redirection

redirection

[glburt]

I have the following program:

	SECTION .data		; data section
msg1:	db "Hello World -- 1",10,0 ; the string to print, 10=cr
msg:	db "Hello World",10	; the string to print, 10=cr
len:	equ $-msg		; "$" means "here"
				; len is a value, not an address

	SECTION .text		; code section
        global main		; make label available to linker 
        extern printf
main:				; standard  gcc  entry point
	
	mov	edx,len		; arg3, length of string to print
	mov	ecx,msg		; arg2, pointer to string
	mov	ebx,1		; arg1, where to write, screen
	mov	eax,4		; write command to int 80 hex
	int	0x80		; interrupt 80 hex, call kernel
	
        push    msg1
        call    printf
        add	esp, 4

        mov	ebx,0		; exit code, 0=normal
	mov	eax,1		; exit command to kernel
	int	0x80		; interrupt 80 hex, call kernel


When I assembled/linked it, I used:

nasm -f elf burtp1.asm
gcc burtp1.o

when i ran it, I got two lines of output.:

[burt@linux2 ~]$ ./a.out
Hello World
Hello World -- 1


When I redirected it, I got:

[burt@linux2 ~]$ ./a.out > foo
[burt@linux2 ~]$ cat foo
Hello World

Why was the output from using printf not redirected?

Re: redirection

[Marcin Kościelnicki]

> I have the following program:
>
> 	SECTION .data		; data section
> msg1:	db "Hello World -- 1",10,0 ; the string to print, 10=cr
> msg:	db "Hello World",10	; the string to print, 10=cr
> len:	equ $-msg		; "$" means "here"
> 				; len is a value, not an address
>
> 	SECTION .text		; code section
>         global main		; make label available to linker
>         extern printf
> main:				; standard  gcc  entry point
>
> 	mov	edx,len		; arg3, length of string to print
> 	mov	ecx,msg		; arg2, pointer to string
> 	mov	ebx,1		; arg1, where to write, screen
> 	mov	eax,4		; write command to int 80 hex
> 	int	0x80		; interrupt 80 hex, call kernel
>
>         push    msg1
>         call    printf
>         add	esp, 4
>
>         mov	ebx,0		; exit code, 0=normal
> 	mov	eax,1		; exit command to kernel
> 	int	0x80		; interrupt 80 hex, call kernel
>
>
> When I assembled/linked it, I used:
>
> nasm -f elf burtp1.asm
> gcc burtp1.o
>
> when i ran it, I got two lines of output.:
>
> [burt@linux2 ~]$ ./a.out
> Hello World
> Hello World -- 1
>
>
> When I redirected it, I got:
>
> [burt@linux2 ~]$ ./a.out > foo
> [burt@linux2 ~]$ cat foo
> Hello World
>
> Why was the output from using printf not redirected?

It's because printf buffers output when it goes to file.

In c program, it cannot do any wrong, since library exit() function flushes 
all buffers before actually exitting.

However, you exit with direct kernel syscall. It's REALLY bad idea, as it 
terminates the process immediately, not allowing anything to run before it 
happens -- including the flush-buffers-on-exit thingie.

If you're not using libc, direct syscall is alright. However, if you use libc, 
you have to use the whole framework, or something's bound to break 
eventually. Will people never learn?

To make it do the Right Thingâ„¢, change this:

>         mov	ebx,0		; exit code, 0=normal
> 	mov	eax,1		; exit command to kernel
> 	int	0x80		; interrupt 80 hex, call kernel

to this:

push dword 0 ;exit code
call exit

Should work now.

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