<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - __attribute__((constructor)) doesn't run on AVR because __do_global_ctors isn't linked"
href="https://bugs.llvm.org/show_bug.cgi?id=51278">51278</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>__attribute__((constructor)) doesn't run on AVR because __do_global_ctors isn't linked
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Driver
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>mhjacobson@me.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>Compiling and running this program:
__attribute__((constructor))
void ctor(void) {
printf("ctor\n");
}
int main(void) {
printf("hello\n");
for (;;);
}
results in "ctor" *not* being printed.
===
This is because `__do_global_ctors`, the symbol from libgcc.a responsible for
calling `ctor()`, is not emitted into the binary.
In turn, that is because nothing requests any symbol from the object file
containing `__do_global_ctors`. Other bits from libgcc.a are used, but not
these:
$ nm -n /opt/local/lib/gcc/avr/10.3.0/avr5/libgcc.a
...
_ctors.o:
U __ctors_end
U __ctors_start
U __tablejump2__
00000000 T __do_global_ctors
_dtors.o:
U __dtors_end
U __dtors_start
U __tablejump2__
00000000 T __do_global_dtors
...
===
The way this works in GCC is that, when it sees a constructor function, it
emits:
.globl __do_global_ctors
into the assembly. This causes the linker to resolve the resulting undefined
symbol.
===
One way to work around this is to pass
`-Wl,--require-defined,__do_global_ctors` to the driver. I'm not sure if
that's the right fix here (i.e., change the driver always to pass
`--require-defined __do_global_ctors` to the linker). Or perhaps this should
be fixed in the backend, so that `__do_global_ctors` is only linked in the
presence of a constructor function.</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>