Video4linux: /dev/video0 open failed

I am new to linux and am trying to run the ARToolKit simpleTest program. I am using Ubuntu 14.04.
I try doing ./Configure selecting the video4linux option. When I finally run the ./simpleTest command, it gives me this output:

Using supplied video config string [-dev=dev/video0 ].
video device (dev/video0) open failed

I have already modified the simpleTest.c file as such

#ifdef _WIN32
char            *vconf = "Data\WDM_camera_flipV.xml";
char            *vconf = "-dev=dev/video0 ";

Please help me understand the problem! My objective is to use the inbuilt webcam of the laptop. Would appreciate any suggestions

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Author: pranp2001