CHAPTER 23. POINTERS TO FUNCTIONS CHAPTER 23. POINTERS TO FUNCTIONS
Breakpoint 1, comp (_a=0xbffff0f8, _b=_b@entry=0xbffff0fc) at 17_1.c:11
11 if (a==b)
(gdb) p a
$1 = 1892
(gdb) p b
$2 = 45
(gdb) c
Continuing.
Breakpoint 1, comp (_a=0xbffff104, _b=_b@entry=0xbffff108) at 17_1.c:11
11 if (a==b)
(gdb) p a
$3 = -98
(gdb) p b
$4 = 4087
(gdb) bt
#0 comp (_a=0xbffff0f8, _b=_b@entry=0xbffff0fc) at 17_1.c:11
#1 0xb7e42872 in msort_with_tmp (p=p@entry=0xbffff07c, b=b@entry=0xbffff0f8, n=n@entry=2)
at msort.c:65
#2 0xb7e4273e in msort_with_tmp (n=2, b=0xbffff0f8, p=0xbffff07c) at msort.c:45
#3 msort_with_tmp (p=p@entry=0xbffff07c, b=b@entry=0xbffff0f8, n=n@entry=5) at msort.c:53
#4 0xb7e4273e in msort_with_tmp (n=5, b=0xbffff0f8, p=0xbffff07c) at msort.c:45
#5 msort_with_tmp (p=p@entry=0xbffff07c, b=b@entry=0xbffff0f8, n=n@entry=10) at msort.c:53
#6 0xb7e42cef in msort_with_tmp (n=10, b=0xbffff0f8, p=0xbffff07c) at msort.c:45
#7 GI_qsort_r (b=b@entry=0xbffff0f8, n=n@entry=10, s=s@entry=4, cmp=cmp@entry=0x804844d <⤦
Çcomp>,
arg=arg@entry=0x0) at msort.c:297
#8 0xb7e42dcf in GI_qsort (b=0xbffff0f8, n=10, s=4, cmp=0x804844d
#9 0x0804850d in main (argc=1, argv=0xbffff1c4) at 17_1.c:26
(gdb)
23.2.2 GCC + GDB (no source code).
But often there is no source code at all, so we can disassemble thecomp()function (disas), find the very firstCMP
instruction and set a breakpoint (b) at that address. At each breakpoint, we are going to dump all register contents (info
registers). The stack information is also available (bt), but partially: there is no line number information forcomp().
Listing 23.6: GDB sessiondennis@ubuntuvm:~/polygon$ gcc 17_1.c
dennis@ubuntuvm:~/polygon$ gdb ./a.out
GNU gdb (GDB) 7.6.1-ubuntu
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later http://gnu.org/licenses/gpl.html
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
For bug reporting instructions, please see:
http://www.gnu.org/software/gdb/bugs/...
Reading symbols from /home/dennis/polygon/a.out...(no debugging symbols found)...done.
(gdb) set disassembly-flavor intel
(gdb) disas comp
Dump of assembler code for function comp:
0x0804844d <+0>: push ebp
0x0804844e <+1>: mov ebp,esp
0x08048450 <+3>: sub esp,0x10
0x08048453 <+6>: mov eax,DWORD PTR [ebp+0x8]
0x08048456 <+9>: mov DWORD PTR [ebp-0x8],eax
0x08048459 <+12>: mov eax,DWORD PTR [ebp+0xc]
0x0804845c <+15>: mov DWORD PTR [ebp-0x4],eax
0x0804845f <+18>: mov eax,DWORD PTR [ebp-0x8]
0x08048462 <+21>: mov edx,DWORD PTR [eax]
0x08048464 <+23>: mov eax,DWORD PTR [ebp-0x4]
0x08048467 <+26>: mov eax,DWORD PTR [eax]
0x08048469 <+28>: cmp edx,eax
0x0804846b <+30>: jne 0x8048474 <comp+39>
0x0804846d <+32>: mov eax,0x0