I'm facing an issue with BufReader in Rust when attempting to read data into two separate temporary buffers under different conditions.
Below is a breakdown along with the added code.
first_tmp_buf): It successfully reads 500 bytes of data.
[44, 32, 49, 50, 47, 55, ..., 55] ✅second_tmp_buf): During the first iteration, it's intended to read 1500 bytes. However, only the first 1000 bytes are valid, and the last 500 bytes are unexpectedly zeros. I suspect this issue arises from the initial 500-byte read into first_tmp_buf, which may be influencing the read position.
[44, 32, 49, 50, ..., 0, ..., 0] ❌second_tmp_buf): These operations perform as expected, consistently returning 1500 valid bytes and completely filling the buffer.
[44, 32, 49, 50, 47, 55, ..., 55] ✅I suspect the problem is linked to how the read position is managed across different buffer contexts, but I’m not certain how to address this effectively.
use std::io::{BufReader, Read, Cursor};
fn main() -> std::io::Result<()> {
let huge_string_data = vec![0_u8; 200_00_00]; // Simulating a large dataset
let src_buf = Reader::from_vec(huge_string_data);
let mut first_tmp_buf = vec![0_u8; 500];
// First read into `first_tmp_buf`:
// ✅ Successfully returns 500 bytes of data.
// Eg output: [44, 32, 49, 50, 47, 55, ..., 55]
let bytes_read = src_buf.inner.read(&mut first_tmp_buf)?;
let mut second_tmp_buf = vec![0_u8; 1500];
let mut iter = 0;
while condition {
let bytes_read = src_buf.inner.read(&mut second_tmp_buf)?;
// Second read into `second_tmp_buf` during first iteration:
// When `(iter=0)`
// ❌ Returns 1000 bytes of valid data, but last 500 bytes are zeros, not good.
// Eg output: [44, 32, 49, 50, ..., 0, 0, 0, ..., 0]
// Third read into `second_tmp_buf`:
// When `(iter=1)`
// ✅ Successfully returns 1500 bytes of data.
// Eg output: [44, 32, 49, 50, 47, 55, ..., 55]
iter += 1;
}
Ok(())
}
struct Reader {
pub inner: BufReader<Box<dyn Read>>,
}
impl Reader {
fn from_vec(v: Vec<u8>) -> Self {
let cursor = Cursor::new(v);
let boxed_reader = Box::new(cursor);
Self {
inner: BufReader::with_capacity(0x4000, boxed_reader),
}
}
}
Could anyone shed light on why BufReader behaves this way and suggest the best approach to ensure consistent reads across varying buffer sizes and contexts?
Calling .read() does not guarantee that the buffer you gave it is filled. It is allowed to only partially fill the buffer. That is why it returns the number of bytes that were written to the buffer. So if it returns 1000 when your buffer was size 1500, then only buffer[..1000] is the data that was read.
If your use-case require getting an expected amount of data, you should use .read_exact(). That will fill the entire buffer (by repeatedly calling .read() internally if needed), or else it will error if it reaches EOF before being completely filled.
As to why this happens in your case, you have a BufReader wrapping a Vec (via Cursor). So there are actually two "pools" of bytes: one in the original Vec, the other in the BufReader. So when you .read() from the BufReader, it will read from its internal buffer if it still has data there and won't reach out to the inner reader. That internal buffer may not have enough to fill the buffer, but since you only called .read(), partial filling is allowed, so it will just end there. The BufReader will only read from the inner reader when its internal buffer is empty.