gistfile1.asm

; tee
; Returns non-zero exit code if an error occured

section .bss
        buf: resb 1024
        buflen equ $-buf
        stdin equ 0
        stdout equ 1
        O_WRONLY equ 1q
        O_CREAT equ 100q

section .text
        global _start

_start:
        ; Throw away argc and argv[0]
        pop eax
        pop eax
        
        ; sys_open
        mov eax, 5
        pop ebx
        mov ecx, (O_WRONLY|O_CREAT)
        mov edx, 666q
        int 0x80

        cmp eax, 0
        jz exit

        mov edi, eax
        read_loop:
                ; sys_read
                mov eax, 3
                mov ebx, stdin
                mov ecx, buf
                mov edx, buflen
                int 0x80
                
                ; EOF if bytes read == 0
                cmp eax, 0
                ; Bail out if <= 0
                jle cleanup
                
                ; sys_write
                ; edx = bytes to write == bytes read
                ; ecx already contains buf
                mov edx, eax
                mov eax, 4
                mov ebx, edi
                int 0x80

                ; Check number of bytes written
                cmp eax, edx
                jne cleanup

                ; Do the same for stdout
                mov eax, 4
                mov ebx, stdout
                int 0x80

                cmp eax, edx
                je read_loop

cleanup:
        ; Save exit code
        push eax

        ; sys_close
        mov eax, 6
        mov ebx, edi
        int 0x80

        ; Restore exit code
        pop eax
exit:
        mov ebx, eax
        mov eax, 1
        int 0x80