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Can anybode suggest me where to find the source code for system calls itself?
Download the libc source code of the appropriate system. You'll find libc sources for Linux, bsd and Solaris on the net.
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When ever we quit a file or save and quit a file,
Give some context. Quit and save aren't system calls either. Do you mean typical commands from the file menu? No. The application calls close(). The OS itsself takes care about writing the actual data to disk. Typically it won't write the contents to disk immediately but just send them to some buffer which is cleaned whenever the disk comes idle or the filesystem is unmounted.