1.27. POINTERS TO FUNCTIONS
push esi
push OFFSET _comp
push 4
lea eax, DWORD PTR _numbers$[esp+52]
push 10 ; 0000000aH
push eax
mov DWORD PTR _numbers$[esp+60], 1892 ; 00000764H
mov DWORD PTR _numbers$[esp+64], 45 ; 0000002dH
mov DWORD PTR _numbers$[esp+68], 200 ; 000000c8H
mov DWORD PTR _numbers$[esp+72], -98 ; ffffff9eH
mov DWORD PTR _numbers$[esp+76], 4087 ; 00000ff7H
mov DWORD PTR _numbers$[esp+80], 5
mov DWORD PTR _numbers$[esp+84], -12345 ; ffffcfc7H
mov DWORD PTR _numbers$[esp+88], 1087 ; 0000043fH
mov DWORD PTR _numbers$[esp+92], 88 ; 00000058H
mov DWORD PTR _numbers$[esp+96], -100000 ; fffe7960H
call _qsort
add esp, 16 ; 00000010H...
Nothing surprising so far. As a fourth argument, the address of label_compis passed, which is just a place
wherecomp()is located, or, in other words, the address of the very first instruction of that function.
How doesqsort()call it?
Let’s take a look at this function, located in MSVCR80.DLL (a MSVC DLL module with C standard library
functions):
Listing 1.364: MSVCR80.DLL.text:7816CBF0 ; void cdecl qsort(void *, unsigned int, unsigned int, int (cdecl )(const⤦
Çvoid , const void *))
.text:7816CBF0 public _qsort
.text:7816CBF0 _qsort proc near
.text:7816CBF0
.text:7816CBF0 lo = dword ptr -104h
.text:7816CBF0 hi = dword ptr -100h
.text:7816CBF0 var_FC = dword ptr -0FCh
.text:7816CBF0 stkptr = dword ptr -0F8h
.text:7816CBF0 lostk = dword ptr -0F4h
.text:7816CBF0 histk = dword ptr -7Ch
.text:7816CBF0 base = dword ptr 4
.text:7816CBF0 num = dword ptr 8
.text:7816CBF0 width = dword ptr 0Ch
.text:7816CBF0 comp = dword ptr 10h
.text:7816CBF0
.text:7816CBF0 sub esp, 100h
....
.text:7816CCE0 loc_7816CCE0: ; CODE XREF: _qsort+B1
.text:7816CCE0 shr eax, 1
.text:7816CCE2 imul eax, ebp
.text:7816CCE5 add eax, ebx
.text:7816CCE7 mov edi, eax
.text:7816CCE9 push edi
.text:7816CCEA push ebx
.text:7816CCEB call [esp+118h+comp]
.text:7816CCF2 add esp, 8
.text:7816CCF5 test eax, eax
.text:7816CCF7 jle short loc_7816CD04
comp—is the fourth function argument. Here the control gets passed to the address in thecompargument.
Before it, two arguments are prepared forcomp(). Its result is checked after its execution.
That’s why it is dangerous to use pointers to functions. First of all, if you callqsort()with an incorrect
function pointer,qsort()may pass control flow to an incorrect point, the process may crash and this bug
will be hard to find.