No doubt the program for the previous programming assignment gave you some\n practice in debugging. This assignment will give you more. You are to find\n some bugs we've installed in a program. gdb will again be\n helpful.
\n\n \n Assignment
\n\n The first step in the debugging process is the compilation of the\n program. In the process of compiling code, useful error messages are\n provided that identify problematic parts of the code that are not\n syntactically correct. Very often, these mistakes are typos.
\n\n If you haven't done so already, download generate.c from the starter code. To compile the program type:
\n\n $ gcc -g -Wall generate.c
\n\n \n The -g option links the source code to the executable, so\n that you have extra debugging information when using gdb.\n The -Wall option turns on warnings.\n
\n\n You should see something similar to the following (your output may not\n be exactly the same):
\n\n generate.c: In function ‘main’:\ngenerate.c:34:5: error: expected ‘;’ before ‘puts’\ngenerate.c:37:5: warning: ‘gets’ is deprecated [-Wdeprecated-declarations]\ngenerate.c:46:5: warning: ‘gets’ is deprecated [-Wdeprecated-declarations]\ngenerate.c:54:5: warning: ‘gets’ is deprecated [-Wdeprecated-declarations]\ngenerate.c: In function ‘print_if_good’:\ngenerate.c:165:5: warning: value computed is not used [-Wunused-value]\ngenerate.c: At top level:\ngenerate.c:194:1: warning: return type defaults to ‘int’ [-Wreturn-type]\ngenerate.c:194:1: warning: conflicting types for ‘convert’ [enabled by default]\ngenerate.c:8:6: note: previous declaration of ‘convert’ was here
\n\n The numbers after the filename refer to the numbers of lines that\n contain errors. \"Errors\" typically signify something that is syntactically\n incorrect. \"Warnings\" are issued for code that the C compiler believes to\n be problematic but that is syntactically correct. Warnings will not prevent\n your code from compiling, but they can tip you off to some rather serious\n problems.
\n\n \n Question 1
\n\n Use the text editor of your choice, correct the errors. What changes\n did you make? You may need to make small changes and recompile to see if\n those errors are fixed. This may take several iterations. It is okay not\n to fix all the warnings but all of the non-warning problems will need to\n be fixed.
\n\n If you reach this point:
\n\n generate.c: In function ‘main’:\ngenerate.c:36:5: warning: ‘gets’ is deprecated [-Wdeprecated-declarations]\ngenerate.c:45:5: warning: ‘gets’ is deprecated [-Wdeprecated-declarations]\ngenerate.c:53:5: warning: ‘gets’ is deprecated [-Wdeprecated-declarations]\ngenerate.c: In function ‘print_if_good’:\ngenerate.c:164:5: warning: value computed is not used [-Wunused-value]\n...
\n\n Your program has compiled successfully; don't try to figure out what's\n wrong with this line yet. We'll figure it out eventually.
\n
\n\n Let's see if the program works. To run the compiled program type the\n following (words you type are in bold):
\n\n $ ./a.out\nThis program finds words associated with phone numbers.\nWhat is the maximum number of consonants you would like\nthe generated word to possess?\n(note: 0's and 1's are considered consonants)\n4\nPlease enter a 7 digit phone number with dashes removed
\n\n \n Question 2
\n The program has one or more bugs. How does the output suggest\n something is wrong?
\n
\n\n We will now use gdb to begin debugging. To start\n gdb debugging this program, type:
\n\n $ gdb ./a.out\nGNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.3) 7.7.1\nCopyright (C) 2014 Free Software Foundation, Inc.\nLicense GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>\nThis is free software: you are free to change and redistribute it.\nThere is NO WARRANTY, to the extent permitted by law. Type \"show copying\"\nand \"show warranty\" for details.\nThis GDB was configured as \"x86_64-linux-gnu\".\nType \"show configuration\" for configuration details.\nFor bug reporting instructions, please see:\n<http://www.gnu.org/software/gdb/bugs/>.\nFind the GDB manual and other documentation resources online at:\n<http://www.gnu.org/software/gdb/documentation/>.\nFor help, type \"help\".\nType \"apropos word\" to search for commands related to \"word\"...\nReading symbols from ./a.out...done.\n(gdb)
\n\n Before we begin running the program, we will need to set a\n breakpoint. A breakpoint is a programmer-specified point in\n the code where we would like the program to \"pause\" at while the program is\n running. Why would we want to do this? Using a debugger, we may examine\n variables in this paused state which will give us important information\n about whether or not the program is working correctly. Type:
\n\n (gdb) break main\nBreakpoint 1 at 0x400685: file generate.c, line 18.
\n\n This will ask the program to stop at the first executed line of code of\n the main() function. To run the program and have it stop at\n this point, type:
\n\n (gdb) run\nStarting program: a.out\n\nBreakpoint 1, main () at generate.c:18\n18 {
\n\n Notice that the program has stopped right before it is about to execute\n a line of code. In this case, the line of code in question is line 18 (your\n output may vary), which is just a curly bracket {. Let's go to\n the next line of code to find something more interesting:
\n\n (gdb) next\n24 goahead = 1;
\n\n The next command executes the line of code that was stopped\n at, and then stops the program at the very next line of code.
\n\n \n Notice that the debugger appears to have skipped several lines of code\n from line 18 to line 24 even though we only instructed it to proceed to\n the next line of code. This is because variable declarations\n are processed immediately.\n
\n\n Let's find out what the value of goahead is. Type the\n following:
\n\n (gdb) print goahead\n$1 = -1073785707
\n\n In the above case, the variable goahead is storing a\n garbage integer (your output will definitely vary here!). Remember, if you\n don't assign a variable an initial value, its contents will be\n unpredictable. Now type:
\n\n (gdb) next\n25 while (goahead)
\n\n To make sure the assignment goahead = 1 has actually taken\n place, we can again view the contents of goahead:
\n\n (gdb) print goahead\n$2 = 1
\n\n Sometimes we may want to find out the value of a variable after each\n statement. gdb provides this functionality in its\n display command. Type:
\n\n (gdb) display goahead\n1: goahead = 1\n(gdb) next\n32 puts(\"This program finds words that can be associated with phone numbers.\");\n1: goahead = 1
\n\n Continue executing the program line by line using the next\n command.
\n\n \n In gdb, hitting return re-executes the most\n recent command; this facility simplifies the process of monitoring a long\n stretch of code.\n
\n\n Stop when you see the following:
\n\n (note: 0's and 1's are considered consonants)\n36 gets(input);\n1: goahead = 1
\n\n Type the following:
\n\n (gdb) display input\n2: input = \"38!(@#*(#*!(@#1\"\n(gdb) next\n4\n42 maxNumConsonant = input[0] - '0';\n2: input = \"4\\000!(@#*(#*!(@#1\"\n1: goahead = 1
\n\n Notice that the contents of the string input has changed.\n Since strings are null-terminated, when gets reads a string,\n it copies the user's input into the array passed into the function and\n places a null terminator (denoted by \\000 in the display\n portion) into the array when it reads a carriage return.
\n\n \n Question 3
\n Using the techniques you have learned so far, explain what\n gets copied into maxNumConsonant after the line\n above is executed. What gdb commands did you use to find\n out what was copied into maxNumConsonant? Explain what\n input[0] - '0' does.
\n
\n\n Continue executing the code line by line. You will be prompted to enter\n a phone number with dashes removed somewhere in the process. After the\n program has read the input phone number, you may want to verify that the\n appropriate information was read. Stop when you reach the following\n line:
\n\n 48 generateSpellings(phone, maxNumConsonant);
\n\n generateSpellings is a defined function within this\n program. We may be interested in watching what happens within the\n generateSpellings function. If we type next at\n this point, we will go to the next line of code instead of going inside the\n generateSpellings function. In order to go inside\n generateSpellings, we can use the step\n command:
\n\n (gdb) step\ngenerateSpellings (phone=0x7fffffffe060 \"4460175\", maxNumConsonant=4)\n at generate.c:75\n75 {
\n\n Several things happen now. First, the variables that you painstakingly\n \"displayed\" one by one are no longer refreshed. The reason is because those\n variables are not accessible from within the generateSpellings\n function. The line above shows the contents of the two variables passed\n into the function: phone and maxNumConsonant.\n Your output may be somewhat different.
\n\n For your information, 0x7fffffffe060 is the hexadecimal\n (base 16) equivalent for 140,737,488,347,232. gdb denotes\n memory addresses by the 0x prefix. As you can see, the\n hexadecimal format is a more efficient way of representing large\n numbers.
\n\n \n Question 4
\n What is the value stored in phone? (Hint: Everyone's\n answer will be different.) Why is phone represented by an\n address?
\n
\n\n Continue using the next command until you see:
\n\n 96 convert (phone, phone_int);
\n\n Let's step into this function as well:
\n\n (gdb) step\nconvert (phone=0x7fffffffe060 \"4460175\", phone_int=0x7fffffffdfd0)\n at generate.c:196\n196 for (i = 0; i < 7; i++)
\n\n When stepping into a function, gdb does not display the\n contents of an integer array when it is passed in as an argument. This is\n unlike what it does with character arrays. (Note the difference between\n phone and phone_int.) However, regardless of\n variable type, gdb will always show you what the value of that\n variable is when it is passed into the function. If this doesn't make\n sense, then you didn't answer Question 4 correctly.
\n\n In the next five lines is a for loop. It is often helpful\n to monitor a for loop's progress. Type the following:
\n\n (gdb) display i\n3: i = 74128\n(gdb) display phone_int[0]\n4: phone_int[0] = 268530816\n(gdb) display phone_int[1]\n5: phone_int[1] = 4199972\n(gdb) display phone_int[2]\n6: phone_int[2] = 8192\n(gdb) display phone_int[3]\n7: phone_int[3] = 0\n(gdb) display phone_int[4]\n8: phone_int[4] = 843604063\n(gdb) display phone_int[5]\n9: phone_int[5] = 0\n(gdb) display phone_int[6]\n10: phone_int[6] = 268436928
\n\n Again, because values have not been assigned to the array cells above,\n their contents are in fact completely arbitrary. Your output may differ\n somewhat.
\n\n Monitoring the for loop's progress generally means stepping\n through it via repeated step or next\n commands.
\n\n \n Question 5
\n Use the techniques that you've learned thus far and determine whether\n or not the for loop in this function works. What evidence\n suggests so?
\n
\n\n When you exit the function, you should see the following:
\n\n generateSpellings (phone=0x7fffffffe060 \"4460175\", maxNumConsonant=4)\n at generate.c:103\n103 for (a = 0; a < 3; a++)
\n\n The statement above lets you know that you've returned to the calling\n function (in this case, generateSpellings).
\n\n \n Question 6
\n Starting on the above line, a series of nested loops is encountered.\n How does each nested loop work? Make sure you step through the loop and\n demonstrate that you've actually examined how the loop works.
\n
\n\n Continue executing the code line by line until you reach
\n\n 131 print_if_good(maxNumConsonant, word, &level);
\n\n \n Question 7
\n What is the current value of word? Which gdb\n command did you use to find its value? Is its value correct?
\n
\n\n \n Question 8
\n In line 131, the address of the variable level is passed\n into print_if_good as opposed to the variable's contents.\n The comments explain that this is a technique that allows the function to\n change the contents of a variable passed into a function. Explain how\n passing a pointer to a variable accomplishes this.
\n
\n\n Step into this function using the step command:
\n\n (gdb) step\nprint_if_good (maxNumConsonant=4, word=0x7fffffffe020 \"ggm01pj\",\n level=0x7fffffffdfb0) at generate.c:147\n147 int numVowel = 0;
\n\n In the following code, there is a for loop that counts the\n number of vowels and an if statement that determines whether\n or not the generated word is printed. We are interested in testing what\n happens when the if condition is true. Since we may not enter\n the condition every time, we could end up typing next many\n times before we reach the statements within the if condition.\n One way to skip to the \"interesting\" part of the program is to execute the\n following commands:
\n\n (gdb) break 161\nBreakpoint 2 at 0x400a1e: file generate.c, line 161.\n(gdb) continue\nContinuing.
\n\n In the above gdb statements, we have asked the program to\n stop at line 161. Line 161 is the call to printf within the\n if statement. The continue statement tells the\n program to run at full speed until the next breakpoint is reached-in this\n case, when we next execute line 161.
\n\n You should eventually see something similar to
\n\n Breakpoint 2, print_if_good (maxNumConsonant=4, word=0x7fffffffe020 \"iio01pj\",\n level=0x7fffffffdfb0) at generate.c:161\n161 printf(\"%s \",word);
\n\n Print the word and go to the next line:
\n\n (gdb) next\n164 *level++;
\n\n Now find out what the value of level and\n *level is (your output will vary):
\n\n (gdb) display level\n11: level = (int *) 0x7fffffffdfb0\n(gdb) display *level\n12: *level = 0
\n\n The hexadecimal number 0x7fffffffdfb0 is\n 140,737,488,347,056 in decimal. Let's find out what happens when we execute\n this line.
\n\n 168 if ((*level) % 7 == 0)\n12: *level = 0\n11: level = (int *) 0x7fffffffdfb4
\n\n The value of level is now 0x7fffffffdfb4 or in\n decimal, 140,737,488,347,060. *level is still 0. What result\n did we expect? It seems like we might have a problem here.
\n\n \n Question 9
\n Explain what the problem is in this section of code. Fix the problem\n in the code, and recompile the program. What happened to the warning?
\n
\n\n The quit command exits gdb.
\n\n When you've recompiled the program, verify the rest of the program works\n as described.
\n\n You will need to turn in your corrected version of\n generate.c, and your answers to the boxed questions in this\n assignment (including any notes you took on your interactions with the\n debugger).
\n \n\n ![](\"./images/animal-sample-tree.gif\")
\n ![](\"./images/animal-sample-tree-array.gif\")
\n ![](\"./images/HP10c.jpg\")
\n ![](\"./images/AmoebaFamily.png\")
\n ![](\"./images/blocks-sample-config.gif\")
\n ![](\"./images/blocks-sample-goal.gif\")
\n ![](\"./images/sorted-list.png\")
\n ![](\"./images/MadLibs.jpg\")
\n ![](\"./images/rule30_200.png\")
\n ![\"Cat](\"./images/cat-and-mouse.png\")
\n ![\"The](\"./images/turtle_simple.png\"\n)
\n ![](\"./images/turtle_walking-turtle.gif\"\n)
\n ![\"Cat](\"./images/turtle_cat-and-mouse.png\")
\n ![\"Git](\"./images/git-bash-options.png\")
\n ![](\"./images/terminal-g++.png\")
\n ![\"dialog\"](\"./images/minGW_manager.png\")
\n ![](\"./images/apply_changes.png\")
\n ![](\"./images/windows_var.png\")
\n ![](\"./images/windows_PATH.png\")
\n ![](\"./images/windows_env_var.png\")
\n ![](\"./images/windows_edit_var.png\")
\n ![\"starting](\"./images/terminal.png\")
\n ![\"cs9f](\"./images/terminal_commands.png\")
\n ![](\"./images/terminal-gcc.png\")
\n ![\"Cat](\"./images/cat-and-mouse-gui.png\")
\n ![](\"./images/graph.gif\")
\n ![](\"./images/spanning-trees.gif\")
\n ![](\"./images/prims-algorithm.gif\")
\n ![\"Git](\"./images/setup_git_bash_options.png\")
\n ![\"Windows](\"./images/setup_python-windows-install-path.png\")
\n ![\"starting](\"./images/setup_terminal.png\")
\n ![\"cs9h](\"./images/setup_terminal_commands.png\")
\n ![](\"./images/marble-1.gif\")
\n ![](\"./images/marble-2.gif\")
\n ![](\"./images/marble-3.gif\")
\n ![](\"./images/marble-4.gif\")
\n ![](\"./images/marble-5.gif\")
\n ![](\"./images/marble-6.gif\")
\n ![](\"./images/marble-7.gif\")
\n ![\"Probability](\"./images/probability-distribution.jpg\")
\n ![](\"./images/unit_converter_mco_mapping.gif\")
\n ![](\"./images/hashmap.png\")
\n ![\"Rule](\"./images/rule30.jpg\")
\n ![\"Heat](\"./images/heat-transfer-plate.jpg\")
\n ![](\"./images/cellular_automation_rule30_200.png\")
\n ![comedic references](\"http://www.toothpastefordinner.com/120504/mad-libs-by-myself.gif\"){kind=link}