How to compare a char in a string with another char in NASM x86_64 Linux Assembly

Question

I'm actually trying to understand the basic concepts of NASM Assembly in Intel x64 syntax but facing an issue while trying to make a strchr equivalent...

I've been sailing the web to get the maximum information but I can't understand how to compare the current char of a string (like str[i]) with a simple char.

Here is the test main :

#include <stdio.h>

extern char* my_strchr(char*, char);

int     main(void)
{
  char* str;

  str = my_strchr("foobar", 'b');
  printf("%s\n", str);
  return 0;
}

And here is my assembly code :

I assume that rdi is my string and rsi my one-byte data.

my_strchr:
    push    rcx                  ;Save our counter
    xor     rcx, rcx             ;Set it to 0
loop:
    cmp     rdi, byte 0          ;Check the end of string
    jz      end
    cmp     rsi, [byte rdi+rcx]  ;Here is the point ...
    jz      end
    inc     rcx                  ;increment our counter
    jmp     loop
end:
    mov     rax, [rdi+rcx]       ;Are the brackets needed ? Is it equivalent to '&' in C ? 
    pop     rcx
    ret

Gdb gave me this output for the strchr function written in c and so disassembled :

....
cmp    al,BYTE PTR [rbp-0x1c]
....

But mine is actually doing this :

0x400550 <my_strchr>            push   rcx
0x400551 <my_strchr+1>          xor    rcx,rcx
0x400554 <loop>                 cmp    rdi,0x0
0x400558 <loop+4>               je     0x400566 <end>
0x40055a <loop+6>               cmp    rsi,QWORD PTR [rdi+rcx*1+0x0]

Thank you in advance, Hope someone will know

Jester · Accepted Answer · 2016-03-10 23:22:01Z

3

rdi is a pointer, so cmp rdi, 0 checks for a null pointer. What you meant was cmp byte [rdi + rcx], 0 to check the end of string. Note you need to check the current character so have to add the index obviously.

As for cmp rsi, [byte rdi+rcx] the byte there makes no sense, since you are comparing the whole of rsi which is 8 bytes. That should be cmp sil, [rdi + rcx].

Finally, strchr is supposed to return a pointer so you should change mov rax, [rdi+rcx] to lea rax, [rdi + rcx].

answered Mar 10, 2016 at 23:22

Jester

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2 Comments

Cegree Over a year ago

Thank you ! After your explanation, everything seems clear from now. I've just discovered "lea" instruction and sil (operand ?) how should I know when to use them properly for the future so ?

Jester Over a year ago

You can address smaller parts of registers, consult a manual for the names. You use them whenever you need a smaller unit. lea is useful for addressing or some simple arithmetic. You could have done the same with mov rax, rdi; add rax, rcx.

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