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5.3. Toolchain Technical Notes

This section explains some of the rationale and technical details behind the overall build method. It is not essential to immediately understand everything in this section. Most of this information will be clearer after performing an actual build. This section can be referred back to at any time during the process.

The overall goal of Chapter 5 is to provide a temporary environment that can be chrooted into and from which can be produced a clean, trouble-free build of the target LFS system in Chapter 6. Along the way, we separate from the host system as much as possible, and in doing so, 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, be aware of the name of the working platform, often referred to as the target triplet. Many times, the target triplet will probably be i686-pc-linux-gnu. A simple way to determine the name of the 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.

Also be aware of the name of the platform's dynamic linker, often referred to as the dynamic loader (not to be confused with the standard linker ld that is part of Binutils). The dynamic linker provided by Glibc finds and loads the shared libraries needed by a program, prepares the program to run, and then runs it. The name of the dynamic linker will usually be ld-linux.so.2. On platforms that are less prevalent, the name might be ld.so.1, and newer 64 bit platforms might be named something else entirely. The name of the platform's dynamic linker can be determined by looking in the /lib directory on the host system. A sure-fire way to determine the name is to inspect a random binary from the 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:

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 an entire distribution. A test suite failure will usually alert this error before too much additional work is performed.

Binutils installs its assembler and linker in two locations, /tools/bin and /tools/$TARGET_TRIPLET/bin. 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, an ld --verbose | grep SEARCH will illustrate the current search paths and their order. It shows which files are 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 all the files successfully opened during the linking.

The next package installed is GCC. An example of what can be seen during its run of ./configure is:

checking what assembler to use... 
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. To find out which standard linker gcc will use, run: gcc -print-prog-name=ld.

Detailed information can be obtained from gcc by passing it the -v command line option while compiling a dummy program. For example, gcc -v dummy.c will show detailed information about the preprocessor, compilation, and assembly stages, including gcc's included 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 not an issue since Glibc will always use the gcc found in a PATH directory. The binary tools and kernel headers can be a bit more complicated. Therefore, take no risks and use the available configure switches to enforce the correct selections. After the run of ./configure, check the contents of the config.make file in the glibc-build directory for all important details. Note the use of CC="gcc -B/tools/bin/" to control which binary tools are used and the use of the -nostdinc and -isystem flags to control the compiler's include search path. These items 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, make some adjustments to ensure that searching and linking take place only within the /tools prefix. Install an adjusted ld, which has a hard-wired search path limited to /tools/lib. Then amend gcc's specs file to point to the 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 and Link Format (ELF)-shared executable. This can be inspected by running: readelf -l <name of binary> | grep interpreter. Amending gcc's specs file ensures that every program compiled from here through the end of this chapter will use the new dynamic linker in /tools/lib.

The need to use the new dynamic linker is also the reason why the Specs patch is applied 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 the 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 Chapter 5 packages all build against the new Glibc in /tools.

Upon entering the chroot environment in Chapter 6, the first major package to be installed is Glibc, due to its self-sufficient nature mentioned above. Once this Glibc is installed into /usr, perform a quick changeover of the toolchain defaults, then proceed in building the rest of the target LFS system.

5.3.1. Notes on Static Linking

Besides their specific task, most programs have to perform many common and sometimes trivial operations. These include allocating memory, searching directories, reading and writing files, string handling, pattern matching, arithmetic, and 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, it includes 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 option 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, only one copy of the executable library code is needed on the hard disk, instead of having multiple copies of the same code included in several 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, only the one library needs to be recompiled instead of recompiling all 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. The historical reason is that earlier versions of LFS statically linked every program in this chapter. Educationally, knowing the difference between static and dynamic linking is useful. The technical benefit is a gained element of independence from the host, meaning that those programs can be used independently of the host system. However, it is worth noting that an overall successful LFS build can still be achieved when the first two packages are built dynamically.