link two object files(in diff lang), error on execute

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othername

Joined: 08 Apr 2005
Posts: 26
Location: Lithuania

othername 15 Apr 2005, 22:25

hi, have a problem. First file(in asm) calls function writen in C(second).
I compile them like an objects, then link them:
-----------
fasm first.asm
gcc -c second.c
ld first.o second.o -o outputfile -lc
---------------
what is the problem, when i execute outputfile i have a message "no such file"?

Code:

format ELF
public _start
section '.text' executable 
_start:
extrn hello
    push        message
    call        hello
    mov         eax,1
    xor         ebx,ebx
    int         0x80
section '.data' writeable
message db "hello world", 0xA, 0

and second file in C(sorry for that)

Code:

#include <stdio.h>
int hello(const char* mesg)
{
    char *msg = (char *)mesg;
    for(; *msg!='0'; msg++)
        write(1, msg, 1);
    return 1
}

_________________
Sorry for my bad English Sad

15 Apr 2005, 22:25

crc

Joined: 21 Jun 2003
Posts: 637
Location: Penndel, PA [USA]

crc 16 Apr 2005, 17:48

Try this:

Code:

format ELF
public main
section '.text' executable
main:
extrn hello
    push        message
    call        hello
    mov         eax,1
    xor         ebx,ebx
    int         0x80
section '.data' writeable
message db "hello world", 0xA, 0

and link them by doing:

Code:

gcc first.o second.o -o outputfile

Just using 'ld' doesn't get the C runtime stuff in place; without that Linux can't properly identify and load the C library.

16 Apr 2005, 17:48

othername

Joined: 08 Apr 2005
Posts: 26
Location: Lithuania

othername 16 Apr 2005, 19:09

Works!
But why it must be main insted _start?

_________________
Sorry for my bad English Sad

16 Apr 2005, 19:09

crc

Joined: 21 Jun 2003
Posts: 637
Location: Penndel, PA [USA]

crc 16 Apr 2005, 22:52

The C runtime header provides _start, which sets up argc/argv among other things. The provided _start then jumps to main, which is the main entry point for the program in a C-based world.

16 Apr 2005, 22:52

Endre

Joined: 29 Dec 2003
Posts: 215
Location: Budapest, Hungary

Endre 17 Apr 2005, 11:17

Just one more thing: when a function returns it does not destroy the argument space you have created with push on the stack before calling to it. So you have to do it manually:

Code:

    push    message
    call    hello
    add     esp, 4 ; restore the stack - practically pop message but more general
                   ; even in case of multiple arguments
    mov     ebx, eax ; use return value for exit value
    mov     eax, 1 ; exit follows
    int     0x80

But you can also exploit this behavior:

Code:

    push    message
    call    hello ; message on the stack
    call    hello ; message still on the stack
    add     esp, 4 ; restore the stack
    mov     ebx, eax ; use return value for exit value
    mov     eax, 1 ; exit follows
    int     0x80

17 Apr 2005, 11:17

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