From f8a8ed4826489a7747f492f8ba9795e3121c74f2 Mon Sep 17 00:00:00 2001
From: _ <_@_>
Date: Sat, 9 Sep 2023 13:32:24 -0500
Subject: [PATCH 1/2] expose `bytesused` to users

When capturing JPEG frames, v4l will tell us that the JPEG can be as
big as the decoded YUV frame, like 4 MB for a 1080p frame.
But typically the JPEG is only 500 KB or so. Exposing bytesused
allows callers to copy less data and write precisely-sized JPEG frames
---
 src/stream.rs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/stream.rs b/src/stream.rs
index 8b4ef14..4de58d8 100644
--- a/src/stream.rs
+++ b/src/stream.rs
@@ -224,6 +224,7 @@ impl ReadStream {
             unsafe { slice::from_raw_parts(buffer.ptr as *const u8, buffer.length as usize) };
         let view = ReadBufferView {
             flags: buf.flags,
+            bytesused: buf.bytesused,
             data,
         };
 
@@ -282,10 +283,15 @@ impl AsRawFd for ReadStream {
 /// Dereferences to a byte slice.
 pub struct ReadBufferView<'a> {
     flags: BufFlag,
+    bytesused: u32,
     data: &'a [u8],
 }
 
 impl ReadBufferView<'_> {
+    pub fn bytesused(&self) -> u32 {
+        self.bytesused
+    }
+    
     /// Returns whether the error flag for this buffer is set.
     ///
     /// If this returns `true`, the application should expect data corruption in the buffer data.

From 03e81f5d05d1df784fdc904ddf23846de368d05d Mon Sep 17 00:00:00 2001
From: _ <_@_>
Date: Sat, 9 Sep 2023 13:40:11 -0500
Subject: [PATCH 2/2] update example to use `bytesused`

---
 examples/save-jpeg.rs | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/examples/save-jpeg.rs b/examples/save-jpeg.rs
index 96c4a9e..1ee0d28 100644
--- a/examples/save-jpeg.rs
+++ b/examples/save-jpeg.rs
@@ -32,14 +32,16 @@ fn main() -> anyhow::Result<()> {
         device.capabilities()?.device_capabilities()
     );
 
-    let capture = device.video_capture(PixFormat::new(u32::MAX, u32::MAX, PixelFormat::JPEG))?;
+    let capture = device.video_capture(PixFormat::new(u32::MAX, u32::MAX, PixelFormat::MJPG))?;
+    
     println!("negotiated format: {:?}", capture.format());
 
     let mut stream = capture.into_stream()?;
 
     println!("stream started, waiting for data");
     stream.dequeue(|buf| {
-        file.write_all(&*buf)?;
+        let bytesused = usize::try_from (buf.bytesused ()).unwrap ();
+        file.write_all(&buf [0..bytesused])?;
         println!("wrote file");
         Ok(())
     })?;