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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Mixing objects with thread_local in C++ and _Thread_local in C"
href="https://bugs.llvm.org/show_bug.cgi?id=51814">51814</a>
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<th>Summary</th>
<td>Mixing objects with thread_local in C++ and _Thread_local in C
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>12.0
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C++11
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>simon.marchi@polymtl.ca
</td>
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>Hi,
I'm not sure that this is really a bug, but if I could at least get an
explanation of what is happening I'd be really happy. With clang 12, this
fails to build:
$ cat tls.c
extern __attribute__((visibility("hidden"))) _Thread_local const char
*logger_thread_name;
__attribute__((visibility("hidden"))) _Thread_local const char
*logger_thread_name;
$ cat main.cpp
extern "C" {
extern __attribute__((visibility("hidden")))
// Works if you use _Thread_local
// _Thread_local
thread_local
const char *logger_thread_name;
}
const char *func() {
return logger_thread_name;
}
$ cat build.sh
clang++ main.cpp -c -fPIC -DPIC
clang tls.c -c -fPIC -DPIC
clang++ main.o tls.o -shared
$ bash build.sh
/usr/bin/ld: main.o: relocation R_X86_64_PC32 against undefined hidden symbol
`_ZTH18logger_thread_name' can not be used when making a shared object
/usr/bin/ld: final link failed: bad value
clang-12: error: linker command failed with exit code 1 (use -v to see
invocation)
What we have here is:
- a C source file defining a TLS variable using _Thread_local, with the
attribute visibility("hidden")
- a C++ source file using this TLS variable, the declaration it sees is in an
extern "C", but uses "thread_local", and also has the attribute
visibility("hidden")
- both
- we compile both using -fPIC and link them into a shared library
- the linking fails because of an invalid relocation type
The link is successful is you either:
- change the declaration in the C++ file to use the C keyword _Thread_local
instead
- change the declaration in the C++ file to remove the visibility("hidden")
attribute
Is the code above doing things that we really shouldn't do? Regardless of
that, is there some bug in clang, given that it produces a R_X86_64_PC32
relocation in a file built with -fPIC?</pre>
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