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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - reconsider when we automatically define _GNU_SOURCE"
href="https://bugs.llvm.org/show_bug.cgi?id=35141">35141</a>
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<th>Summary</th>
<td>reconsider when we automatically define _GNU_SOURCE
</td>
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<th>Product</th>
<td>clang
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<th>Version</th>
<td>unspecified
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
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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++
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>richard-llvm@metafoo.co.uk
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<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>Currently, Clang follows GCC in automatically defining _GNU_SOURCE when
compiling C++ on a glibc platform. We should revisit this.
We need _GNU_SOURCE for the following:
1) GNU extensions used by user code.
2) GNU extensions used in the implementation of libstdc++.
3) C99 and C11 extensions.
For case #1, we could ask the user to provide -D_GNU_SOURCE explicitly on the
command line when compiling C++, as they are required to when compiling C. This
would break compatibility with GCC, though, so isn't obviously a win, but would
equally allow us to accept valid code that -D_GNU_SOURCE rejects. Perhaps a
separate flag for controlling this would be best.
For case #2, we may be able to get away with a smaller set of extensions. In
particular, _ISOC99_SOURCE and _ISOC11_SOURCE appear to be sufficient to
support most or all of libstdc++, and also appear to be sufficient for libc++.
Case #3 just needs _ISOC99_SOURCE / _ISOC11_SOURCE. (glibc should really be
able to figure this one out for itself based on the value of __cplusplus, but
it doesn't seem to do so today.)
At the very least, we should define _ISOCxx_SOURCE automatically based on the
C++ language mode, so that users need only pass -U_GNU_SOURCE to get a working
C++ compilation without the extensions. (Unfortunately, even this is not easy
right now, as the code adding this define is scattered throughout our target
definitions.)</pre>
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