CSE 333 25su Exercise 6

out: Monday, July 07, 2025
due: Wednesday, July 09, 2025 10:00 am. No late exercises accepted.

Goals: Learn how to use various parts of the standard C file I/O library, particularly ones that will be needed on later projects. Also reenforce understanding of exactly what is in a file that is stored on the disk.

Description: Write a C program that accepts a filename as a single command-line argument. The program should read the file, copying the contents of the file to stdout in reverse order, character by character (or, more precisely, byte by byte).

For example, consider a file name foo.txt in the current directory that contains the following 7 bytes (the 7th byte is the newline character at the end):

hello!

Running the program should result in the following:

bash$ ./ex6 foo.txt



!ollehbash$

Pay careful attention to where the newline and other characters are in this output.

You will need to use several standard library calls in order to do this. In particular, you should use:

• fopen, with mode "rb", to open the file
• fclose to close the file
• fseek and ftell to figure out how large the file is. (We'd suggest you fseek(f, 0, SEEK_END); and then size = ftell(f); to get the file size, in bytes.)
• fseek and fread to read the file's contents, in reverse order, byte-by-byte.
• printf, with the %c format code, to print out the characters (bytes) of the file one at a time.

Your code should read the bytes one-at-a-time and print them immediately. Do not read the entire file into memory and print it backwards or create a reversed copy of the data and print that. A goal of this exercise is to gain experience with the various file I/O functions described above.

Remember to consult the C/C++ Reference Material and Linux manual pages to learn how to use each of these functions. (Check out the stdio.h header)

Note: Some of these library functions are quite old and have parameters or results whose type is long. Style guides and programs like cpplint generally discourage using long and recommend using types like int64_t instead. But for these library functions it's appropriate to use variables of type long to match function parameter and result types, since that guarantees an exact match regardless of the exact size of long on the underlying machine. Feel free to ignore any cpplint warnings about using long in these situations.

Hint: you can use linux commands like hexdump -C foo.txt or hexdump -c foo.txt to look at the bytes in a file foo.txt in hex and in other formats to see exactly what's there, including invisible characters. Many text editors also have modes to do this.

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Your code must:

• compile without errors or warnings on CSE Linux machines (lab workstations, attu, or CSE home VM)
• have no crashes, memory leaks, or memory errors on CSE linux machines (Suggestion: valgrind)
• be contained in a single file called ex6.c that compiles with the command gcc -Wall -g -std=c17 -o ex6 ex6.c -- do not submit a Makefile.
• be pretty: the formatting, modularization, variable and function names, and so on must make us smile rather than cry. (Suggestion: cpplint --clint)
• be robust: you should think about handling bogus input from the user (if any), and you should handle hard-to-handle cases (if there are any) gracefully.
• have a comment at the top of the source file with your name, and CSE or UW email address.

You should submit your exercise using the Gradescope dropbox linked on the course resources web page.