[libcxx-commits] [libcxx] [libc++] Implements filebuf unbuffered. (PR #76629)

[Louis Dionne via libcxx-commits]

================
@@ -337,6 +377,23 @@ private:
 
     return this;
   }
+
+  _LIBCPP_HIDE_FROM_ABI void __try_set_unbuffered_mode() {
+    if (__cm_ == (__no_io_operations | __use_unbuffered_io)) {
+      std::setbuf(__file_, nullptr);
+      __cm_ = 0;
+    }
+  }
+  _LIBCPP_HIDE_FROM_ABI void __try_set_unbuffered_mode(char_type* __s, streamsize __n) {
----------------
ldionne wrote:

```
// If the file is already open, switch to unbuffered mode. Otherwise, record the request
// to use unbuffered mode so that we use that mode when we eventually open the file.
```

Note that we could also name this function `__request_unbuffered_mode`, which may capture better the fact that we're "placing a request" that may be fulfilled later (when we open).

https://github.com/llvm/llvm-project/pull/76629