<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/73683>73683</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
             [libc++] basic_string move assignment can leak memory
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            libc++
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          daltenty
      </td>
    </tr>
</table>

<pre>
    The following test program exhibits memory leaks when built with libc++ versions <= 6.0 and run against newer libc++ shared/dylibs:

```
$ cat test.cc
#include <string>

std::string* p;

void SetNoArena(::std::string&& value)
{
        *p = std::move(value);
}

int main()
{
        p = new ::std::string("Hello World!");

        const char value1[128] = "A long enough string for heap allocation";

        while(true){
 SetNoArena( ::std::string(value1) );
        }
 delete p;
        return 0;
}
$clang++ -nostdinc++ -isystem ~/libcxx-6.x/usr/local/include/c++/v1/ -nostdlib++ -L ~/libcxx-6.x/usr/local/lib/ -lc++ -lc++abi test.c
$export LD_LIBRARY_PATH=${HOME}/libcxx-17.x/usr/local/lib/:/usr/lib:/lib
$ ./a.out &
[1] 581514
$ ps -o pid,args,pmem
    PID COMMAND %MEM
 581514 ./a.out                      0.7
...
$ ps -o pid,args,pmem
    PID COMMAND                     %MEM
 581514 ./a.out 0.8
...
$ ps -o pid,args,pmem
    PID COMMAND %MEM
 581514 ./a.out 0.9
```

Previous versions of libc++ before 7.0 used clear and shrink_to_fit in the move constructor to release the existing allocation as part of the move:
```
    clear();
 shrink_to_fit();
```
https://github.com/llvm/llvm-project/blob/d359f2096850c68b708bc25a7baca4282945949f/libcxx/include/string#L2106

In the same versions, shrink_to_fit() is implemented as an inline call to reduce().

Unfortunately, the C++ standard changed the semantics of reduce() in https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0966r1.html to never deallocate, which libc++ now implements (https://github.com/llvm/llvm-project/commit/4d64d7dd641d385439a354db1f98cbb0ab7167f9).

The deallocation was non-binding according to the standard, but unfortunately the string header implementation prior to LLVM 6.0 depends on that behaviour being present in the shared/dylib.
</pre>
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