<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/155752>155752</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            std::print() can unexpectedly clobber errno
        </td>
    </tr>

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

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

    <tr>
      <th>Reporter</th>
      <td>
          m2
      </td>
    </tr>
</table>

<pre>
    print() invokes __vprint_unicode() invokes __is_terminal() invokes isatty(), which is subject to clobbering errno with ENOTTY when not writing to a terminal.

C++23 working draft in §31.7.10 bullet 7 specifies logic (flushing, etc.) conditional on stream referring to a terminal and use of isatty() specifically, so invocation of isatty() is motivated.

For the case of logging with print() between a system call and logic checking errno (e.g. differentiating EAGAIN from other errors), this leads to a hassle diagnosing mysterious ENOTTY errors.

I couldn't find anything in the standard about expectations for this and maybe there's a good reason not to, but I'd propose a store/restore of errno around the isatty() call would avoid frustration. Otherwise the same thing has to be done in any use of print() not wanting errno to be affected.
</pre>
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