Android：在/ dev / video0上设置666权限

Question

I have built an Android app which uses an external USB camera. The camera image is shown full-screen app takes screenshots, and stores them on the local file system. When the USB camera is connected, it creates and streams to the following folder;

/dev/video0

the folder is destroyed each time the camera is disconnected.

This issue is that my app doesn't have permission to read from this folder. So I have to set the permissions each time the camera is connected.

I have tried to get the app to set the permissions automatically, using the following code;

Process sh = Runtime.getRuntime().exec(new String[]{"su", "-c", "system/bin/sh"});
sh = Runtime.getRuntime().exec(new String[]{"chmod", "666", "/dev/video0"});        
sh.waitFor();

...which asks the user to grant Super User permissions, and gives me feedback telling me that the user is now root - but I can see via terminal that the permissions haven't been changed on the folder.

Any help would be greatly appreciated

Answer 1

the shell supports reading commands via stdin (like you are doing with the OutputStream) or as arguments to it's -c command line option. For example, this should work also if you'd rather not create the extra object:

Process sh = Runtime.getRuntime().exec(new String[]{"su", "-c", "/system/bin/sh -c \"chmod 666 /dev/video0\""});

this translates to executing in the shell:

su -c "/system/bin/sh -c \"chmod 666 /dev/video0\""

which you can try out first in adb if you want.

In this particular case, because you are only executing one command anyway, you don't really need the shell at all and can probably just call this:

Process sh = Runtime.getRuntime().exec(new String[]{"su", "-c", "chmod 666 /dev/video0"});

Also to add to the comment that you can edit the ueventd.rc file: that should work, but those files are either in the read-only /system partition (which is easily edited by remounting it with read-write) or more likely some ramdisk (gzipped cpio archive) that is flashed to a partition on the device. The latter won't be quite as easy unless you can find that ramdisk image on the flash chip, extract it, edit the file, recreate the cpio, and write it back to the flash.

Answer 2

If you are not concerned about the risks of giving those permissions to the camera, you can add

/dev/video*               0666   system     camera

to /ueventd.rc .

Answer 3

I eventually found the following code which works great;

Process sh = Runtime.getRuntime().exec("su", null,null);
OutputStream  os = sh.getOutputStream();
os.write(("chmod 666 /dev/video0").getBytes("ASCII"));
os.flush();
os.close();
sh.waitFor();

For some reason, the command has to be sent via an OutputStream.

Answer 4

也许您只需要在AndroidManifest.xml中添加摄像头权限：

<uses-permission android:name="android.permission.CAMERA"/>