Question $4$: Files in xv$6($Max Mark: $40/100)$

In Unix$/$Linux$,$ there is a system call named lseek $()($accessed through the standard C library$)$ that repositions the offset of an open file for reading and writing. Look at the man page of lseek to understand more.

Task: Implement a lseek $()$ system call for xv$6-$riscv. This system call is a lot more restricted than the Linux version. The function prototype is

int lseek$($int fd$,$ int offset$)$;

where fd is the file descriptor, and the file offset is set of offset bytes. Offset is always computed from the beginning of the file. It returns the resulting offset location in bytes if successful. Otherwise, it should return $-1.$

Note that if the offset is larger than the size of the file, then the offset location will be set to the end of the file and this value is what should be returned. This is different from the behaviour of the lseek $()$ in Unix$/$Linux$.$

Write your own user program filetest $($in a file filetest.c$)$ to test your lseek $()$ system call. It should take two arguments, the first one is a text file name and the second is an integer $($the offset$).$ Make sure that it test for the case where the offset is larger than the file size. Otherwise, it should read a few characters from the file and print them out to show that the offset has been set correctly.

Chapter $8$ of the xv$6$ book contains useful information.

Submission Requirements: Clean up $($remove$)$ all the object files and executable files using the command make clean $($in the directory where the Makefile is located$).$ Then, in the parent directory of xv$6-$riscv, run the tar command to archive the whole xv$6$riscv directory $($with its sub$-$directories including user, kernel, and mkfs$).$ Name this tar file q$4-.$tar where is your student ID$.$ Submit this file to Canvas.

The code you added$/$modified should be documented with appropriate comments.