Syslinux - Jun 2009 - error on implementing printf

hello, all.

here is my implementation of printf for use in syslinux core.

#include <stdio.h>
#include <unistd.h>

#define BUF_SIZE 1024
char buf[BUF_SIZE];


extern void myputs(const char *);

int printf(const char *format, ...)
{
    va_list ap;
    int rv;

#if 1
    myputs("DEBUG:the string we want fomart is\n\r");
    myputs(format);
#endif

    va_start(ap, format);
    rv = vsprintf(buf, format, ap);
    va_end(ap);

    myputs(buf);

    return rv;
}

well, this program is simple. but it can't work; then i debugged it, found
something error really happened.
it's called from hello() in hello.c, here is the snippet::

void hello(void)
{
    static char hello_str[] = "Hello, World!  (hello.c)\r\n";

    myputs(hello_str);
    printf(hello_str);
    printf("testing with format output %d\n\r", 2);
}



here is the disassemble:

sub esp, 0x0000000c   ; seems it want allocate some memory, but I didn't see
anything pushed on it..
mov eax, 0x00100fb0   ; the address of hello_str.

call 0x100170(myputs)  ; the myputs() function.  Seems the function passed
the parameter by register but not stack.
                                    ; then I would be really confused, why
it don't use the stack after allocating the stack memory.

; and for now, it goes well, the hello_str displayed well.

mov eax, 0x00100fb0  ; also use the eax to pass the parameter.
call 0x1001b8(printf)     ; the printf function. trace in.....

-------------printf------------
push ebx
sub esp 0x00000008  ; allocate memory again...
mov eax, 0x00100f68
call 0x100170(myputs) ; puts the debug message, work wee too.

; then here we go
mov eax, dword ptr ss:[esp+0x10]   ; well, as we haven't push the fomart
address, so how can we get the address by stack?
call 0x100170(myputs)

; so eax would point to a unknow value, in there, I don't know why it point
to "linux".
; and as we can think, it will output "linux" but not the hello_str
string.
; so it failed.

I don't know why, but I'm think of the gcc flags.

any ideas?

Thanks!

-- 
regards
liu Aleaxander

liu Aleaxander wrote:
> 
> well, this program is simple. but it can't work; then i debugged it,
found
> something error really happened.
> it's called from hello() in hello.c, here is the snippet::
> 
> void hello(void)
> {
>     static char hello_str[] = "Hello, World!  (hello.c)\r\n";
> 
>     myputs(hello_str);
>     printf(hello_str);
>     printf("testing with format output %d\n\r", 2);
> }
> 
> 
> 
> here is the disassemble:
> 
> sub esp, 0x0000000c   ; seems it want allocate some memory, but I
didn't see
> anything pushed on it..
> mov eax, 0x00100fb0   ; the address of hello_str.
> 
> call 0x100170(myputs)  ; the myputs() function.  Seems the function passed
> the parameter by register but not stack.
>                                     ; then I would be really confused, why
> it don't use the stack after allocating the stack memory.
> 
> ; and for now, it goes well, the hello_str displayed well.
> 
> mov eax, 0x00100fb0  ; also use the eax to pass the parameter.
 
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> call 0x1001b8(printf)     ; the printf function. trace in.....
> 
The 32-bit code in Syslinux is compiled with -mregparm=3, which means
that it uses registers to pass in the arguments to *non-varadic*
functions.  However, it should not use registers to pass in the argument
to a varadic function like printf().  The easiest explanation for this
problem is that it is missing the prototype, i.e. <stdio.h> isn't
included.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.