simple upper-casifier loop infinitely - linux

Question

simple upper-casifier loop infinitely
what's wrong with my code?
any advice?
my programming environment is linux, emacs, assembly, at&t syntax
.section .data
.section .bss
.lcomm buffer,1
.section .text
.global _start
_start:
movl %esp,%ebp
subl $8,%esp
#8(%ebp) is 2nd arg == input
#12(%ebp) is 3rd arg == output
#open,read,open,write,close
movl $5,%eax
movl 8(%ebp),%ebx
movl $0,%ecx
movl $0666,%edx
int $0x80
#%eax contains input's fd
#movl to first local var
movl %eax,-4(%ebp)
movl $5,%eax
movl 12(%ebp),%ebx
movl $03101,%ecx
movl $0666,%edx
int $0x80
#eax contains output's fd
#movl to second local var
movl %eax,-8(%ebp)
loop:
#read 1 byte from file 1st byte of data
movl $3,%eax
movl -4(%ebp),%ebx
movl $buffer,%ecx
movl $1,%edx
int $0x80
#buffer contains 1 byte of file
cmpb $0,buffer
je program_exit
pushl buffer
call convert #will return converted to %al
addl $4,%esp
movb %al,buffer
#write 1 byte from buffer to file
movl $1,%edx
movl $buffer,%ecx
movl -8(%ebp),%ebx
movl $4,%eax
int $0x80
jmp loop
program_exit:
movl buffer,%ebx
movl $1,%eax
int $0x80
.type convert,#function
convert:
pushl %ebp
movl %esp,%ebp
movb 8(%ebp),%al #1 byte data in the buffer
cmpb $'a',%al
jl convert_end
cmpb $'z',%al
jg convert_end
addb $32,%al #convert to upper
convert_end:
movl %ebp,%esp
popl %ebp
ret

Answer 1

Note that read(2) signals an end-of-file condition by returning 0. You're trying to find the end of the file by looking for an ascii NUL, which is very rare on Unix-derived systems. (If you want an easy way to make a terabyte-sized file, dd if=/dev/zero of=/tmp/huge bs=1048576 seek=1048576 count=1. The whole thing will be filled with ascii NUL characters. (Or the integer 0, however you want to interpret it.)
You need to modify your code to find the end of file by comparing the return value of the read(2) system call, not by looking at the data you receive into your buffer.