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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Swapping a filebuf with a small buffer (or disabled buffering) discards characters"
href="https://bugs.llvm.org/show_bug.cgi?id=49938">49938</a>
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<th>Summary</th>
<td>Swapping a filebuf with a small buffer (or disabled buffering) discards characters
</td>
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<th>Product</th>
<td>libc++
</td>
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<th>Version</th>
<td>11.0
</td>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>All Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>alex@grundis.de
</td>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
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<p>
<div>
<pre>When a filebuf with a small buffer size is used, then the internal buffer
`__extbuf_min_` is used instead:
<a href="https://github.com/llvm/llvm-project/blob/82e537a9d28a2c18bd1637e2eac0e0af658ed829/libcxx/include/fstream#L896">https://github.com/llvm/llvm-project/blob/82e537a9d28a2c18bd1637e2eac0e0af658ed829/libcxx/include/fstream#L896</a>
This buffer may be filled by (e.g.) read operations. When the filebuf is then
swapped with another filebuf, the contents of this buffer will be ignored and
the swapped-to filebuf will use its own, unfilled internal buffer instead. See
<a href="https://github.com/llvm/llvm-project/blob/82e537a9d28a2c18bd1637e2eac0e0af658ed829/libcxx/include/fstream#L432-L436">https://github.com/llvm/llvm-project/blob/82e537a9d28a2c18bd1637e2eac0e0af658ed829/libcxx/include/fstream#L432-L436</a>
Example code triggering the bug:
std::filebuf buf1, buf2;
buf1.pubsetbuf(0, 0);
assert(buf1.open(filepath, std::ios_base::in) == &buf1);
assert(buf2.open(filepath2, std::ios_base::in) == &buf2);
// Peek
assert(buf1.sgetc() == 'B');
assert(buf2.sgetc() == 'H');
swap(buf1, buf2);
assert(buf2.sgetc() == 'B'); // Boom!</pre>
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