C/C++ Inline assembly [with C loops]

0
votes
#include <stdio.h>

#define OPT //define for assembly

int main()
{
 char chr;
 for(chr = 'A' ; chr <= 'Z' ; chr++)
 {
  #ifdef OPT
  __asm
  {
   lea eax,chr
   push eax
   mov eax, putchar
   call eax
   pop ebx
  }
  #endif

  #ifndef OPT
  putchar(chr);
  #endif 
 }
 return 0;
}

Upon usage of the assembly code, all that happens is a random trash-character is printed. Note this is in intel syntax.

Also: I'm learning inline assembly, how would you grab the return value of a called function (through mov eax,func then call eax or equiv?)

2
c++cassembly
What's wrong with just call putchar ? – TonyK
TonyK, you cannot call putchar, because putchar is a variable. I think this is mainly for debugging purposes and can be turned on or off in C compiler settings. – Al Kepp
@Aleš Keprt: That's a new one on me! – TonyK

2 Answers

4
votes

It doesn't work because lea instruction is intended to get the address of a variable. (+1 to zebarbox for this note.) We need the value of chr, not its address, so we use this instead:

movsx eax,chr

This pseudoinstruction will compile to something like this:

movsx eax,[ebp-4]

You can also write putchar(chr), put there a breakpoint, run the application and look into disassembly window to see how it is compiled.

Note that I use movsx because chr is char and I need here a dword. If chr was int, I would simply use mov instruction.

Also, you are not allowed to use pop ebx, because ebx must not be changed here. Use pop eax or add esp,4 instead.

1
votes

Here is your correct code:

int c = chr;
__asm
{
   mov  eax, c
   push eax
   mov  eax, putchar
   call eax
   pop  ebx
}

You cannot move chr on eax, due to size-conflict. So I used 'c' of int type!