程序代写案例-CS3214

  • January 26, 2021

CS3214 Spring 2021 Exercise 0 Due Date: see website In this class, you are required to have familiarity with Unix commands and Unix pro- gramming environments. The first part of this exercise is related to making sure you are comfortable in our Unix environment; the second and third part asks for familiarity with how to invoke programs in a Unix environment and how to make use of the standard input and output streams. 1 Using Linux It is crucial that everybody become productive using a Unix command line, even if the computer you are using daily is a Windows or OSX machine. Working on the command line requires working knowledge of a shell such as bash or zsh, but it also requires an understanding of how the shell interacts with system commands and programs from the user’s perspective. Please do the following, then answer the questions below. • Make sure your personal machine has an ssh client installed. Set your machine up for public key authentication when logging on to rlogin.cs.vt.edu. Use ssh-keygen to create a key. There is also an web interface provided by the department that allows you to create a key pair at https://admin.cs.vt.edu/keys.pl. However, in this case, you do not maintain continuous possession of the private key from its inception. • Make sure you can use the command line editing facilities of your shell. For bash users, which most of you are by default, examine the effect of the following keys when editing: dˆ, TAB, aˆ, eˆ, rˆ, kˆ, and wˆ. Examine the effect of the following keys when you invoke a program: cˆ, sˆ, qˆ (ˆx stands for Ctrl-x.) • Customize your shell and create a custom prompt and any aliases you may need. • Make sure you know how to use at least one command line editor, such as vim, nano, pico, or emacs. • Many students set up a remote environment that allows them to use an IDE on their computer. Notably, Microsoft’s Visual Studio Code provides an extension that presents a remote environment within the IDE that is well integrated. Although not mandatory, we highly recommend that you do this as well. Answer the following questions: 1. To which file did you need to add your public key to set up public key authentica- tion? 2. Which key offers filename completion in your shell? 1 CS3214 Spring 2021 Exercise 0 3. How many machines are part of the rlogin cluster (Hint: visit http://rlogin.cs.vt.edu/) this semester? Include only those whose names are derived from trees, e.g. “birch.” 4. Which command do you use to find out the name of the machine on which your shell runs? 5. Make sure your bash prompt includes your username, the name of the current ma- chine, and a suffix of the current directory. Copy the value of your $PS1 variable here. 6. What is the significance of filenames that start with a dot? 7. Why do many people define an alias for rm using alias rm=’rm -i’? 8. Which Unix command compares two files line by line? 9. Which switch makes grep recurse down directories? 10. What is the difference between the command date and the bash shell built-in time? 11. Which Unix group or group(s) are you currently a member of on our cluster? 12. What Unix permissions does a file or directory need to have to make sure that no one besides yourself can access it? 2 Understanding Command Line Arguments and Standard I/O in Unix In the past, we observed that some students coming into CS 3214 did not understand how programs access their command line arguments and how they make use of the stan- dard input/output facilities, which present one of the basic abstractions provided by an operating system. 2 CS3214 Spring 2021 Exercise 0 Application Side Note. Deep knowledge of Unix is an absolute prerequisite for anyone wanting to learn or work with containers. As an example, consider this ex- cerpt [link] from a script used to set up the container in which this semester’s Dis- course server runs: run_image=‘cat $config_file | $docker_path run $user_args \ –rm -i -a stdin -a stdout $image ruby -e \ “require ’yaml’; puts YAML.load(STDIN.readlines.join)[’run_image’]”‘ This command sets a variable run_image to the standard output that results from running the pipeline that is enclosed in backquotes. This pipeline consists of 2 com- mands: the command cat, which is given 1 argument (taken from the value of $config_file) and whose standard output is “piped” into the command given by the $docker_path variable (probably docker), which is invoked with 12 argu- ments, the last one being a Ruby program that will be run inside the container, but which can access as its standard input (STDIN) the data written to cat’s standard output. Being able to understand what commands like this one do is a motivation for this exercise (and hopefully, the following exercise and project will provide an even deeper understanding). To practice this knowledge, write a C program that concatenates a combination of given files and/or its standard input stream to its standard output stream. The exact specification is as follows. Your program should be simply called concatenate.c. When invoked without arguments, it should copy the content of its standard input stream to its standard output stream. “Standard input” and “standard output” are standard streams that are set up by a control program that starts your program (often, the control program is a shell). When invoked with arguments, it should process the arguments in order. Each argument should be treated as the name of a file. These files should be opened and their content should be written to the standard output stream, in the order in which they are listed on the command line. If the name given is – (a single hyphen), the program should read and output the content of its standard input stream instead. If any of the files whose names are given on the command line do not exist, the program’s behavior is undefined. Your C program may make use of C’s stdio library (e.g., the family of functions including fgetc, etc.), or it may use system calls such as read() or write() directly. You should buffer the data, but you may not assume that it is possible to buffer the entire file content in memory. Implementation Requirement: to make sure you understand the uniformity provided by the POSIX C API, we require that your program use the same function to copy the data contained in files and for the data it reads from its standard input stream. Your program’s main() function will then call this single function multiple times, as needed. 3 CS3214 Spring 2021 Exercise 0 In other words, do not make use of facilities such as getchar() that implicitly refer to the standard input stream. You may use the script test-concat.sh to test your code. 3 Understanding how to access the Standard Input and Out- put Streams in your Preferred Language Standard input and output are concepts that are not specific to the use of C. Choose a language of your choice that is not C (e.g. C++, Go, Ruby, Java, Python 2, Python 3, JavaScript, etc. etc.) and implement the concatenate program in your language.1 If your language cannot be compiled into an executable, and also cannot be executed directly by an interpreter using the Shebang/Hash-bang convention, you need to create a wrapper script for testing it. This wrapper script may be required for Java, it should invoke your program, passing any command line arguments to it. You may use test-concat.sh to test, by passing the name of your script or executable as an argument. Hint: most higher-level language allow compact implementations of these tasks. For instance, my Python 2 implementation is 13 lines long. Implementation Requirement: the implementation requirement is the same. Do not special case standard input/output, use a single function. Implementation Notes For both parts 3 and 2, your program should not attempt to interpret the content of the streams it reads and writes in any way. In other words, it should output the bytes (octets) that appear in the input as they appear without making assumptions or processing them in any way. This includes the possible occurrence of the byte value 0x00, which may occur any number of times in the input and must be copied into the output. Similarly, the byte value 0x0A (aka LF, or LINEFEED character) may occur any number of times. Your program should not assign special significance to either of them, so do not assume (a) that data read can be represented as zero-terminated C-style strings, and (b) do not assume that the input can be broken into lines efficiently. (The worst case input would be a sequence consisting of only LF characters.) Many programs process characters, which has contributed to the fact that the I/O li- braries of some higher-level languages default to the assumption that programmers will 1If you choose C++, you should use C++’s standard library, not C’s. 4 CS3214 Spring 2021 Exercise 0 want to input and/or output character streams in some valid encoding when accessing file streams. The most commonly used character set today is the Unicode character set, and the encoding that is most commonly used is UTF-8. For instance, the unicode char- acter U+263A is encoded as a 3-byte sequence 0xE2 0x98 0xBA in UTF-8. While any sequence of Unicode characters can be encoded into a sequence of bytes, the opposite is not true: not every sequence of bytes represents a valid encoding of some characters. 2 For this problem, do not assume that the input represents characters in any valid encod- ing. Specifically, the input data may not represent a valid UTF-8 encoding, and therefore, attempts to interpret it as UTF-8 data and decode it will fail for some tests, resulting in exceptions and/or data corruption. Efficiency. You should use buffered forms of input and output in order to reduce the number of system calls your program makes. For instance, in C, the stdio library provides such buffering by default if you use fgetc() or fread(), whereas if you use the lower- level read() call directly you will need to make sure that you do buffering yourselves (in other words, read multiple bytes at once rather than a single byte in each call). The autograder may run your program under a suitable timeout. What to submit: Submit a tar file with your answers. The tar file must contains a UTF-8 text file answers.txt with your answers to part 1, a C file concatenate.c containing your implementation for part 2. In addition, include the file or files needed to answer part 3. Do not submit compiled executables. 2For those wanting to learn more about the rationale behind UTF-8, I recommend The history of UTF-8 as told by Rob Pike which describes how Ken Thompson invented UTF-8 in one evening and how they together built the first system-wide implementation in less than a week. 5 欢迎咨询51作业君

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