2019-07-24 18:20:27 +00:00
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<html>
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<head>
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2019-07-25 00:30:20 +00:00
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<title>Lab: xv6</title>
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2019-07-24 18:20:27 +00:00
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<link rel="stylesheet" href="homework.css" type="text/css" />
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</head>
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<body>
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<h1>Lab: xv6</h1>
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This lab makes you familiar with xv6 and its system calls.
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<h2>Boot xv6</h2>
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<p>Login to Athena (e.g., ssh -X athena.dialup.mit.edu) and attach the course
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locker: (You must run this command every time you log in; or add it to your
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~/.environment file.)
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<pre>
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$ add -f 6.828
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</pre>
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<p>Fetch the xv6 source:
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<pre>
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$ mkdir 6.828
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$ cd 6.828
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$ git clone git://github.com/mit-pdos/xv6-riscv.git
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Cloning into 'xv6-riscv'...
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...
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$
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</pre>
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<p>XXX pointer to an update tools page
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<p>Build xv6 on Athena:
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<pre>
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$ cd xv6-public
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$ makeriscv64-linux-gnu-gcc -c -o kernel/entry.o kernel/entry.S
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riscv64-linux-gnu-gcc -Wall -Werror -O -fno-omit-frame-pointer -ggdb -MD -mcmodel=medany -ffreestanding -fno-common -nostdlib -mno-relax -I. -fno-stack-protector -fno-pie -no-pie -c -o kernel/start.o kernel/start.c
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...
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$ make qemu
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...
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mkfs/mkfs fs.img README user/_cat user/_echo user/_forktest user/_grep user/_init user/_kill user/_ln user/_ls user/_mkdir user/_rm user/_sh user/_stressfs user/_usertests user/_wc user/_zombie user/_cow
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nmeta 46 (boot, super, log blocks 30 inode blocks 13, bitmap blocks 1) blocks 954 total 1000
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balloc: first 497 blocks have been allocated
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balloc: write bitmap block at sector 45
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qemu-system-riscv64 -machine virt -kernel kernel/kernel -m 3G -smp 3 -nographic -drive file=fs.img,if=none,format=raw,id=x0 -device virtio-blk-device,drive=x0,bus=virtio-mmio-bus.0
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hart 0 starting
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hart 2 starting
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hart 1 starting
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init: starting sh
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$
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</pre>
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<p>
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If you type <tt>ls</tt> at the prompt, you should output similar to the following:
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<pre>
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$ ls
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. 1 1 1024
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.. 1 1 1024
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README 2 2 2181
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cat 2 3 21024
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echo 2 4 19776
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forktest 2 5 11456
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grep 2 6 24512
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init 2 7 20656
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kill 2 8 19856
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ln 2 9 19832
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ls 2 10 23280
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mkdir 2 11 19952
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rm 2 12 19936
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sh 2 13 38632
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stressfs 2 14 20912
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usertests 2 15 106264
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wc 2 16 22160
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zombie 2 17 19376
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cow 2 18 27152
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console 3 19 0
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</pre>
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These are the programs/files that <tt>mkfs</tt> includes in the
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initial file system. You just ran one of them: <tt>ls</tt>.
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<h2>sleep</h2>
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2019-07-24 18:26:05 +00:00
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<p>Implement the UNIX program sleep, which sleeps for a user-specified
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number of ticks.
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2019-07-24 18:20:27 +00:00
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<p>Some hints:
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<ul>
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<li>Look at some of the other programs in <tt>user/</tt> to see
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how you can obtain the arguments passed to a program. If the user
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forgets to pass an argument, sleep should print an error message.
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<li>The argument is passed as a string; you can convert it to an
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integer using <tt>atoi</tt> (see user/ulib.c).
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2019-07-24 18:26:05 +00:00
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<li>Use the system call <tt>sleep</tt> (see user/usys.S and kernel/sysproc.c).
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<li>Make sure <tt>main</tt> calls <tt>exit()</tt> in order to exit
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your program.
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<li>Add the program to <tt>UPROGS</tt> in Makefile and compile
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user programs by typing <tt>make fs.img</tt>.
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</ul>
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<p>Run the program from the xv6 shell:
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<pre>
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$ make qemu
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...
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init: starting sh
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2019-07-25 11:07:03 +00:00
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$ sleep 10
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2019-07-24 18:20:27 +00:00
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(waits for a little while)
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$
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</pre>
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<p>Optional: write an uptime program that prints the uptime in terms
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2019-07-25 00:30:20 +00:00
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of ticks using the <tt>uptime</tt> system call.
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2019-07-24 18:20:27 +00:00
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<h2>pingpong</h2>
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2019-07-25 00:30:20 +00:00
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2019-07-24 18:20:27 +00:00
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<p> Write a program that uses UNIX system calls to ``ping-pong'' a
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byte between two processes over a pair of pipes, one for each
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direction. The parent sends by writing a byte to <tt>fd[1]</tt> and
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the child receives it by reading from <tt>fd[0]</tt>. After
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receiving a byte from parent, the child responds with its own byte
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by writing to <tt>fd[1]</tt>, which the parent then reads.
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<p>Some hints:
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<ul>
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<li>Use <tt>pipe</tt> to create a pipe.
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<li>Use <tt>fork</tt> to create a child.
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<li>Use <tt>read</tt> to read from the pipe, and <tt>write</tt> to write to the pipe.
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</ul>
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2019-07-25 00:30:20 +00:00
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<h2>primes</h2>
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<p>Write a concurrent version of prime sieve using pipes. This idea
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is due to Doug McIlroy, inventor of Unix pipes. The picture
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halfway down <a href="http://swtch.com/~rsc/thread/">the page</a>
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and the text surrounding it explain how to do it.
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<p>Your goal is to use <tt>pipe</tt> and <tt>fork</tt> to set up
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the pipeline. The first process feeds the numbers 2 through 35
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into the pipeline. For each prime number, you will arrange to
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create one process that reads from its left neighbor over a pipe
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and writes to its right neighbor over another pipe. Since xv6 has
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limited number of file descriptors and processes, the first
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process can stop at 35.
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<p>Some hints:
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<ul>
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<li>Be careful to close file descriptors that a process doesn't
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need, because otherwise your program will run xv6 out of resources
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before the first process reaches 35.
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<li>Once the first process reach 35, you should arrange that the
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pipeline terminates cleanly (Hint: read will return an end-of-file
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when the write-side of the pipe is closed).
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</ul>
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2019-07-24 18:20:27 +00:00
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<h2>find</h2>
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2019-07-24 18:26:05 +00:00
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<p>Write a simple version of the UNIX find program: find all the files
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2019-07-24 22:00:32 +00:00
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in a directory tree whose name matches a string. For example if the
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file system contains a file <tt>a/b</tt>, then running find as
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follows should produce:
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<pre>
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$ find . b
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./a/b
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$
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</pre>
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2019-07-24 18:26:05 +00:00
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<p>Some hints:
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<ul>
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<li>Look at user/ls.c to see how to read directories.
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2019-07-24 22:00:32 +00:00
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<li>Use recursion to run find in sub-directories.
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<li>Don't recurse into "." and "..".
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</ul>
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2019-07-24 18:26:05 +00:00
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2019-07-25 10:50:12 +00:00
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<p>Optional: support regular expressions in name matching. Grep has some
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primitive support for regular expressions.
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<h2>xargs</h2>
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<p>Write a simple version of the UNIX xargs program: read lines from
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standard in and run a command for each line, supplying the line as
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arguments to the command. The following example illustrates xarg's
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behavior:
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<pre>
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$ xargs echo bye
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hello too
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bye hello too
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<ctrl-d>
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$
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</pre>
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Note that the command here is "echo bye" and the additional
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arguments are "hello too", making the command "echo bye hello too",
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which outputs "bye hello too".
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<p>xargs and find combine well:
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<pre>
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find . b | xargs grep hello
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</pre>
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will run "grep hello" on each file named b in the directories below ".".
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<p>Some hints:
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<ul>
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<li>Use <tt>fork</tt> and <tt>exec</tt> system call to invoke the
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command on each line of input. Use <tt>wait</tt> in the parent
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to wait for the child to complete running the command.
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<li>Read from stdin a character at the time until the newline
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character ('\n').
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<li>kernel/param.h declares MAXARG, which may be useful if you need
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to declare an argv.
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</ul>
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2019-07-25 11:47:22 +00:00
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<h2>System call tracing</h2>
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2019-07-25 10:50:12 +00:00
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2019-07-25 11:47:22 +00:00
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<p>In this exercise you will modify the xv6 kernel to print out a line
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for each system call invocation. It is enough to print the name of the
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system call and the return value; you don't need to print the system
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call arguments.
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<p>
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When you're done, you should see output like this when booting
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xv6:
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<pre>
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...
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fork -> 2
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exec -> 0
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open -> 3
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close -> 0
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$write -> 1
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write -> 1
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</pre>
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<p>
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That's init forking and execing sh, sh making sure only two file descriptors are
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open, and sh writing the $ prompt. (Note: the output of the shell and the
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system call trace are intermixed, because the shell uses the write syscall to
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print its output.)
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2019-07-25 00:30:20 +00:00
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2019-07-25 11:47:22 +00:00
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<p> Hint: modify the syscall() function in kernel/syscall.c.
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<p>Run the programs you wrote in the previous exercises and inspect
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the system call trace. Are there many system calls? Which systems
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calls correspond to code in the applications you wrote above?
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<p>Optional: print the system call arguments.
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2019-07-24 18:20:27 +00:00
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2019-07-25 11:47:22 +00:00
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<h2>Optional: modify the shell</h2>
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2019-07-24 18:20:27 +00:00
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2019-07-25 11:47:22 +00:00
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There are endless ways in which the shell could be extended. Here are
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some suggestions:
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2019-07-24 22:00:32 +00:00
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2019-07-25 11:47:22 +00:00
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<ul>
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<li>Modify the shell to support wait.
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2019-07-24 18:20:27 +00:00
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2019-07-25 11:47:22 +00:00
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<li>Modify the shell to support lists of commands, separated by ";"
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<li>Modify the shell to support sub-shells by implementing "(" and ")"
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<li>Modify the shell to allow users to edit the command line
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</ul>
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2019-07-24 18:20:27 +00:00
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</body>
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</html>
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