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

[Louis Dionne via llvm-branch-commits]

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@@ -276,6 +276,30 @@ private:
   state_type __st_;
   state_type __st_last_;
   ios_base::openmode __om_;
+  // Used to track the currently used mode and track whether the output should
+  // be unbuffered.
+  // [filebuf.virtuals]/12
+  //   If setbuf(0, 0) is called on a stream before any I/O has occurred on
+  //   that stream, the stream becomes unbuffered. Otherwise the results are
+  //   implementation-defined.
+  // This allows calling setbuf(0, 0)
+  // - before opening a file,
+  // - after opening a file, before
+  //   - a read
+  //   - a write
+  //   - a seek.
+  // Note that opening a file with ios_base::ate does a seek operation.
+  // Normally underflow, overflow, and sync change this flag to
+  // ios_base::in, ios_base_out, or 0.
+  //
+  // The ios_base::trunc and ios_base::ate flags are used in the following way:
+  // - ios_base::trunc is set upon construction to indicate the unbuffered
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ldionne wrote:

We should maybe introduce aptly-named values instead of reusing `trunc` and `ate`?

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