python - v4l2 文档的示例代码原样(打开为 'rw' )？

Question

我刚开始使用v4l2 (对于 Python 3 )在 Ubuntu 16.04. 上

我正在尝试从 v4l2 Python 文档中运行示例，即:

$ cat demo4SO.py
#!/usr/bin/env python3

import v4l2
import fcntl

vd = open('/dev/video0', 'rw')
#vd = open('/dev/video0', 'r')
cp = v4l2.v4l2_capability()
fcntl.ioctl(vd, v4l2.VIDIOC_QUERYCAP, cp)

print("driver:",cp.driver)
print("card:",cp.card)

但我得到:

$ python3 demo4SO.py
Traceback (most recent call last):
  File "demo4SO.py", line 6, in <module>
    vd = open('/dev/video0', 'rw')
ValueError: must have exactly one of create/read/write/append mode

即使使用 sudo. 运行，我也会遇到相同的错误

尝试使用“w+”(或“r+”)打开，我得到:

$ python3 demo4SO.py
Traceback (most recent call last):
  File "demo4SO.py", line 8, in <module>
    vd = open('/dev/video0', "w+")
io.UnsupportedOperation: File or stream is not seekable.

如果我将“rw”(或“w+”或“r+”)更改为“r”，代码将按预期工作。

$ cat demo4SO.py
#!/usr/bin/env python3

import v4l2
import fcntl

#vd = open('/dev/video0', 'rw')
vd = open('/dev/video0', 'r')
cp = v4l2.v4l2_capability()
fcntl.ioctl(vd, v4l2.VIDIOC_QUERYCAP, cp)

print("driver:",cp.driver)
print("card:",cp.card)


$ python3 demo4SO.py
driver: b'uvcvideo'
card: b'Intel(R) RealSense(TM) 410'

我的用户名属于视频设备的组，所以问题似乎与权限无关:

$ groups
openstack root sudo video staff
$ ls -ls /dev/video0
0 crw-rw----+ 1 root video 81, 0 פבר 14 11:31 /dev/video0

知道为什么我不能按原样运行文档的示例代码(使用“rw”)吗？

Answer 1

改变

vd = open('/dev/video0', 'rw')

至

vd = open('/dev/video0', 'rb+', buffering=0)

启用以 O_RDWR 权限打开文件。