6.10. Adjusting the Toolchain

#### Linux From Scratch - Version 7.10-rc1

### Chapter 6. Installing Basic System Software

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# 6.10. Adjusting the Toolchain

Now that the final C libraries have been installed, it is time to adjust the toolchain so that it will link any newly compiled program against these new libraries.

First, backup the `/tools` linker, and replace it with the adjusted linker we made in chapter 5. We'll also create a link to its counterpart in `/tools/$(uname -m)-pc-linux-gnu/bin`:

    mv -v /tools/bin/{ld,ld-old}
    mv -v /tools/$(uname -m)-pc-linux-gnu/bin/{ld,ld-old}
    mv -v /tools/bin/{ld-new,ld}
    ln -sv /tools/bin/ld /tools/$(uname -m)-pc-linux-gnu/bin/ld

Next, amend the GCC specs file so that it points to the new dynamic linker. Simply deleting all instances of “/tools” should leave us with the correct path to the dynamic linker. Also adjust the specs file so that GCC knows where to find the correct headers and Glibc start files. A sed command accomplishes this:

    gcc -dumpspecs | sed -e 's@/tools@@g'                   \
        -e '/\*startfile_prefix_spec:/{n;s@.*@/usr/lib/ @}' \
        -e '/\*cpp:/{n;s@$@ -isystem /usr/include@}' >      \
        `dirname $(gcc --print-libgcc-file-name)`/specs

It is a good idea to visually inspect the specs file to verify the intended change was actually made.

It is imperative at this point to ensure that the basic functions (compiling and linking) of the adjusted toolchain are working as expected. To do this, perform the following sanity checks:

    echo 'int main(){}' > dummy.c
    cc dummy.c -v -Wl,--verbose &> dummy.log
    readelf -l a.out | grep ': /lib'

There should be no errors, and the output of the last command will be (allowing for platform-specific differences in dynamic linker name):

    [Requesting program interpreter: /lib/ld-linux.so.2]

Note that `/lib` is now the prefix of our dynamic linker.

### Note

On 64-bit systems the interpreter should be /lib64/ld-linux-x86-64.so.2.

Now make sure that we're setup to use the correct start files:

    grep -o '/usr/lib.*/crt[1in].*succeeded' dummy.log

The output of the last command should be:

    /usr/lib/crt1.o succeeded
    /usr/lib/crti.o succeeded
    /usr/lib/crtn.o succeeded

### Note

On 64-bit systems, the path above will be /usr/lib/gcc/x86_64-unknown-linux-gnu/5.3.0/../../../../lib64/. This reduces to /usr/lib64 and /usr/lib64 is a symlink that points to /usr/lib.

Verify that the compiler is searching for the correct header files:

    grep -B1 '^ /usr/include' dummy.log

This command should return the following output:

    #include <...> search starts here:
     /usr/include

Next, verify that the new linker is being used with the correct search paths:

    grep 'SEARCH.*/usr/lib' dummy.log |sed 's|; |\n|g'

References to paths that have components with '-linux-gnu' should be ignored, but otherwise the output of the last command should be:

    SEARCH_DIR("/usr/lib")
    SEARCH_DIR("/lib");

Next make sure that we're using the correct libc:

    grep "/lib.*/libc.so.6 " dummy.log

The output of the last command (allowing for a lib64 directory on 64-bit hosts) should be:

    attempt to open /lib/libc.so.6 succeeded

Lastly, make sure GCC is using the correct dynamic linker:

    grep found dummy.log

The output of the last command should be (allowing for platform-specific differences in dynamic linker name and a lib64 directory on 64-bit hosts):

    found ld-linux.so.2 at /lib/ld-linux.so.2

If the output does not appear as shown above or is not received at all, then something is seriously wrong. Investigate and retrace the steps to find out where the problem is and correct it. The most likely reason is that something went wrong with the specs file adjustment. Any issues will need to be resolved before continuing on with the process.

Once everything is working correctly, clean up the test files:

    rm -v dummy.c a.out dummy.log

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