I am opening the /dev/ptmx file on MacOS 14.2.1. When I open this file,it by default opens in non-blocking mode, I am not sure why because I do not specify O_NONBLOCK flag while opening it. When I try to read from /dev/ptmx I get an EAGAIN error. I even invoke the Fd() method on this file to get the file descriptor, the source code of Fd() puts the file on blocking mode, but in spite of that I get the EAGAIN error. I have found two fixes but I am unable to understand why these fixes work.

Go Version: 1.22

Code which gives me EAGAIN error

ptmx, err := os.OpenFile("/dev/ptmx", syscall.O_RDWR, 0)
    if err != nil {
        panic(err)
    }
err = Unlockpt(ptmx) // this function call the `ptmx.Fd()` method
    if err != nil {
        fmt.Println("unlock err", err)
        return
    }
name, err := Ptsname(ptmx) // this function call the `ptmx.Fd()` method
    if err != nil {
        fmt.Println("err", err)
        return
    }
    
go func() {
  _, err = io.Copy(os.Stdout, ptmx)
    if err != nil {
       // EAGAIN error is received here
        fmt.Println("copy err", err)
    }
      close(done)
}()   

Fix1: Open the file in O_NONBLOCK mode, somehow this sets the file in blocking mode

ptmx, err := os.OpenFile("/dev/ptmx", syscall.O_RDWR|syscall.O_NONBLOCK, 0)
// this works and I do not get the `EAGAIN` error on read
go func() {
        _, err = io.Copy(os.Stdout, ptmx)
        if err != nil {
            // NO EGAIN error and works fine
            fmt.Println("copy err", err)
        }
        close(done)
    }()

Fix2: Invoke the syscall.SetNonblock(int(ptmx.Fd()), false)
This also works because I am manually setting the file into blocking mode.