how to write this in C

Question

how to write this ASM code in C?

loc_536FB0:
mov cl, [eax]
cmp cl, ' '
jb short loc_536FBC
cmp cl, ','
jnz short loc_536FBF

loc_536FBC:
mov byte ptr [eax], ' '

loc_536FBF
mov cl, [eax+1]
inc eax
test cl, cl
jnz short loc_536FB0

I have figured out that it is a for loop counting to 23 then exiting.

have a look [here](http://reverseengineering.stackexchange.com) — 0xC0000022L, May 25 '13 at 19:10
It seems like you've already reverse-engineered it, since you say you know what it does. So what's the problem? You don't know how to write a `for` loop in C? — Barmar, May 25 '13 at 19:29
i do know how to write a for loop in C but i dont know what this code does besides that it counts to 23 and quits. — user1365830, May 25 '13 at 19:35

score 4 · Accepted Answer · answered May 25 '13 at 19:44

char *str; // = value of eax
int i = 0;
while (str[i]) {
    if (str[i] < ' ' || str[i] == ',')
        str[i] = ' ';
    i++;
}

It traverses a c-string and replaces all characters below ' ' and commas ',' with a space ' '. See an ASCII table: characters "below" space are all the controll characters. The function probably wants to erase them to get a "clean" string. The string is passed via a pointer in eax.

I don't know what this would have to do with 23, but maybe this is what you got for some specific input while debugging.

Small nitpick: given the assembly code in the question it would be more accurately transcribed as a `do { } while();`, since the NULL terminator check is the last thing performed during each iteration. — Michael, May 25 '13 at 20:50

how to write this in C

1 Answers1