<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/60892>60892</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            libcpp: Use pthread_cond_timedwait_relative_np for condvar waits when available
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

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

<pre>
    On Apple platforms, `pthread_cond_clockwait` isn't available, but `pthread_cond_timedwait_relative_np` is. This function takes the same args as `pthread_cond_timedwait`, but treats the `timespec` arg as relative to the time of the call instead of relative to the UNIX epoch.

The underlying syscall takes a relative time, so `pthread_cond_timedwait` is implemented by subtracting the passed-in value from `__gettimeofday`, so there's no particular advantage to libpthread perform that subtraction instead of doing it ourselves when relevant (and passing a relative time all the way through in cases like `wait_for` calls). This would avoid a race condition where the system time could be updated between when we internally generate an absolute time and when libpthread reads it to subtract.

`pthread_cond_timedwait_relative_np` was introduced in macOS 10.4 and iOS 2.0, so there shouldn't be any substantial version-support-conditional issues to worry about here.
</pre>
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