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Suppose I have a program compiled that's linked against libc.so.6 on one system (e.g., Ubuntu x64). If I take this binary to another system that has the same libc version 6 (e.g., Fedora x64), it seems to run fine.

My question is, is it more-or-less a contract that the ABI is stable as long as the so versions are identical?

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No, it's not necessary that shared libraries be ABI compatible cross-distro. I think it's a reasonable expectation, but ABI breaks happen and you can't rely on ABI compatibility blindly.

An example would be libstdc++. On Ubuntu 16.04 (with GCC 5) and CentOS 7 (with GCC 4.8), the respective sos fall on opposite sides of an ABI break. Software compiled on Ubuntu 16.04 with C++ but without _GLIBCXX_USE_CXX11_ABI set would not run on CentOS 7. They're both still libstc++.so.6. ABI breaks may also happen with bug-fix patches applied by distro maintainers, who would not usually bump the so major version numbers for bug fixes.

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    Out of interest, would this depend on the compiler alone or on the combination of compiler and Linux distribution? I.e., could this still be a problem within a single Linux distribution? – Kusalananda Apr 12 '18 at 5:43
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    @Kusalananda well, the bug-fix breaking ABI would be within the same release of the same distro, but I don't have any good examples of that handy. – muru Apr 12 '18 at 7:46
  • @muru Thank you. Just out of curiosity, is there a similar ABI rule for libc? I tried googling this but my google-fu is weak :( – wbkang Apr 12 '18 at 14:29
  • @wbkang there's the musl vs glibc problem. – muru Apr 12 '18 at 17:56

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