Thread: works with devices in /dev/*

  1. #1
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    works with devices in /dev/*

    hi,i am new in c programming and lately i have been wondering:

    if in linux, all devices are considered as files, why some devices like usb webcam connected to linux machine are as not simple to open and use as opening a normal file in a c program?
    ex: i connected a usb webcam on my linux(ubuntu) laptop and i tried to open it and read from it as normal file but i could work, and i wonder why i can open a camera as normal file and read from it if linux consider it as a file. thank you.

    ps: i apologise in advance if i asked a stupid question .

  2. #2
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    i tried to open it and read from it as normal file but i could [not get it to] work
    What exactly did you do? (Post code/script.)
    How exactly did it not work?
    If you want the truth to stand clear before you, never be for or against. - Sent-ts'an

  3. #3
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    May be you need to have root Permissions to use usb-devices.
    For example if you want to use an usb-stick with windows emulated on qemu-kvm
    you need first to login as superuser.
    Then, start terminal and type 'lsusb' to detect what for a bus and what for a device
    you must use to access an usb-stick with an emulated operating system.

    If you want to get only informations you can use 'lsusb' without root permissions.

    I don't know if you can change the permissions with chmod for the device, but it is
    possible that the permissions will change to the former status after a new start of
    the system.

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    Quote Originally Posted by j3ph View Post
    hi,i am new in c programming and lately i have been wondering:

    if in linux, all devices are considered as files, why some devices like usb webcam connected to linux machine are as not simple to open and use as opening a normal file in a c program?
    Yep.. use open() to get a file desciptor and...

    Quote Originally Posted by j3ph View Post
    ex: i connected a usb webcam on my linux(ubuntu) laptop and i tried to open it and read from it as normal file but i could work, and i wonder why i can open a camera as normal file and read from it if linux consider it as a file. thank you.
    ... there is a special syscall to deal with devices: ioctl(), to send "commands" to control the device. Those commands are device dependent...

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