如何从程序集x86中的文件读取

Question

I have a code in c language that needs to be translated to assembly x86. Here is the c code:

int rb (FILE *f){
    int s;
    char c;
    s = fr(&c, 1, 1, f);
    if (s <= 0) return -1;
    return (int)c;
}

So far I got to this assembly code that gives me Segmentation fault:

rb:
    pushl %ebp
    movl %esp,%ebp
    pushl 8(%ebp)
    pushl $1
    pushl $1
    leal 12(%ebp), %eax
    pushl %eax
    call fr
    jz ng
    jns ex
ng:
    pushl $1
    negl %eax
ex:
    popl %ebp
    ret

Can anyone help me to solve this? :)

Answer 1

Both Gcc and Clang can generate the assember for you. It might not always be easy to read but this is how to do it:

Make the snippet you want to inspect compilable with no errors. Note, I've changed your example to take a pointer to an integer as an argument because in your example you were declaring a char on the stack and then returning it ie Undefined Bahaviour.

Create a file called foo.c with this in it:

#include <stdio.h>
extern size_t fr(void *restrict ptr, size_t size, size_t nitems, FILE *restrict stream);
int rb (FILE *f, int *c){
  int s;
  s = fr(c, 1, 1, f); 
  if (s <= 0) return -1; 
  return *c; 
}

Compile it using the S flag to gcc ie

gcc-5 -S -O0 -Wall -pedantic -std=c11 foo.c

The open the following file foo.s

.text
  .globl _rb 
_rb:
LFB1:
  pushq %rbp
LCFI0:
  movq  %rsp, %rbp
LCFI1:
  subq  $32, %rsp
  movq  %rdi, -24(%rbp)
  movq  %rsi, -32(%rbp)
  movq  -24(%rbp), %rdx
  movq  -32(%rbp), %rax
  movq  %rdx, %rcx
  movl  $1, %edx
  movl  $1, %esi
  movq  %rax, %rdi
  call  _fr 
  movl  %eax, -4(%rbp)
  cmpl  $0, -4(%rbp)
  jg  L2  
  movl  $-1, %eax
  jmp L3
L2:
  movq  -32(%rbp), %rax
  movl  (%rax), %eax
L3:
  leave
LCFI2:
  ret 
LFE1:
  .section __TEXT,__eh_frame,coalesced,no_toc+strip_static_syms+live_support
....
.... snipped
....

Now you have the assembler on x86 for the code you wanted. Note, you can play around with various options to change the output in particular the optimization levels will drastically change the output.