Function aligned_pwrite

block/src/qcow_common.rs:172–190

Write `buf` via an aligned bounce buffer when O_DIRECT requires it.

(fd: RawFd, buf: &[u8], offset: u64, alignment: usize)

/// Write `buf` via an aligned bounce buffer when O_DIRECT requires it.
pub fn aligned_pwrite(fd: RawFd, buf: &[u8], offset: u64, alignment: usize) -> io::Result<()> {
    if alignment == 0
        || ((buf.as_ptr() as usize).is_multiple_of(alignment)
            && buf.len().is_multiple_of(alignment)
            && (offset as usize).is_multiple_of(alignment))
    {
        return pwrite_all(fd, buf, offset);
    }

    let aligned_offset = offset & !(alignment as u64 - 1);
    let head = (offset - aligned_offset) as usize;
    let aligned_len = (head + buf.len()).next_multiple_of(alignment);
    let mut bounce = AlignedBuf::new(aligned_len, alignment)?;

    // Read-modify-write: read the existing aligned region, overlay our data.
    pread_exact(fd, bounce.as_mut_slice(aligned_len), aligned_offset)?;
    bounce.as_mut_slice(aligned_len)[head..head + buf.len()].copy_from_slice(buf);
    pwrite_all(fd, bounce.as_slice(aligned_len), aligned_offset)
}

// -- iovec helper functions --
//