6 +1,6 @@ - + diff --git a/index.xml b/index.xml index a8e4ce6fe..97fc7157c 100644 --- a/index.xml +++ b/index.xml @@ -3,8 +3,8 @@ "/usr/share/docbook/docbookx.dtd" [ - - + + -- cgit v1.2.3-54-g00ecf