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58 lines
2.5 KiB
Markdown
58 lines
2.5 KiB
Markdown
# The problem
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Some projects require newer C++ standards to build them. To keep the glibc
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dependency low you can build a newer GCC version on an older distro and use it
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to compile the project. This project however will now require a newer version of
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the `libstdc++.so.6` library than available on that distro.
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Blindly bundling `libstdc++.so.6` however will in most cases break compatibility
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with distros that have a newer library version installed into their system than
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the bundled one.
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By the way, while this is primarily an issue with `libstdc++.so.6` in some cases
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this might also occur with `libgcc_s.so.1`. That's because both libraries are
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part of GCC.
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# The solution
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You would have to know the library version of the host system and decide whether
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to use a bundled library or not before the application is started. This is
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exactly what the `checkrt` tool does. It will search for a bundled `libstdc++.so.6`
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and `libgcc_s.so.1` library inside sub-directories next to it. If found it will
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compare their internal versions with the ones found on the system and return a
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path that can be prepended to `LD_LIBRARY_PATH`.
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# Usage
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Compile by running `make` inside the `src` directory.
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Inside your AppDir create the directory `usr/optional/`
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and put the binary `checkrt` inside there. Execute `usr/optional/checkrt --copy-libraries`
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to bundle your system's `libstdc++.so.6` and `libgcc_s.so.1` libraries.
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Use the provided AppRun script or write/modify your own. `checkrt` will either
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return a path that can be added to `LD_LIBRARY_PATH` or an empty string.
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Error messages, help and verbosity will always be returned to stderr.
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# exec.so
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You might also want to bundle `exec.so` inside `usr/optional/`. This library is
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intended to restore the environment of the AppImage to its parent. This is done
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to avoid library clashing of bundled libraries with external processes, e.g. when
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running the web browser.
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The intended usage is as follows:
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1. This library is injected to the dynamic loader through LD_PRELOAD
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automatically in AppRun **only** if `usr/optional/exec.so` exists:
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e.g `LD_PRELOAD=$APPDIR/usr/optional/exec.so My.AppImage`
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2. This library will intercept calls to new processes and will detect whether
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those calls are for binaries within the AppImage bundle or external ones.
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3. In case it's an internal process, it will not change anything.
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In case it's an external process, it will restore the environment of
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the AppImage parent by reading `/proc/[pid]/environ`.
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This is the conservative approach taken.
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