How can ejecting an audio CD by using cdaudio's cd_eject () method produce errno #5?

问题

I'm trying to playback an audio CD from my app by using the cdaudio library + a USB DVD drive attached to a Raspi 3B. Trying to eject the CD after playback always makes me end up with errno #5. This is my code:

void sound::Eject ()
{
  struct disc_status cd_stat;

  if (sound::current_sound_source == CD) {
    sound::Stop ();
    cd_poll (sound::cd_drive_handler, &cd_stat);

    if (sound::is_cd_stopped && cd_stat.status_present == 1) {
      if ((cd_eject (sound::cd_drive_handler)) < 0) cout << "Ejecting CD failed! Error: " << strerror (errno) << endl;
    }
  }
}

This is the output I get:

ioctl returned -1
Ejecting CD failed! Error: Input/output error

When trying to eject the CD, I hear a noise in the drive, as if it was about to access the CD, for about half a second. This is the drive I'm using:

pi@autoradio:~ $ ls -al /dev/sr*
brw-rw----+ 1 root cdrom 11, 0 Mai  1 21:38 /dev/sr0

Ejecting the CD from the command line (eject /dev/sr0), does work, though.

Does anybody know what may cause this error? Thank you.

UPDATE #1: I gave cdcd (the command-line tool for audio CDs) a try, and I could reproduce the error there, too (even under sudo):

cdcd> eject
ioctl returned -1

UPDATE #2: I found out that cdaudio calls ioctl with the CDAUDIO_EJECT command (see sourcecode), but I can't find such a command anywhere in the linux/cdrom.h file. According to one of the developers of the cdaudio library, this is just an alias for CDROMEJECT and not a bug.

UPDATE #3: strace give me this output. I hope this is sufficient:

ioctl(3, CDROM_DISC_STATUS, 0)          = 100
ioctl(3, CDROMSUBCHNL, 0x7e93e308)      = 0
ioctl(3, CDROMEJECT, 0x1)               = -1 EIO (Input/output error)
write(1, "ioctl returned -1\n", 18)     = 18

In contrast, when tracing the eject utility, I get something slightly different:

geteuid32()                             = 1000
open("/dev/sr0", O_RDWR|O_NONBLOCK)     = 3
ioctl(3, CDROMEJECT, 0x1)               = 0
close(3)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++

A comparison of the open () calls reveals that the cdaudio library apparently opens the drive on read-only mode (which is theoretically correct, but, on the other hand, seems to choke the eject command):

open("/dev/sr0", O_RDONLY|O_NONBLOCK)   = 3

SEE ALSO: Question #26240195

回答1:

OK, after a couple of weeks of studying the eject utility, I found out that at least some CD drives wouldn't accept a CDROMEJECT command via ioctl (), but require a bunch of SCSI commands. In fact, eject contains a method, which is used as a fallback in such situations: eject_scsi (). I implanted this method into cdaudio. Tests were successful. So I asked the maintainers of cdaudio for a respective patch.

来源：https://stackoverflow.com/questions/55942194/how-can-ejecting-an-audio-cd-by-using-cdaudios-cd-eject-method-produce-errno

标签

ioctl