- GDB (GNU Debugger) is a powerful tool for debugging your program. For more information, please refer to https://darkdust.net/files/GDB%20Cheat%20Sheet.pdf
Invoke gdb with giving the name of your executable file as an argument. Your program should be compiled with "-Og" or "-g" flag to be used with gdb.
$ gdb ./bmpdiag
GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.5) 7.11.1
Copyright (C) 2016 Free Software Foundation, Inc.
...
Reading symbols from ./bmpdiag...done.
The break command is used to set the breakpoint in your code. The following shows how to set a breakpoint at the beginning of the function bmp_diag() which is located in the file bmpdiag.s.
(gdb) break bmp_diag
Breakpoint 1 at 0x400b50: file bmpdiag.s, line 33.
If you want to run your program, use the run command followed by any arguments as shown below. Because we have set a breakpoint in the entry of the function bmp_diag(), you can see that the execution stopped at that point (just before executing the first instruction in bmp_diag()).
(gdb) run bordeaux.bmp out.bmp 100
Starting program: /home/jinsoo/ca-pa3/bmpdiag bordeaux.bmp out.bmp 100
BMP file: bordeaux.bmp (640 x 359 pixels, 24 bits/pixel)
Breakpoint 1, bmp_diag () at bmpdiag.s:33
33 movb $0x55, (%rdi)
Now you can browse the source code around the current breakpoint using the list command.
(gdb) list
29
30 # --> FILL HERE <--
31
32
33 movb $0x55, (%rdi)
34 movb $0x88, 1(%rdi)
35 movb $0xff, 2(%rdi)
36
37
You can examine the content of a register using the print (or simply p) command as follows. Note that the name of the register is given as $rax, not %rax.
(gdb) print $rdi
$1 = 140737353273414
If you want to print the value in a hexadecimal format, use the print/x (or simply p/x) command. The %rdi register contains the first argument for bmp_diag(). Therefore, the following address 0x7ffff7f2d046 is the address that points to the bitmap data.
(gdb) print/x $rdi
$2 = 0x7ffff7f2d046
You can print the value of any register. The following shows the content of the %rsi register that has the second argument. You can see that it has the value 640 which is the width of the input image file.
(gdb) p $rsi
$3 = 640
The %rcx register is used to pass the fourth argument. In this case, it has the value 100 which is the gap between adjacent diagonal lines (in pixels).
(gdb) p $rcx
$4 = 100
You can print the content of the memory using the x command. Esepcially, you can print 8 bytes in a hexademical format using the x/8b command as shown below. If you want to see 32 bytes, use the x/32b command. Initially, the %rdi register points to the memory address of the first pixel in the last row of the given input image. The first three bytes, (0x04, 0x04, 0x04), represents the color value for blue, green, and red for that pixel, respectively.
(gdb) x/8b $rdi
0x7ffff7f2d046: 0x04 0x04 0x04 0x03 0x03 0x02 0x04 0x03
The 'step' command executes one instruction at a time. Now it has executed the first instruction in the bmp_diag() function (e.g., movb $0x55, (%rdi)), and shows the next instruction to be executed.
(gdb) step
34 movb $0x88, 1(%rdi)
If you examine the memory contents pointed to by the %rdi register, you can see that the first byte now has been modified to 0x55 as a result of executing the first instruction.
(gdb) x/8b $rdi
0x7ffff7f2d046: 0x55 0x04 0x04 0x03 0x03 0x02 0x04 0x03
Now, let's execute the second instruction. Note that the instruction shown after using step command is the instruction to be executed next.
(gdb) step
35 movb $0xff, 2(%rdi)
Again, you can see that the second byte has been changed to 0x88 as a result of executing the second instruction in bmp_diag().
(gdb) x/8b $rdi
0x7ffff7f2d046: 0x55 0x88 0x04 0x03 0x03 0x02 0x04 0x03
You can just use s for the step command. The following executes the third instruction in bmp_diag().
(gdb) s
bmp_diag () at bmpdiag.s:49
49 ret
Again, the third byte has been changed to 0xff. In this way, you can modify the color values of any pixel you want.
(gdb) x/8b $rdi
0x7ffff7f2d046: 0x55 0x88 0xff 0x03 0x03 0x02 0x04 0x03
The memory address can be given in the following way also.
(gdb) x/8b $rdi+600
0x7ffff7f2d29e: 0x04 0x04 0x03 0x04 0x03 0x03 0x02 0x03
The cont command continues the execution until it meets another breakpoint or until the program terminates.
(gdb) cont
Continuing.
[Inferior 1 (process 8747) exited normally]
It is always a good idea to ask for a help to gdb.
(gdb) help
List of classes of commands:
aliases -- Aliases of other commands
breakpoints -- Making program stop at certain points
data -- Examining data
files -- Specifying and examining files
internals -- Maintenance commands
obscure -- Obscure features
running -- Running the program
stack -- Examining the stack
status -- Status inquiries
support -- Support facilities
tracepoints -- Tracing of program execution without stopping the program
user-defined -- User-defined commands
Type "help" followed by a class name for a list of commands in that class.
Type "help all" for the list of all commands.
Type "help" followed by command name for full documentation.
Type "apropos word" to search for commands related to "word".
Command name abbreviations are allowed if unambiguous.
You know what the following means.
(gdb) quit
$
Enjoy :)
Jin-Soo Kim
Systems Software and Architecture Laboratory
Dept. of Computer Science and Engineering
Seoul National University