ii. Toolchain Technical Notes

About Cross-Compilation

Cross-compilation involves some concepts that deserve a section of their own. Although this section may be omitted on a first reading, coming back to it later will help you gain a fuller understanding of the process.

Let us first define some terms used in this context.

As an example, let us imagine the following scenario (sometimes referred to as “Canadian Cross”): we have a compiler on a slow machine only, let's call it machine A, and the compiler ccA. We also have a fast machine (B), but no compiler for (B), and we want to produce code for a third, slow machine (C). We will build a compiler for machine C in three stages.

Then, all the programs needed by machine C can be compiled using cc2 on the fast machine B. Note that unless B can run programs produced for C, there is no way to test the newly built programs until machine C itself is running. For example, to run a test suite on ccC, we may want to add a fourth stage:

In the example above, only cc1 and cc2 are cross-compilers, that is, they produce code for a machine different from the one they are run on. The other compilers ccA and ccC produce code for the machine they are run on. Such compilers are called native compilers.

Implementation of Cross-Compilation for LFS

In order to fake a cross compilation in LFS, the name of the host triplet is slightly adjusted by changing the "vendor" field in the LFS_TGT variable so it says "lfs". We also use the --with-sysroot option when building the cross linker and cross compiler to tell them where to find the needed host files. This ensures that none of the other programs built in Chapter 6 can link to libraries on the build machine. Only two stages are mandatory, plus one more for tests.

In the preceding table, “on pc” means the commands are run on a machine using the already installed distribution. “On lfs” means the commands are run in a chrooted environment.

Now, there is more about cross-compiling: the C language is not just a compiler, but also defines a standard library. In this book, the GNU C library, named glibc, is used (there is an alternative, "musl"). This library must be compiled for the LFS machine; that is, using the cross compiler cc1. But the compiler itself uses an internal library implementing complex subroutines for functions not available in the assembler instruction set. This internal library is named libgcc, and it must be linked to the glibc library to be fully functional! Furthermore, the standard library for C++ (libstdc++) must also be linked with glibc. The solution to this chicken and egg problem is first to build a degraded cc1-based libgcc, lacking some functionalities such as threads and exception handling, and then to build glibc using this degraded compiler (glibc itself is not degraded), and also to build libstdc++. This last library will lack some of the functionality of libgcc.

This is not the end of the story: the upshot of the preceding paragraph is that cc1 is unable to build a fully functional libstdc++, but this is the only compiler available for building the C/C++ libraries during stage 2! Of course, the compiler built during stage 2, cc-lfs, would be able to build those libraries, but (1) the build system of gcc does not know that it is usable on pc, and (2) using it on pc would create a risk of linking to the pc libraries, since cc-lfs is a native compiler. So we have to re-build libstdc++ later as a part of gcc stage 2.

In Chapter 8 (or “stage 3”), all packages needed for the LFS system are built. Even if a package is already installed into the LFS system in a previous chapter, we still rebuild the package unless we are completely sure it's unnecessary. The main reason for rebuilding these packages is to settle them down: if we reinstall a LFS package on a complete LFS system, the installed content of the package should be same as the content of the same package installed in Chapter 8. The temporary packages installed in Chapter 6 or Chapter 7 cannot satisify this expectation because some of them are built without optional dependencies installed, and autoconf cannot perform some feature checks in Chapter 6 because of cross compilation, causing the temporary packages to lack optional features or use suboptimal code routines. Additionally, a minor reason for rebuilding the packages is allowing to run the testsuite.

Other procedural details

The cross-compiler will be installed in a separate $LFS/tools directory, since it will not be part of the final system.

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 realize at first. 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 highlight this error before too much additional work is performed.

Binutils installs its assembler and linker in two locations, $LFS/tools/bin and $LFS/tools/$LFS_TGT/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, $LFS_TGT-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, $LFS_TGT-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... /mnt/lfs/tools/i686-lfs-linux-gnu/bin/as
checking what linker to use... /mnt/lfs/tools/i686-lfs-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: $LFS_TGT-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.

Next installed are sanitized Linux API headers. These allow the standard C library (glibc) to interface with features that the Linux kernel will provide.

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 compiler relating to the --host parameter passed to its configure script; e.g. in our case, the compiler will be $LFS_TGT-gcc. The binary tools and kernel headers can be a bit more complicated. Therefore, we take no risks and use the available configure switches to enforce the correct selections. After the run of configure, check the contents of the config.make file in the build directory for all important details. Note the use of CC="$LFS_TGT-gcc" (with $LFS_TGT expanded) 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.

As mentioned above, the standard C++ library is compiled next, followed in Chapter 6 by other programs that need to be cross compiled for breaking circular dependencies at build time. The install step of all those packages uses the DESTDIR variable to force installation in the LFS filesystem.

At the end of Chapter 6 the native LFS compiler is installed. First binutils-pass2 is built, in the same DESTDIR directory as the other programs, then the second pass of gcc is constructed, omitting some non-critical libraries. Due to some weird logic in gcc's configure script, CC_FOR_TARGET ends up as cc when the host is the same as the target, but different from the build system. This is why CC_FOR_TARGET=$LFS_TGT-gcc is declared explicitly as one of the configuration options.

Upon entering the chroot environment in Chapter 7, the temporary installations of programs needed for the proper operation of the toolchain are performed. From this point onwards, the core toolchain is self-contained and self-hosted. In Chapter 8, final versions of all the packages needed for a fully functional system are built, tested and installed.