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From:	Jose Miguel =?ISO-8859-1?B?UOlyZXo=?= <NOSPAM_josemiguel_ AT _NOSPAM_perezruiz DOT com_NOSPAM>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Need to hook INT 21h from a DJGPP TSR
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Date:	Thu, 06 Oct 2005 09:37:54 GMT
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp
Reply-To:	djgpp AT delorie DOT com

Hi All!

I'm developing a DJGPP TSR which will be a installable FS to access Ext2 
partitions from DOS. I have in place all the code to keep the TSR, hook some 
interrupts and such. However, I'm stuck with INT 21h.

I want to hook INT 21h, to provide LFN support and some more functions. The 
following is a sample of the actual code I'm using to hook INT 2F, 
but it does not work with INT 21h.

Please note before replying: This is a protected mode TSR, which returns to 
DOS. The problem is similar to calling system() with INT 21h hooked.


//-------------------------------------------------------------------------
// All memory is LOCKED. No virtual memory.
int _crt0_startup_flags = _CRT0_FLAG_LOCK_MEMORY;

//-------------------------------------------------------------------------
void my_handler(_go32_dpmi_registers *r)
{
    if (r->h.ah == OUR_FUNCTION) {
        // Do work
        return;
    }

    r->x.cs = oldInt2F.rm_segment;
    r->x.ip = oldInt2F.rm_offset;
    r->x.ss = r->x.sp = 0;
    _go32_dpmi_simulate_fcall_iret(r);
}

//-------------------------------------------------------------------------
void install_handler()
{
    _go32_dpmi_get_real_mode_interrupt_vector(0x2F, &oldInt2F);
    info.pm_offset = (unsigned long)my_handler;
    _go32_dpmi_allocate_real_mode_callback_iret(&info, &regs);
    _go32_dpmi_set_real_mode_interrupt_vector(0x2F, &info);
}


I'm aware of the reentrance problem, however, I don't know how to handle it. 
It will be very useful to see some sample code which already solves this.

I will not call any INT 21h functions inside my handler, at least I'm not 
planning to, so reentrancy should not be a problem for me besides DJGPP 
itself calling INT 21h.

Please, I'm in a hurry, since the entire project is stopped until I can 
solve this. It will be very useful if you have some links to source code 
that already does this or some documentation I can read. I'm not very good 
at assembly code, but if you know of a solution in assembly it will be 
welcome.

Cheers,
Jose Miguel.

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