<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/124637>124637</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc][unistd] provide STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            good first issue,
            libc
      </td>
    </tr>

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

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

<pre>
    building clang against llvm-libc fails with:
```
In file included from /llvm-project-main/llvm/lib/Support/Process.cpp:122:
/llvm-project-main/llvm/lib/Support/Unix/Process.inc:231:30: error: use of undeclared identifier 'STDIN_FILENO'
  231 |   const int StandardFDs[] = {STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO};
      | ^
/llvm-project-main/llvm/lib/Support/Unix/Process.inc:231:44: error: use of undeclared identifier 'STDOUT_FILENO'
  231 |   const int StandardFDs[] = {STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO};
      | ^
/llvm-project-main/llvm/lib/Support/Unix/Process.inc:231:59: error: use of undeclared identifier 'STDERR_FILENO'
  231 |   const int StandardFDs[] = {STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO};
      | ^
```
We should define these in include/llvm-libc-macros/linux/unistd-macros.h with the values of 0, 1, 2 respectively.
</pre>
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