Most in need of debugging

Yusuke Endoh
Twitter: @mametter

Judges' comments:

To use:

make

Try:

./prog

gdb ./prog || llvm ./prog

Selected Judges Remarks:

The purpose of this program is to crash. You’ll want to have memorized “man ascii” when debugging it to reveal its purpose.

Author’s comments:

backtrace quine

Compile prog.c with no optimization.

$ gcc -g -O0 -o prog prog.c

Then, run it. You will see a segmentation fault.

$ ./prog
Segmentation fault (core dumped)

Let’s debug. As usual, run it under gdb.

$ gdb ./prog
(gdb) run
Starting program: /home/.../prog

Program received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()

Okay, check the backtrace.

(gdb) backtrace
#0  0x0000000000000000 in ?? ()
#1  0x0000555555555201 in x23 () at prog.c:35
#2  0x000055555555571a in x64 () at prog.c:100
#3  0x0000555555555747 in x65 () at prog.c:101
#4  0x0000555555555774 in x66 () at prog.c:102
#5  0x00005555555557ce in x69 () at prog.c:105
#6  0x0000555555555828 in x6e () at prog.c:110
#7  0x0000555555555747 in x65 () at prog.c:101
...

See the line numbers and lookup the ASCII table.

 35 = '#'
100 = 'd'
101 = 'e'
102 = 'f'
105 = 'i'
110 = 'n'
101 = 'e'
...

One more thing

The original program can be used as a GDB command file.

$ gdb -q -x prog.c ./prog | cat
Reading symbols from ./prog...done.

Program received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
#0  0x0000000000000000 in ?? ()
#1  0x0000555555555201 in x23 () at prog.c:35
#2  0x000055555555571a in x64 () at prog.c:100
#3  0x0000555555555747 in x65 () at prog.c:101
#4  0x0000555555555774 in x66 () at prog.c:102
#5  0x00005555555557ce in x69 () at prog.c:105
#6  0x0000555555555828 in x6e () at prog.c:110
#7  0x0000555555555747 in x65 () at prog.c:101
...

By using this, you can confirm that it is actually a quine.

$ gdb -q -x prog.c ./prog | sed -n -r 's/#.*:([0-9]+)/\1/p' | awk '{printf "%c",$1}' > prog2.c
$ diff prog.c prog2.c