Accessing the first byte of buffer in assembly?

Question

I am trying to call isdigit, and to do so I need first byte of my buffer, which is defined as following.

...

.equ ARRAYSIZE, 20

    .section ".bss"
buffer:
    .skip ARRAYSIZE

...

input:
    pushl $buffer
    pushl $scanFormat
    call  scanf
    addl  $8, %esp

Thus, buffer is allocated a memory space of 20 byte and I put some value using scanf, as shown in input.

Now I want to access the first 4 bytes in order to call isdigit. How can I access them?

My initial guess is to use movl buffer, %eax, since eax register is 4 byte-size, and will store the first 4 bytes in buffer. But I am not sure this is how it works.

Please let me know if I can only access the first 4 bytes of buffer, or any other methods to apply isdigit to those first 4 bytes. Thank you.

Normally .section .bss wouldn't have quotes around ".bss". I'm not sure that even works. — Peter Cordes
– Peter Cordes, Commented May 19, 2019 at 16:38
libc / POSIX isdigit is locale-aware, but if you can assume that the current locale's only "digit" characters are '0' .. '9' then it's much more efficient to manually check with sub $'0', %al / cmp $10, %al / jae not_a_digit — Peter Cordes
– Peter Cordes, Commented May 19, 2019 at 19:22

Sep Roland · Accepted Answer · 2019-05-19 19:56:48Z

2

You will want to apply isdigit to those 4 bytes separately. You can fetch them from the buffer one by one using a loop that does 4 iterations. The counter is setup in the %ecx register and a pointer to the buffer is setup in the %esi register.

    movl    $4, %ecx          ; Counter
    movl    $buffer, %esi     ; Pointer
More:
    movsbl  (%esi), %eax      ; Get next byte sign-extending it
    push    %eax
    call    isdigit
    addl    $4, %esp
    ...
    incl    %esi              ; Advance the pointer
    decl    %ecx              ; Decrement the counter
    jnz     More              ; Continu while counter not exhausted

Alternatively

    xorl    %esi, %esi        ; Offset start at 0
More:
    movsbl  buffer(%esi), %eax ; Get next byte sign-extending it
    push    %eax
    call    isdigit
    addl    $4, %esp
    ...
    incl    %esi              ; Advance the offset
    cmpl    $4, %esi          ; Test for max offset
    jb      More              ; Continu while offset not max-ed

edited May 19, 2019 at 19:56

answered May 19, 2019 at 15:53

Sep Roland

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4 Comments

Peter Cordes Over a year ago

libc isdigit(3) takes its arg on the stack, in the i386 System V calling convention. And its arg is an int, so you should be sign-extending your byte. Also, probably most sensible to assume that buffer holds a 0-terminated string because it was written by scanf.

Jimmy Suh Over a year ago

May I ask why we need to call isdigit for each byte separately? As far as I know, the argument of isdigit should be 4 bytes.

Sep Roland Over a year ago

@JimmySuh Your buffer contains a zero-terminated string that you got from scanf. Each byte is just one character for which you want to test if it is a digit. Reading 4 bytes at once would also retrieve 4 characters at once, but isdigit can process just 1 character each time. Sure, the argument of isdigit is a dword, but it has to contain a single character that you want verified. Hence extending the byte into a dword.

Jimmy Suh Over a year ago

Oh now I get it. Thank you.

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