varargs call frames

[Nick Clifton <nickc@redhat.com>]

Hi anix,

> my source file: > > .section .data> msg:  .asciz "Address after MALLOC: 0x%x.\n"> p:    .int 0x0
> .section .text
> .globl _start
> _start:
>       pushl $0x14
>       call malloc
> k:    movl %eax, p
>       pushl %eax
>       pushl %ebx
>       pushl $msg
>       call printf

> // after assembling and loading, I fond that the result while running the file:
> [nwsh@localhost ch06]$ ./mmaddr
> *Address after MALLOC:**0xb774dfbc.*
>
> // in gdb environment:
> Breakpoint 1, k () at mmaddr.s:9
> 9     k:    movl %eax, p
> (gdb) p/x $eax
> $2 = 0x804b008 >
> // i do not know why the results under running and gdb different very mach.
> *// please tell me, Thank you very much!!!*

The answer is that you have a mistake in your code.  The call to 'malloc'
returns the allocated address in %eax, but you then destroy this pointer
with the movl instruction at label 'k'.  So when you call 'printf' you
are displaying whatever value happened to be in the %ebx register.  Which
could be anything.

The gdb commands are correct.  Upon return from malloc, the %eax register
holds the correct value.  It is just that you are not preserving this value
and passing it on to the call to printf.

Cheers
   Nick