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• Why can’t two programs access my webcam simultaneously?

  - by qdii

  I first launch cheese and my webcam turns on. I then run vlc to grab the output of /dev/video0 but it fails with: [0x7f3ea40012e8] v4l2 demux error: cannot set input 0: Device or resource busy [0x7f3ea40012e8] v4l2 demux error: cannot set input 0: Device or resource busy [0x7f3ea4002168] v4l2 access error: cannot set input 0: Device or resource busy [0x7f3ea4002168] v4l2 access error: cannot set input 0: Device or resource busy [0x7f3eb4000b78] main input error: open of `v4l2:///dev/video0' failed Whatever pair of video programs I run (skype, cheese, vlc, etc.), the result is always the same: the second program can no longer use the webcam when the first one has already grabbed the output. However I find it curious as video4linux states: In general, V4L2 devices can be opened more than once. When this is supported by the driver, users can for example start a "panel" application to change controls like brightness or audio volume, while another application captures video and audio. My webcam is seen in lspci as 058f:a014 Alcor Micro Corp. Asus Integrated Webcam, but I don’t even know what the underlying driver is, so I can’t check whether my problem is driver-related or not. Any input would be more than welcome!

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• Pinging an external server through OpenVPN tunnel doesn’t work

  - by qdii

  I have an OpenVPN server and a client, and I want to use this tunnel to access not only 10.0.8.0/24 but the whole internet. So far, pinging the server from the client through the tun0 interface works, and vice versa. However, pinging www.google.com from the client through tun0 doesn’t work (all packets are lost). I figured that I should configure the server so that any packet coming from tun0 in destination of the internet be forwarded, so I came up with this iptables config line: interface_connecting_to_the_internet='eth0' interface_openvpn='tun0' internet_ip_address=`ifconfig "$interface_connecting_to_the_internet" | sed -n s'/.*inet \([0-9.]*\).*/\1/p'` iptables -t nat -A POSTROUTING -o "${interface_connecting_to_the_internet}" -j SNAT --to-source "${internet_ip_address}" echo '1' > /proc/sys/net/ipv4/ip_forward Yet, this doesn’t work, the packets are still lost and I am wondering what could possibly be wrong with my setup. Some details: ip route gives on the server: default via 176.31.127.254 dev eth0 metric 3 10.8.0.0/24 via 10.8.0.2 dev tun0 10.8.0.2 dev tun0 proto kernel scope link src 10.8.0.1 127.0.0.0/8 via 127.0.0.1 dev lo 176.31.127.0/24 dev eth0 proto kernel scope link src 176.31.127.109 ip route gives on the client: default via 192.168.1.1 dev wlan0 proto static 10.8.0.1 via 10.8.0.5 dev tun0 10.8.0.5 dev tun0 proto kernel scope link src 10.8.0.6 127.0.0.0/8 via 127.0.0.1 dev lo scope link 192.168.1.0/24 dev wlan0 proto kernel scope link src 192.168.1.109 client uses wifi adapter wlan0 and TUN adapter tun0. server uses ethernet adapter eth0 and TUN adapter tun0. the VPN spans on 10.0.8.0/24 both client and linux are using Linux 3.6.1.

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• Why should I use $[ EXPR ] instead of $(( EXPR ))?

  - by qdii

  On the paragraph explaining arithmetic expansion, Bash's user guide uncovers 2 different ways of evaluating an expression, the first one uses $((?EXPRESSION?)) and the second one uses $[?EXPRESSION?]. The two ways seem pretty similar as the only difference I have found is: $[?EXPRESSION?] will only calculate the result of EXPRESSION, and do no tests: Yet, I am intrigued because the same document recommends using $[?EXPRESSION?] rather than $((?EXPRESSION?)). Wherever possible, Bash users should try to use the syntax with square brackets: Why would you want that if less tests are being done?

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• Do control groups improve system performances?

  - by qdii

  According to this website, enabling cgroups in the kernel can boost performances by sharing resources in a better way. In particular, the conclusion states that:  Nevertheless, with a little trial and error, cgroups can help you improve the efficiency of your systems’ resource usage and avoid downtime due to overusage of a single service. Kernel seeds, however, recommend to deactivate them altogether. They say: Consider these [kernel] settings poison. They remain nothing but system slow-downs. They are all off by default [in the proposed kernel config file]. Who should I trust?

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• GetMessage with a timeout

  - by qdii

  I have an application which second thread calls GetMessage() in a loop. At some point the first thread realizes that the user wants to quit the application and notifies the second thread that he should terminate. As the first thread is stuck on GetMessage(), the program never quits. Is there a way to wait for messages with a timeout? I’m open to other ideas too. EDIT: (additional explanations) The second thread runs that snippet of code: while ( !m_quit && GetMessage( &msg, NULL, 0, 0 ) ) { TranslateMessage( &msg ); DispatchMessage( &msg ); } The first thread sets m_quit to true.

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