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Qt + C++ + C gurus, I could some advice:

I started writing a Qt6 GUI C++ app that includes some extern C code. Much to my surprise, upon launch the ui->setupUi(this) call immediately segfaulted and I found myself inside of one of the extern C functions that was imported! This is literally at the first line of my MainWindow constructor - with no calls to any of the C code even added yet.

The C function where the segfault dumped me was named "crc32". I temporarily renamed it (and all calls to it) to test my theory and sure enough the program started correctly afterwards. So... Qt appears to invoke "crc32" when doing things like adding QMenu items... due to some name collision.

I'm puzzled how to fix it. I don't want to change the C code because it's actually a git submodule for a project I do not want to maintain a fork of forever. Since it's Qt calling it from the C++ side, I can't exactly specify the namespace on the call either.

Weird sidenote: When I was doing testing this morning with Qt5 (instead of 6), I was not running into this issue. The crc32 name collision may be specific to Qt6.

Jesse Litton
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    Just don't declare c functions with common short names, there's not much more you can do, especially when that name is a system function (as crc32 is on Linux) – Alan Birtles Oct 05 '22 at 21:32
  • What OSs are we dealing with? And can you compile the external lib static? In any case, you may be able to work around this by using adding intermediary library that re-exports only the symbols you actually need. See https://stackoverflow.com/questions/6940384/how-to-deal-with-symbol-collisions-between-statically-linked-libraries/6940389#6940389 and https://stackoverflow.com/questions/6538501/linking-two-shared-libraries-with-some-of-the-same-symbols – Homer512 Oct 05 '22 at 21:43

2 Answers2

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Looks like you are on Linux (or similar) and encountering a violation of the One Definition Rule. When the shared lib loader tries to load symbols from a shared lib, it will first check to see if it already has one with that name loaded. If so, it skips it. Thus, if you try to load two symbols with an identical name, you'll only get one of them, and then problems can ensue.

Possible options:

  1. Hide symbols you don't need in the library. For this, consider using GCC's visibility controls.
  2. Separate the symbols into different executables and share data with some interprocess communication like shared mem or a socket.
  3. Use dlopen() on one of the libs along with the RTLD_LOCAL flag so it does not override the symbol.
  4. Add a namespace to the project (or use its build options to control that, if available -- like Boost does). Yes, this violates your wish not to modify it, but it could be done with a patch or similar that does not modify the original checkout if you switched to using a package manager like Conan instead of a git submod.
metal
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crc32 is a symbol from zlib. Qt does use either the system zlib, or a built-in version of it. If it uses the built-in version, symbols are prefixed with z_ (so z_crc32) to avoid exactly such a clash.

So, if you want to use crc32, you have the options to configure Qt to either not use zlib at all (-no-zlib), or use the built-in version (-qt-zlib).

kkoehne
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