3.3. Using Sourcery G++ Lite on uClinux Targets

In order to run and debug programs produced by Sourcery G++ on a uClinux target, you must install runtime support files on the target. You may also need to set appropriate build options so that your executables can find the correct dynamic linker and libraries at runtime.

The runtime support files, referred to as the sysroot, are found in the sh-uclinux/libc directory of your Sourcery G++ Lite installation. The sysroot consists of the contents of the etc, lib, sbin, and usr directories.

Note for Windows Host Users

The sysroots provided in Windows host packages for Sourcery G++ are not directly usable on the uClinux target because of differences between the Windows and uClinux file systems. Some files that are hard links, or copies, in the sysroot as installed on the Windows file system should be symbolic links on the uClinux target. Additionally, some files in the sysroot that should be marked executable on the uClinux target are not marked executable on Windows. If you intend to use the sysroot provided with Sourcery G++ on a Windows host system as the basis for your uClinux target filesystem, you must correct these issues after copying the sysroot to the target.

You have these choices for installing the sysroot on the target:

Setting the environment variable LD_LIBRARY_PATH on the target is not sufficient, since executables produced by Sourcery G++ depend on the Sourcery G++ dynamic linker included in the sysroot as well as the Sourcery G++ runtime libraries.

3.3.1. Installing the Sysroot

If you are modifying an existing system, rather than creating a new system from scratch, you should place the sysroot files in a new directory, rather than in the root directory of your target system.

If you choose to overwrite your existing C library, you may not be able to boot your system. You should back up your existing system before overwriting the C library and ensure that you can restore the backup even with your system offline.

The mechanism you use for copying the sysroot to your target board depends on its hardware and software configuration. You may be able to use FTP or SSH with a server already running on your target. If your target board does not have networking configured, you may be able to copy files using an SD card or USB memory stick, or via a file transfer utility over a serial line. The instructions that come with your board may include specific suggestions.

When running Sourcery G++ on a GNU/Linux host, as an alternative to copying files to the target system, you may be able to NFS-mount the Sourcery G++ Lite installation directory from your host system on the target system. It is especially convenient for debugging if you can make the sysroot pathname on the target system be identical to that on the GNU/Linux host system; refer to Section 3.4.3, “Setting the Sysroot in the Debugger” for further discussion of this issue.

Otherwise, you must copy files from the sh-uclinux/libc directory of your Sourcery G++ Lite install to the target system. In many cases, you do not need to copy all of the files in the sysroot. For example, the usr/include subdirectory contains files that are only needed if you will actually be running the compiler on your target system. You do not need these files for non-native compilers. You also do not need any .o or .a files; these are used by the compiler when linking programs, but are not needed to run programs. You should definitely copy all .so files and the executable files in usr/bin and sbin.

3.3.2. Using Linker Options to Specify the Sysroot Location

If you have installed the sysroot on the target in a location other than the file system root, you can use the -rpath and --dynamic-linker linker options to specify the sysroot location.

If you are using Sourcery G++ from the command line, follow these steps:

  1. First find the correct sysroot directory, dynamic linker, and library subdirectory for your selected multilib. Refer to Section 3.2, “Library Configurations”. In the following steps, sysroot is the absolute path to the sysroot directory on the target corresponding to your selected multilib. For the default multilib, the dynamic linker path relative to the sysroot is lib/ld-uClibc.so.0, and the library subdirectory is lib. This is used in the example below.

  2. When invoking sh-uclinux-gcc to link your executable, include the command-line options:

    -Wl,-rpath=sysroot/lib:sysroot/usr/lib \
    -Wl,--dynamic-linker=sysroot/lib/ld-uClibc.so.0

    where sysroot is the absolute path to the sysroot directory on the target corresponding to your selected multilib.

  3. Copy the executable to the target and execute it normally.

Note that if you specify an incorrect path for --dynamic-linker, the common failure mode seen when running your application on the target is similar to

> ./factorial
./factorial: No such file or directory

or

> ./factorial
./factorial: bad ELF interpreter: No such file or directory

This can be quite confusing since it appears from the error message as if it is the ./factorial executable that is missing rather than the dynamic linker it references.