How to read from a file in assembly x86 - c

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.