Linux NASM detect EOF - linux

Question

I'm trying to learn the basics asm on linux and I can't find a very good reference. The NASM docs seem to assume you already know masm... I found no examples in the documentation of the cmp (outside the Intel instruction reference).
I'd written a program that reads a single byte from stdin and writes it to stdout. Below is my modification to try to detect EOF on stdin and exit when EOF is reached. The issue is it never exits. I just keeps printing the last char read from stdin. The issue is either in my EOF detection (cmp ecx, EOF) and/or my jump to the _exit label (je _exit) I think.
What am I doing wrong?
%define EOF -1
section .bss
char: resb 1
section .text
global _start
_exit:
mov eax, 1 ; exit
mov ebx, 0 ; exit status
int 80h
_start:
mov eax, 3 ; sys_read
mov ebx, 0 ; stdin
mov ecx, char ; buffer
cmp ecx, EOF ; EOF?
je _exit
mov edx, 1 ; read byte count
int 80h
mov eax, 4 ; sys_write
mov ebx, 1 ; stdout
mov ecx, char ; buffer
mov edx, 1 ; write byte count
int 80h
jmp _start
For the sake of sanity, I verified EOF is -1 with this C:
#include <stdio.h>
int main() { printf("%d\n", EOF); }

Answer 1

You are comparing the address of the buffer to EOF (-1) instead of the character stored in the buffer.
Having said that, the read system call does not return the value of EOF when end of file is reached, but it returns zero and doesn't stick anything in the buffer (see man 2 read). To identify end of file, just check the value of eax after the call to read:
section .bss
buf: resb 1
section .text
global _start
_exit:
mov eax, 1 ; exit
mov ebx, 0 ; exit status
int 80h
_start:
mov eax, 3 ; sys_read
mov ebx, 0 ; stdin
mov ecx, buf ; buffer
mov edx, 1 ; read byte count
int 80h
cmp eax, 0
je _exit
mov eax, 4 ; sys_write
mov ebx, 1 ; stdout
mov ecx, buf ; buffer
mov edx, 1 ; write byte count
int 80h
jmp _start
If you did want to properly compare the character to some value, use:
cmp byte [buf], VALUE
Also, I renamed char to buf. char is a basic C data type and a bad choice for a variable name.