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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 - link error when overriding common symbols with weak symbols in library"
href="https://bugs.llvm.org/show_bug.cgi?id=51082">51082</a>
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<th>Summary</th>
<td>link error when overriding common symbols with weak symbols in library
</td>
</tr>
<tr>
<th>Product</th>
<td>lld
</td>
</tr>
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<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>ELF
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
<tr>
<th>Reporter</th>
<td>yabinc@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, smithp352@googlemail.com
</td>
</tr></table>
<p>
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<pre>If I define a common symbol in main object file, and define a weak symbol with
the same name in a library. lld may not be able to override the common symbol
correctly with the weak symbol, and report link error.
Below is an example:
main.c:
#include <stdio.h>
extern int A;
const char* B;
int main() {
printf("A = %d, B = %s\n", A, B);
}
lib.c:
int A;
const char* B __attribute__((weak));
build.sh:
CLANG_DIR=/ssd/opensource/llvm/llvm-build/install/bin
$CLANG_DIR/clang -c -o main.o main.c -fPIE -fcommon
$CLANG_DIR/clang -c -o lib.o lib.c -fPIC
$CLANG_DIR/llvm-ar crsPD -format=gnu lib.a lib.o
$CLANG_DIR/clang main.o lib.a -pie -fuse-ld=lld -v
link error:
ld.lld: error: relocation R_X86_64_PC32 cannot be used against symbol B;
recompile with -fPIC
<span class="quote">>>> defined in lib.a
>>> referenced by main.c
>>> main.o:(main)</span >
The cause of the error seems to be below:
1. lld parses main.o, and adds undefined symbol A and common symbol B.
2. lld parses lib.a, reads LazyArchive symbol A, and decides to read lib.o
to fetch symbol A. By the way, it reads weak symbol B in lib.o, but
decide not to override common symbol B.
3. lld parses lib.a, reads lazyArchive symbol B, and decides to override
common symbol B. So it replace common symbol B with lazyArchive symbol B.
But not further read weak symbol B, because lib.o has been read before.
4. As a result, we have a LazyArchive symbol B, which can't do relocation.</pre>
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</p>
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