Code suddenly stops at self.cam = QCamera() , PyQt5.9.2, Qt5.9.3, Python3.5

JonB

@Xenoshell
Sorry, I know nothing about "camera device access/groups under Ubuntu" other than what @jsulm wrote. (I use Ubuntu, but not Mate/Pi, and I don't have a camera.) He may return to offer more information, or he may just be giving you a hint as to what you need to look for.

If you're new to Ubuntu/Linux, here's just a word about sudo/root user.

Linux (like Windows) has users & groups, who have different permissions. Files and devices have permissions, and if your user/groups doesn't have permission to access something you'll be blocked from it.
Ubuntu doesn't really have a root user that you can log in as, per se, but you can use the sudo command to gain root privileges.
You are saying that if you run your code normally as you, it "hangs" at QCamera().
But if you run it as root via sudo it does not hang.
That would imply you as you lack permission to access the camera (perhaps a device), while root does not have that problem.
Therefore we are thinking there might be a "group" who have access to the camera, and you need to be a member of that group.
You can list all groups with cat /etc/groups. You'd have to look through yours to see if there looks like anything "likely". You add users to groups via adduser user group.
You can run sudo python3 qt5.py if you really want. But it's a really bad idea (haven't got time to list myriad reasons), and I think you'll really regret it as you try to do other stuff in your app.

BTW, did you have to install your "camera device/software"? If so did it come with any instructions about this?

Your Python code looks OK, and I doubt it will have anything to do with Python/PyQt.

I am a little surprised that just plain QCamera() hangs/requires permissions, because you're not even naming a device there, but it might do, or it might try to access the "default" camera device, i don't know. @jsulm / @SGaist know more than me.

Try @SGaist's suggestion of just enumerating the available cameras. You're going to need to learn soon how to turn his bit of Qt C++ code into Python to get anywhere much with Qt.... (Not meaning to be rude or disheartening, just a heads-up, but if you think you could just write code QCamera(0x72dbe3b0) in Python or C++ you have quite a bit still to learn before you'll be able to do stuff.)

Finally, I don't like the look of gdb python3 itself generating a SIGSEGV, but I don't know exactly what you did, could be a red-herring.

Your "screen going black and locking up" might be to do with it requesting a password to access, I really don't know. I assume you've got your camera all working and you can play with it OK outside of your application?

jsulm

@Xenoshell Please take a look at https://www.tutorialspoint.com/unix/unix-file-permission.htm
Each device is represented by a device file which. To read from such a device you read from the device file to write to the device you write into this file. UNIX/Linux access rights apply to device files as well. So, you have user, user groups and others. You have read/write and execute rights. My guess is that the user you are using on your machine does not have access rights to the camera device file. Usually to solve this you just need to add this user to the correct user group (probably this group is called "video"). To see in which groups your user is member execute the command "groups" in a terminal. To add a user to a group see https://askubuntu.com/questions/79565/how-to-add-existing-user-to-an-existing-group

Xenoshell

@JNBarchan

input: cat /etc/group -> output (i am in the group video)
I just post this if i dont see a group i should be in:

root:x:0:
daemon:x:1:
bin:x:2:
sys:x:3:
adm:x:4:syslog,blz
tty:x:5:
disk:x:6:
lp:x:7:
mail:x:8:
news:x:9:
uucp:x:10:
man:x:12:
proxy:x:13:
kmem:x:15:
dialout:x:20:
fax:x:21:
voice:x:22:
cdrom:x:24:blz
floppy:x:25:
tape:x:26:
sudo:x:27:blz
audio:x:29:pulse,blz
dip:x:30:blz
www-data:x:33:
backup:x:34:
operator:x:37:
list:x:38:
irc:x:39:
src:x:40:
gnats:x:41:
shadow:x:42:
utmp:x:43:
video:x:44:blz
sasl:x:45:
plugdev:x:46:blz
staff:x:50:
games:x:60:
users:x:100:
nogroup:x:65534:
systemd-journal:x:101:
systemd-timesync:x:102:
systemd-network:x:103:
systemd-resolve:x:104:
systemd-bus-proxy:x:105:
input:x:106:blz
crontab:x:107:
syslog:x:108:
netdev:x:109:
messagebus:x:110:
uuidd:x:111:
mlocate:x:112:
ssh:x:113:
ssl-cert:x:114:
lpadmin:x:115:blz
lightdm:x:116:
nopasswdlogin:x:117:
ntp:x:118:
avahi-autoipd:x:119:
avahi:x:120:
bluetooth:x:121:
scanner:x:122:saned
colord:x:123:
pulse:x:124:
pulse-access:x:125:
rtkit:x:126:
saned:x:127:
whoopsie:x:128:
gpio:x:999:blz
i2c:x:998:blz
spi:x:997:blz
blz:x:1000:
sambashare:x:129:blz

QCamera(0x72dbe3b0)

From the QCamera documentation: "QCamera::QCamera(const QByteArray &deviceName, QObject *parent = Q_NULLPTR) Construct a QCamera from deviceName and parent."
Correct me if i'm wrong but that means if i know the deviceName that i can just initialize the Camera with QCamera(0x72dbe3b0).
3)
gdb is a debug program which can be used for segmentation fault errors.

@jsulm
4)
Input:

ls -l /home/blz/Schreibtisch/qt5.py

Output:
-rwxrwxrwx 1 blz blz 1537 Dez 7 10:33 /home/blz/Schreibtisch/qt5.py
I guess that means that i have full accessibility to qt5.py

Thanks for your help. I appreciate your feedback.

Greets

jsulm

@Xenoshell 1. Well, the question is: what is the group of the camera device file?
2. You can use this code to see all the camera device names (http://doc.qt.io/qt-5/qcamera.html):

QList<QCameraInfo> cameras = QCameraInfo::availableCameras();
foreach (const QCameraInfo &cameraInfo, cameras) {
    qDebug() << cameraInfo.deviceName();
}

Yes, you can use GDB to get more information if your app crashes
Everyone has full access to Schreibtisch/qt5.py. Not sure how is this related?

JonB

@Xenoshell

Nothing looks interesting in your groups. Never mind, it was only an idea of @jsulm's, maybe or maybe not relevant.

Correct me if i'm wrong but that means if i know the deviceName that i can just initialize the Camera with QCamera(0x72dbe3b0).

It's so wrong. Fortunately in C++ it won't compile, in Python I hope it will spit it back at you. You really need to understand why this is plain wrong in any language/circumstance, if you're new to programming. You must pass a string which has the value of a device name to QCamera(), e.g. QCamera("camera-device-name"), or a device name picked up from a QCameraInfo.deviceName() (which itself is a string).

Please try @jsulm's suggestion of enumerating the available cameras you have. In Python it'll be like:

for cameraInfo in QCameraInfo.availableCameras():
    print(cameraInfo.deviceName())

QString QCameraInfo::deviceName() const
Returns the device name of the camera
This is a unique ID to identify the camera and may not be human-readable.

So a device name might come out like abc1234 (I don't know 'coz I haven't got one to test). Then the actual Linux device will be /dev/abc1234, or something like that. We want you to ls -l that, and look at its owner & group permissions, and see if you have access to it under your own user, not sudo. This is what we mean about "permissions", not the permissions you list for /home/blz/Schreibtisch/qt5.py.

Xenoshell

@JNBarchan, @jsulm
The output for the for-loop is:

/dev/video0

Here the output is:

blz@blz-desktop:~$ ls -l /dev/video0
crw-rw----+ 1 root video 81, 0 Dez 11 10:55 /dev/video0

To me it looks like i dont have the permission to everything.

jsulm

@Xenoshell Add yourself to the video group if it's not already the case

JonB

@Xenoshell
You can see if you're already a member of video group via executing command groups.

If your username is blz, I think you already are....

If you are a member, it would then look like: as yourself, not root, you do have access to the camera device, hence you say it seems to "initially open", but then something else is happening which works as root but not as you.... You could temporarily sudo chmod 666 /dev/video0, see if that helps, then revert to sudo chmod 660 /dev/video0.

Xenoshell

@jsulm @JNBarchan ,
i am already member of the video group.
The camera device only opens if i use sudo. If i dont it just gets stuck at self.cam = QCamera().
After the command sudo chmod 666 /dev/video0 i tried using qt5.py without sudo but it just got stuck.
So there has to be a group which i am not a member of that uses QCamera(). Am i at least right with this assumption?

Xenoshell

Ok i cant reproduce the gdb output anymore. It just stops at 1 and then somehow locks up and i have to relog again.
I somehow have this feeling that the code/the usb-camera use audio or at least try to use it. When i want to turn off the pi i always get the message that "Pulse Audio Sound System" is currently running and if i want to terminate it.
Tomorrow i will sit down and try to get the gdb output without sudo /with sudo and look what is possible.
Debugging can really be stressful...

Xenoshell

@jsulm, @JNBarchan
Here i am again, this is the new gdb output:

(gdb) run qt5.py
Starting program: /usr/bin/python3 qt5.py
Cannot parse expression `.L1185 4@r4'.
warning: Probes-based dynamic linker interface failed.
Reverting to original interface.

[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/arm-linux-gnueabihf/libthread_db.so.1".
1
[New Thread 0x72b39470 (LWP 4134)]
2
3
[New Thread 0x6de10470 (LWP 4139)]
[Thread 0x6de10470 (LWP 4139) exited]
[New Thread 0x6de10470 (LWP 4140)]
[Thread 0x6de10470 (LWP 4140) exited]

At this output is libpulse.so and also libasound.so.2 i talked about in my post above. It could actually be that QCamera() tries to also initalise Audio but obviously i dont have any audiooutput plugged in and thats why it stops at QCamera

(gdb) bt
#0  __libc_do_syscall () at ../sysdeps/unix/sysv/linux/arm/libc-do-syscall.S:46
#1  0x76f21c0a in __GI_ppoll (fds=0x7001a8, nfds=1, timeout=<optimized out>, 
    sigmask=0x0) at ../sysdeps/unix/sysv/linux/ppoll.c:50
#2  0x73389e12 in pa_mainloop_poll ()
   from /usr/lib/arm-linux-gnueabihf/libpulse.so.0
#3  0x7338a290 in pa_mainloop_iterate ()
   from /usr/lib/arm-linux-gnueabihf/libpulse.so.0
#4  0x6de2888c in conf_pulse_hook_load_if_running ()
   from /usr/lib/arm-linux-gnueabihf/alsa-lib/libasound_module_conf_pulse.so
#5  0x6e01c9f2 in ?? () from /usr/lib/arm-linux-gnueabihf/libasound.so.2
Backtrace stopped: previous frame identical to this frame (corrupt stack?)

Here the info threads:

(gdb) info threads
  Id   Target Id         Frame 
* 1    Thread 0x76ff6300 (LWP 4204) "python3" __libc_do_syscall ()
    at ../sysdeps/unix/sysv/linux/arm/libc-do-syscall.S:46
  2    Thread 0x72b39470 (LWP 4206) "QXcbEventReader" 0x76f21b90 in poll ()
    at ../sysdeps/unix/syscall-template.S:84

Can anyone tell me whats going on in those gdb outputs? Are they even important or are they just there and we cant really do anything with them?

JonB

@Xenoshell
Because you are using Python, not a standalone executable of your program compiled from C++, if you use gdb you have to gdb the Python executable, not your app running as a Python script. This means gdb probably is not of any interest to you, per se, for general debugging of your app; though it may give us some clues in this particular case.

To clarify, for your app script completely. Just run gdb against Python without any mention of your qt5.py script. For the record, here is my output under Ubuntu not Pi:

jon@ubuntu:~$ gdb python3
Reading symbols from python3...(no debugging symbols found)...done.
(gdb) run
Starting program: /usr/bin/python3 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Python 3.5.3 (default, Nov 23 2017, 11:34:05) 
[GCC 6.3.0 20170406] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

Does yours produce much the same? Does it only give the libpulse/libasound if a certain line is in your Python script, and not if it is removed, then you'd have an idea what is related to what? I wish you'd show what that line is now, because we no longer know whether you are enumerating available cameras or opening a camera?

Xenoshell

@JNBarchan
This is the whole output:

blz@blz-desktop:~$ gdb python3
GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.5) 7.11.1
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "arm-linux-gnueabihf".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from python3...Reading symbols from /usr/lib/debug/.build-id/d7/14ad8d8b52ca34a8a81f10b4917027977b05ca.debug...done.
done.
(gdb) run
Starting program: /usr/bin/python3 
Cannot parse expression `.L1185 4@r4'.
warning: Probes-based dynamic linker interface failed.
Reverting to original interface.

[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/arm-linux-gnueabihf/libthread_db.so.1".
Python 3.5.2 (default, Nov 23 2017, 16:37:01) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

My code always stops at self.cam = QCamera() otherwise it would also print 4 and not suddenly stop
As a reminder, here is my code:

import sys
from PyQt5 import QtCore , QtWidgets, QtGui, QtMultimedia, QtMultimediaWidgets
from PyQt5.QtCore import QObject, pyqtSignal, pyqtSlot
from PyQt5.QtWidgets import QApplication, QPushButton, QMainWindow
from PyQt5.QtMultimedia import QCamera, QCameraInfo, QMediaObject, QCameraViewfinderSettings, QCameraImageCapture
from PyQt5.QtMultimediaWidgets import QCameraViewfinder


class Camera(QObject):
    def __init__(self, parent = QObject()):
        super(Camera, self).__init__(parent)
        print("3")
        self.cam = QCamera()
        print("4")
        self.caminfo = QCameraInfo(self.cam)
        self.camvfind = QCameraViewfinder()
        self.camvfindset = QCameraViewfinderSettings()
        self.cammode = self.cam.CaptureMode(2)
        self.camimgcap = QCameraImageCapture(self.cam)

    def iniCamera(self):
        #print(self.caminfo.description())
        #print(self.caminfo.availableCameras())
        
        for caminfo in QCameraInfo.availableCameras():
            print(caminfo.deviceName())
        
        
        if self.cam.isCaptureModeSupported(self.cammode):
            print("Capturemode supported")
    
    def startVid(self):
        self.camimgcap.CaptureDestination(2)
        
        self.camvfind.show()
        
        self.cam.setViewfinder(self.camvfind)
        
        self.cam.setCaptureMode(self.cammode)
        
        self.cam.start()
        
        

if __name__ == '__main__':
    print("1")
    app = QtWidgets.QApplication(sys.argv)
    print("2")
    cam = Camera()
    print("4")
    cam.iniCamera()
    
    cam.startVid()
    
    sys.exit(app.exec_())

JonB

@Xenoshell
I asked earlier:

Does it only give the libpulse/libasound if a certain line is in your Python script, and not if it is removed, then you'd have an idea what is related to what?

So, if I were you, under gdb, I'd try commenting in & commenting out the QCamera() line, and report whether your debugger only shows the libpulse error if & only if you have that line in there. then you'd know for sure whether QCamera() has anything to do with libpulse....

I'd also try QCamera("video0") or QCamera("/dev/video0") or whatever it is, instead of plain QCamera(). I'd probably also try QCamera("rubbish"). These are all things for you to play with to try to understand just what causes the problem/hang, it's up to you....

Xenoshell

@JNBarchan
I commented QCamera() and obviously i also need to comment the stuff that is in correlation to self.cam because otherwise i would get a simple error because self.cam is not there. Then i also dont get the libpulse error, well tbh there is not much to compile because about half the code is commented.
I tried using QCamera("/dev/video0) and also video0, this results in the error:

TypeError: arguments did not match any overloaded call:
  QCamera(QObject parent=None): argument 1 has unexpected type 'str'
  QCamera(QByteArray, QObject parent=None): argument 1 has unexpected type 'str'
  QCamera(QCameraInfo, QObject parent=None): argument 1 has unexpected type 'str'
  QCamera(QCamera.Position, QObject parent=None): argument 1 has unexpected type 'str'

I can follow you that this should be right but how are you supposed to initialize QCamera if you need the QCameraInfo or a QByteArray?
Are you supposed to initalize without anything -> find out the QByteArray -> initialize QCamera with the correct QByteArray?

Thanks again for your help

JonB

@Xenoshell
For the way to invoke QCamera(), sorry, I misremembered the constructor, and thought it took a string. It takes a byte array of the name instead. From Python, you'll use str.encode(), e.g. "/dev/video0".encode().

Maybe it's not a good idea to try to create an "empty" QCamera(). Try using a constructor which does take an actual camera. One of:

`QCamera(QCameraInfo.defaultCamera())
QCamera("/dev/video0".encode())
One of the available cameras returned by the loop:

for caminfo in QCameraInfo.availableCameras():
    print(caminfo.deviceName())
    acam = QCamera(caminfo)

I hope one of the above works instead of the default constructor. Maybe only root can create the empty one (though have to say I'm dubious)....

Now that I think I understand what your code is intending to do, I believe you always intended QCamera(QCameraInfo.defaultCamera()). Don't forget the docs admonition:

QCameraInfo QCameraInfo::defaultCamera()
Returns the default camera on the system.
The returned object should be checked using isNull() before being used, in case there is no default camera or no cameras at all.
See also availableCameras().

Xenoshell

@JNBarchan
well that didnt do anything... When i use QCamera("/dev/video0".encode()) or QCamera(QCameraInfo.defaultCamera()) it just prints till 3 and then stops.
Maybe i need to add myself to the audio group (EDIT: ok already am)

JonB

@Xenoshell
Then at this point I'm afraid I'm stumped. QCamera(QCameraInfo.defaultCamera()) should definitely not hang. I don't know what is going on in the Qt code which will cause something to do so unless run as root. (I just wonder whether something might be prompting for, say, root password to allow access, and that's why it hangs/goes black....)

You need one of the experts who knows what the Qt code does to get you anywhere now, I think....

Xenoshell

@JNBarchan
No worries, i think it should work too and am baffled... Maybe i can summon @Lifetime-Qt-Champion @SGaist ? He has also helped me alot in the past. But i am quite sure it has something to do with the gdb output

SGaist

You can't without lots of chocolate...

Minimal PyQt5 example that shows a viewfinder using the default camera:

import sys

from PyQt5.QtWidgets import QApplication
from PyQt5.QtMultimedia import QCamera, QCameraInfo
from PyQt5.QtMultimediaWidgets import QCameraViewfinder

if __name__ == '__main__':

    app = QApplication(sys.argv)
    camera = QCamera(QCameraInfo.defaultCamera());
    viewfinder = QCameraViewfinder()
    viewfinder.show()
    camera.setViewfinder(viewfinder);
    camera.start()

    sys.exit(app.exec_())

Does it work for you ?