2017-05-04 11:12:49 +00:00
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# The problem
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Some projects require newer C++ standards to build them. To keep the glibc dependency low you can
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build a newer GCC version on an older distro and use it to compile the project.
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This project however will now require a newer version of the `libstdc++.so.6` library than available on that distro.
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Bundling `libstdc++.so.6` however will in most cases break compatibility with distros that have a newer library
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version installed into their system than the bundled one. So blindly bundling the library is not reliable.
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By the way, while this is primarily an issue with `libstdc++.so.6` in some rare cases this might also occur with `libgcc_s.so.1`.
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That's because both libraries are 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 to use a bundled library or not before the
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application is started. This is exactly what the patched AppRun binary does.
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It will search for `usr/optional/libstdc++/libstdc++.so.6` and `usr/optional/libgcc_s/libgcc_s.so.1` inside the AppImage or AppDir.
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2017-12-06 13:37:47 +00:00
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If found it will compare their internal versions with the ones found on the system and prepend their paths to `LD_LIBRARY_PATH` if necessary.
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2018-01-11 12:17:08 +00:00
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You should also put `exec.so` into `usr/optional`. This exec.so library is intended to restore the environment of the AppImage to its parent.
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This is done to avoid library clashing of bundled libraries with external processes. e.g when 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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