call fgets from assembly code

Question

I would like to know how to call fgets from assembly code. I read these questions which are exactly the same as the this one: How to call fgets in x86 assembly? and this one: How to use c library function fgets in assembly language? But both of them aren't satisfying for two reasons: 1. I really want to use fgets since i want my code to run on both windows and linux ( I'm using NASM ) 2.I looked at the disassembled version of fgets, unfortunately it doesn't provide the necessary details to reproduce it naming how is stdin represented in assembly. Here are my C and assembly codes i got by using gcc -S fgets.c .

fgets.c

#include <stdio.h>
int main()
{
    char name[15];
    fgets(name, 16, stdin);
    return 0;
}

fgets.s

    .file   "fgets.c"
    .def    ___main;    .scl    2;  .type   32; .endef
    .text
.globl _main
    .def    _main;  .scl    2;  .type   32; .endef
_main:
    pushl   %ebp
    movl    %esp, %ebp
    andl    $-16, %esp
    subl    $32, %esp
    call    ___main
    movl    __imp___iob, %eax
    movl    %eax, 8(%esp)
    movl    $16, 4(%esp)
    leal    17(%esp), %eax
    movl    %eax, (%esp)
    call    _fgets
    movl    $0, %eax
    leave
    ret
    .def    _fgets; .scl    2;  .type   32; .endef

First, i'm not good at reading AT&T syntax and thus understanding the above assembly source easily. So can anyone help me figure out: (i) where is my local variable name located? in ESP+17 ? (ii) If __imp___iob is representing stdin, where is coming from so that i can use it?

Thanks

Answer 1

Yes, name is at esp+17. You'd be wise to initialize it. Arguments in the cdecl calling conventions are passed right-to-left with the right-most deepest on the stack. The __imp___iob is exported from the CRT, you'll find it back in the stdio.h header file. Search for stdin. The __imp prefix is a Microsoft convention to make exports from DLLs faster. Getting this right is clearly the job of a compiler.