辅导案例-A3
CSE 30, Winter 2020 A3: GDB and Printify Due: Wednesday, Jan 29 @ 11:59PM Please read over the entire assignment before starting to get a sense of what you will need to get done in the next week. Your program must be able to run on ieng6 Part 0: Degree Planning Assignment [1 pt] Please complete the degree planning assignment found here: https://docs.google.com/document/d/1vrYQav_7lBePZpHp9KkTE2kCha8p_Q8uyvfdypEF8jI/edit Part 1: GDB Core Dump [4 pts] ● What is GDB? ● Getting Started ● How the program works ● Submission and Grading Part 2: Printify [45 pts] ● Getting Started ● Description ● Arguments ● Source Files ● ASCII cGrams ● Global Buffers ● Functions and Behavior to Implement ● Submission and Grading ● Extra Notes on Printify CSE 30, Winter 2020 Part 1: GDB Core Dump When dealing with tools such as GDB you are free to search online to find any information that may help you. GDB is an extremely important tool and will save you HOURS of time debugging. Learning Goals ● Gain practice using the gdb interface and commands ● Understand how useful gdb is ● Use gdb in Part 2 of this assignment and all future work in C and Assembly! ● Reading and understanding code written in C What is GDB? GDB is the GNU debugger. It allows you to see what is going on inside another program while it executes or the moment the program crashed. (https://www.gnu.org/software/gdb/) Why use GDB? Can’t I just use print statements? Well, you can and should use print statements in some cases BUT gdb is an extremely useful tool for debugging that can help you determine exactly where errors are occuring. It allows you to execute a program step by step, print intermediate values, and set breakpoints to pause code execution at a specific line. Getting Started For this assignment you will need to run a simple program and determine what is causing a segmentation fault when you run it. Compile the program: $ gcc -g -O0 -Wall -Wextra -o Argument Description gcc Runs the GNU Compiler Collection (GCC) -g Flag to produce debugging information CSE 30, Winter 2020 -O0 Used to reduce compiler optimizations -Wall (optional) This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid -Wextra (optional) This enables some extra warning flags on top of -Wall -o (optional) Name of the output executable (will be a.out if unspecified) Name of the source code files (just coredump.c in our case) How the program works For this assignment, you will run the program located in coredump.c . This is a small program that, when run, will lead to a segmentation fault. It is your job to use gdb to debug this program and determine what is causing the program to segfault. There starter code can be found here: https://github.com/cse30-wi20/hw3gdb When you compile and run this program, the output will be: Segmentation fault (core dumped) We have provided you with an example of the commands you should run in gdb to find the issue but expect you to spend time playing around with gdb as well. GDB Commands You Will Use https://gcc.gnu.org/onlinedocs/gcc-3.3.6/gnat_ug_unx/Introduction-to-GDB-Commands.html Argument Description run executes the program from the beginning where shows the next statement that will be executed up used to examine the content of frames p (or print) print the value of a given expression q (or quit) used to quit out of gdb CSE 30, Winter 2020 What You Should Run $ gdb a.out (gdb) run Starting program: /home/core/a.out Program received signal SIGSEGV, Segmentation fault. strlen () at ../sysdeps/x86_64/strlen.S:106 106 ../sysdeps/x86_64/strlen.S: No such file or directory. (gdb) (gdb) where #0 strlen () at ../sysdeps/x86_64/strlen.S:106 #1 0x00007ffff7a7c69c in _IO_puts (str=0x0) at ioputs.c:35 #2 0x00005555555546ee in printListFwd (curPt=0x555555756010) at coredump.c:26 #3 0x0000555555554786 in main (argc=1, argv=0x7fffffffdc78) at coredump.c:41 (gdb) up #1 0x00007ffff7a7c69c in _IO_puts (str=0x0) at ioputs.c:35 35 ioputs.c: No such file or directory. (gdb) up #2 0x00005555555546ee in printListFwd (curPt=0x555555756010) at coredump.c:26 26 //! This function returns a pointer to the new record which becomes (gdb) print curPt->name … what is printed here? In the above example, the where command prints which functions have been called to get to the failure point. Typing up moves gdb to the function that called a function. The error occurs in an internal library's functions (strlen() and _IO_puts ), which are called from our program. So we type up twice to get to our code. CSE 30, Winter 2020 Submission and Grading Submitting You will submit a PDF to the assignment the “HW 3: GDB” assignment on Gradescope. The PDF should contain: 1. Take a screenshot of the terminal where you ran gdb (it should look similar to what is shown above). It is okay if you need multiple screenshots to show everything you ran. 2. Provide a short explanation on what is causing program to have a segmentation fault. What does the final command you run (print curPt->name ) print? What does it mean? What does it say about the issue with our program? How can the program be fixed? Grading Breakdown ● [2 point] Screenshots of GDB output ● [2 point] Explanation of why the segmentation fault is occurring CSE 30, Winter 2020 Part 2: Printify Learning Goals: ● File IO ● Pointer arrays ● 2D Arrays ● Flags with optional arguments ● Multiple .c files and .h files Getting Started The starter code is at https://github.com/cse30-wi20/hw3starter. You can either clone the repo or download the zip. The Makefile provided will create a printify executable from provided source files. Compile it by typing make into the terminal. Warnings are on by default (run make no_warnings to compile without warnings). Run make clean to remove all files generated by make . Description Printify is a commonly used spell that generates fancy announcements at venues like Quidditch matches and tournaments. Given a message string, it conjures large ASCII-art characters for every character in the input. (Handy ASCII Table.) Make sure your terminal window is large enough to hold the entire buffer or it may wrap around and mess up the display (150x16 chars by default; defined in globals.h ). Input: ● A font file ● A message string* Output: ● A MAXWIDTH x MAXWIDTH symbol message printed using cGrams (ASCII-art) Ex. ./printify fonts/cse30.font Printify! CSE 30, Winter 2020 Printify Arguments Format for calling this executable with arguments (up to one flag at a time): ./printify [-h] [-f fillSymbol] Argument Description -h Prints a help message and exits the program. -f fillSymbol Specifies a char (fillSymbol) to use as background. The default fillSymbol is a single space character (ASCII 32). To debug we recommend using the ‘-’ as fillSymbol The path to a font file. printify> The words you want to appear in the output. If no input text provided, the program prints nothing. Source Files The following files are required to compile and run, but globals.h must not be edited. Filename Description globals.h do NOT edit Defines global constants, and externs variable and function declarations to interlink all of the other .c files (so that they can use each other's variables). ● MAXFONTSIZE - max size of fontBuffer ● MAXFONTLINESIZE - max size of one line in font file ● MAXNUMCGRAMS - number of distinct cGrams ● MAXWIDTH - max width of the 2D displayBuffer ● MAXHEIGHT - max height of the 2D displayBuffer ● FONTDELIM - char used to indicate end of cGram ● SPACING - # of cols between cGrams in displayBuffer ● SMILEY - represents emoji :) as ascii value 127 ● FIRSTCHAR - char of first cGram in font file continued on the next page CSE 30, Winter 2020 Externed globals from other files: ● char fontBuffer[MAXFONTSIZE] - holds all of the cGrams in flattened 1D form ● char *cGramLookup[MAXNUMCGRAMS] - maps a char to its cGram's location in fontBuffer ● int cGramWidth[MAXNUMCGRAMS] - maps a char to its cGram's width ● char displayBuffer[MAXHEIGHT][MAXWIDTH] - a 2D array for displaying cGrams fontBuffer.c ● int readFontBuffer(const char *fontFileName) - reads font file; updates cGramLookup , cGramWidth ● void printCGram(const char c) - looks up and prints out appropriate cGram displayBuffer. c ● int copyCGram(const char c, int xPos) - starting at column xPos , looks up and copies the appropriate cGram char by char into displayBuffer (not exceeding MAXWIDTH ) - returns where *the next cGram* should start (i.e. the next xPos ) ● void fillDisplayBuffer(const char c) - fills the display buffer with the given symbol ● void printDisplayBuffer() - prints the entire display buffer to stdout main.c ● void printHelp() - prints the help message; called with the -h flag ● int main(int argc, char** argv) - processes options, reads in the font file, reads in the input string, and prints out the message CSE 30, Winter 2020 ASCII cGrams We have implemented a font file in fonts/cse30.font . This file contains ASCII art of all the printable ASCII characters, which we call cGrams. Our cse30.font cGrams are separated by a single line containing the '#' delimiter character (no delim at the start of the file), which is not considered part of the cGram. Here is a subset of the file with two cGrams shown, where ↵ represents the '\n' character: .oooo. ↵ d8P'`Y8b ↵ 888 888 ↵ 888 888 ↵ 888 888 ↵ `88b d88' ↵ `Y8bd8P' ↵ ↵ ↵ ↵ # ↵ .o ↵ o888 ↵ 888 ↵ 888 ↵ 888 ↵ 888 ↵ o888o ↵ ↵ ↵ ↵ # ↵ Global Buffers (Arrays) fontBuffer : A big char array defined in fontBuffer.c that is populated with the constituent symbols of all cGrams by the readFontBuffer() function, given some font file. The 2D cGrams are flattened in the fontBuffer , stored as sequences of concatenated rows. Note: cGrams are of varying sizes and occupy different amounts of storage in fontBuffer . cGramLookup : An array of char pointers defined in fontBuffer.c . Each pointer points to the start of a cGram in fontBuffer . This is effectively a char-to-cGram map. ● Indexing: Our font file contains ASCII values 32 - 126 ( ' ' - ~ ), so the location of the cGram for ' ' is stored in cGramLookup[0] . We thus define FIRSTCHAR to be ' ' , and subtract its ASCII value (32) when indexing into cGramLookup . For some char c , its cGram data will be at the address given by cGramLookup[c - FIRSTCHAR] . cGramWidth : An array of ints defined in fontBuffer.c that specify the width of each cGram. ● Indexing: The indexing scheme here is identical to that of cGramLookup . displayBuffer : A 2D char array defined in displayBuffer.c used as a canvas to store the individual symbols that make up a series of cGrams. To print a message “ab”, the symbols that make up the cGrams for ‘a’ and ‘b’ are copied into displayBuffer , with additional SPACING between them. Then displayBuffer is printed to stdout. CSE 30, Winter 2020 Functions and Behavior to Implement You need to implement all of the functions provided in all of the files in the starter code without changing their signatures. Your functions must work with our provided globals.h . We propose the following workflow and guidelines. 1) int readFontBuffer() [10 points] This function is partially completed in the starter code. The font file is read into the FILE* fontFilePtr pointer. Line 32 assigns the beginning of the 0th cGram (the character ' ' ) to be at index 0 of the fontBuffer array for convenience; feel free to change this if you wish. The while loop starting on line 34 is the bulk of this code. The loop iterates over every line in the font file, copying the line into the fontBuffer array at the location of the fontBufferPtr pointer, and additionally updating the fontLine pointer to point to the same thing. For the moment, fontBufferPtr and fontLine are pointing to exactly the same memory, but they serve logically different purposes so we keep both: think of fontBufferPtr as the location in fontBuffer that should be updated, and think of fontLine as the string that just got copied. Process the fontLine to correctly update the cGramLookup and cGramWidth arrays when necessary. (How do you know when you've reached the end of a cGram?) After this readFontBuffer() function returns: - The cGram for char c must begin at address given by cGramLookup[c - FIRSTCHAR] , stored as a concatenated sequence of rows from top to bottom (including '\n' characters) and ending on the FONTDELIM char, which is just '#' defined in globals.h . - The width of the cGram for char c must be given by cGramWidth[c - FIRSTCHAR] 2) void printCGram() [2 points] Given a char c , this function should print its cGram to stdout. You can then use it to check that your readFontBuffer() is working correctly, for instance with a simple loop like below: for(char c=' '; c < '~"; c++) { printf("\n\'%c\' :\n", c); printCGram(c); } $ ./printify cse30.font > foobar $ less foobar Once you correctly parse the font file, you are ready to move onto displaying a given message! CSE 30, Winter 2020 3) void copyCGram() [10 points] This function will copy the cGram representing char c into the outputBuffer starting at row 0 and column xPos . It returns an int representing what the xPos of the next cGram should be, which should account for the current cGram's width and the additional inter-cGram SPACING . 4) void fillDisplayBuffer() [2 points] This function must fill the entire 2D displayBuffer array with the specified char. This char will serve as a background symbol for the cGrams. Hint: use a non-' ' character (like a dash '-' for example) to verify that your copyCGram() function is positioning the cGrams correctly! 5) void printDisplayBuffer() [4 points] Prints the displayBuffer to stdout. Since the terminal prints along a row, you should print one row of the displayBuffer at a time followed by a '\n' newline to return the terminal's cursor to the leftmost position on the next line. This is done for MAXHEIGHT rows. When printing the displayBuffer, you may reach the MAXWIDTH of the buffer before printing out all of the symbols of the cGrams in a line. Nothing should be printed beyond these dimensions; any remaining cGram symbols should just be simply cut off! 6) int main() [15 points] You’re ready to put it all together now! main() should process the options (see the getopt example at bottom of doc), fill in the displayBuffer with the appropriate symbol (' ' by default if not specified), read the font file into the fontBuffer , process the input string copying its cGrams into the displayBuffer , and print the displayBuffer out to stdout. 7) Support for a custom cGram [2 points] At the end of cse30.font , we have defined a custom SMILEY cGram. If you see the string ':)' in the input (must be enclosed in single quotes), instead of printing the two cGrams for ':' and ')' , print the cGram pointed to by cGramLookup[SMILEY] . Ex: $ ./printify fonts/big.font 'hi :)' CSE 30, Winter 2020 Submission and Grading Submitting 1. Submit your files to Gradescope under the assignment titled HW3. You will submit the following files (you don’t need to submit globals.h ): ● main.c ● fontBuffer.c ● displayBuffer.c To upload multiple files to gradescope, zip all of the files and upload the zip to the assignment. Ensure that the files you submit are not in a nested folder. Gradescope can extract from a zip, but not a folder. 2. After submitting, the autograder will run a few tests: a. Checks that all required files were submitted b. Checks that your source code compiles c. Runs functional tests on all of the functions you need to complete d. Compares output from your program to one from a solution implementation Grading Breakdown Make sure to check the autograder output after submitting! We will be running additional tests after the deadline passes to determine your final grade. We will test your code both at a function wise level and at an input--output level. You will receive points for each test that you pass on Gradescope. Ensure your code compiles and runs on ieng6 with the provided Makefile and globals.h . Any assignment that does not compile will receive 0 credit. Style Guidelines No points are given for style, but teaching staff won't be able to provide assistance or regrades unless code is readable. Please take a look at the following Style Guidelines. CSE 30, Winter 2020 Extra Notes for Printify Spaces between words Separate words are printed with exactly one cGram of the space char ' ' between them: *note that for the long string above, we temporarily increased the MAXWIDTH value in globals.h. Note that this happens regardless of the number of spaces in your input, as the shell (bash / terminal) removes all whitespace between strings before passing them into your program's argv: If you do want to print multiple spaces however, then surround the input by single quotes to force the shell to treat your entire quoted string literally. In the example below, the shell will remove the single quotes and pass the exact string a wizard as one argument (one single argv entry) to your program: Spaces between consecutive cGrams Between any two consecutive cGrams (including the cGram for the sapce character ' ' itself), there should be SPACING number of columns of the unaltered background character in the displayBuffer . To observe this, examine the following example: Ex. ./printify -f '-' fonts/big.font 'a b c' CSE 30, Winter 2020 A note on getopt() You will be parsing not just flags, but also values associated with the flags whose types differ depending on the flags. Below is an example of how you can parse two flags -a and -c , where -a takes an int argument, and -c takes a char argument, a font file, and then input text. Getopt has an external variable called optind that stores the index of the argument to an option that it uses to parse argv , so you can access the argument with argv[optind] as you are currently processing an option flag. The rest of the input arguments will be in order after index optind . So, for example, if the command was: $ ./myProgram -a 5 -c c cse30.font Hello there then as you process the -a flag, argv[optind] will be 5 ; then as you process the -c flag, argv[optind] will be "c" , and after getopt finishes parsing the flags, optind will point to the next value of "cse30.font" , meaning that argv[optind+1] is "hello" and argv[optind+2] is "there" . optarg will store the integer values passed in for -a and -c . atoi converts an int to a char. The string "a:c:" in the while loop means that optional flags -a and -c will have an accompanying argument passed afterwards (an integer or a character in our example). // parse optional commands int opt; while((opt = getopt(argc, argv, "a:c:")) != -1){ switch (opt){ case ('a'): printf("Value of arg a is: %d", atoi(optarg)); // do something for case a with optarg break; case ('b'): printf("Value of arg c is: %c", optarg[0]); // do something for case a with optarg break; default: break; } } // access font file name char* fontFile = argv[optind];
