測驗說明 GDB

Description

Exercise 3

測驗說明
GDB
問題 1 100 分數
For this exercise, you will gain familiarity with the command line debugging
tool gdb.
On CLEAR (ssh netID@ssh.clear.rice.edu, (mailto:netID@ssh.clear.rice.edu,)
replacing netID with your own), make a folder called factorial, and change your
working directory to that folder
Copy the code of this (https://canvas.rice.edu/courses/59111/files/4776048?
wrap=1) (https://canvas.rice.edu/courses/59111/files/4776048/download?
download_frd=1) file into a new file on CLEAR in the factorial folder
Compile the c file with the -g (gdb) flag:
gcc -g factorial.c
Use ls to view that there is an executable, which by default is called a.out
Now run it:
./a.out
It should ask you for a number to compute the factorial for, give it a number
and notice that the output is incorrect (zero)
Now we will debug the program with gdb by running the following command:
gdb a.out
You are in gdb, and can now play around. Start by entering the following
command, which will show you the code
list
It only shows you part of the code (10 lines), you can hit enter again (which
repeats the previous command, list) to see more of the code
Let’s run it inside gdb:
run
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Notice we are still getting zero, so let’s inspect what is happening with the
program using gdb’s features – start by setting a break point at main:
b main
For sanity, let’s list the breakpoints we’ve enabled:
info break
Now let’s run again, which will stop at the breakpoint that we enabled:
run
Now we want to watch some variables to see how they change – let’s watch f
(factorial) and n (number to compute the factorial for)
watch f
watch n
Do info break again – notice that now f and n are included
Hit the following to continue running the program:
c
Enter 4, and then hit enter again multiple times to notice how f and n change
throughout the program
Notice that eventually f goes from 24 to 0, this is not good!
Let’s verify f really is 0 by using print:
print f
Now figure out how to change the code so that it correctly computes the
factorial, and submit a screenshot displaying the output of your corrected
factorial code working for two different inputs
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