From: Ying Qin <yqin AT ece DOT uci DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Callback problem
Date: Thu, 28 Aug 1997 14:00:09 -0700
Organization: DREAM Lab
Lines: 83
Message-ID: <3405E6D9.98D@ece.uci.edu>
Reply-To: yqin AT ece DOT uci DOT edu
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To: djgpp AT delorie DOT com
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I am writing a program to use packet driver to receive packets. During
the initialization of the packet driver, the real mode address of the
packet receiver function(the application) must be passed to the packet
driver. Then, when a packet arrives, the packet driver will call the
application twice. AX=0 on the first call. The application should return
a pointer to the buffer in ES:DI. On the second call, AX=1 and the
packet was copied to DS:SI.

My program is as follows. Compiled by gcc(DJGPP). CWSDPMI is
automatically loaded when running. The problem is: on the first call, it
returns well, but on the second call, it freezes. 

Thx. 

static _go32_dpmi_seginfo call_back_hdlr, buf_info;
static _go32_dpmi_registers r, call_back_reg;

/* called by the packet driver if a packet was received */
void PacketReceive()
{
        if (call_back_reg.x.ax == 1)
	{ /* Seconde Call */
                total++;
                printf("Total: %d\n", total);
	}
        else
	{ /* First call */
		...
                call_back_reg.x.es = buf_info.rm_segment;
                call_back_reg.x.di = buf_info.rm_offset;
		...
	}
        _go32_dpmi_simulate_fcall_iret(&call_back_reg);
}

int PacketSetup (void)
{
        int i;

        total = 0;
        buf_info.size = (BUFFER_LEN + 15) / 16 ;
        _go32_dpmi_allocate_dos_memory(&buf_info); /* allocat dos memory
for packet driver's use */

	...

	/* Use real mode callback to get the real mode pointer of the receiver
*/
        call_back_hdlr.pm_selector = _go32_my_cs();
        call_back_hdlr.pm_offset = (unsigned long)PacketReceive;
        _go32_dpmi_allocate_real_mode_callback_iret(&call_back_hdlr,
&call_back_reg);

        r.x.es = call_back_hdlr.rm_segment;
        r.x.di = call_back_hdlr.rm_offset;
        r.h.ah = 2;
	...
        r.x.cx = 0;
        __dpmi_int( 0x60, &r);   /* packet driver -> access_type() */

	...

        return (NO_ERROR);
}


void main()
{
	...

        if (PacketSetup()==ERROR)
        {
                printf("Packet Setup failure!\n");
                return;
        }
        while (1)
        {
                if (kbhit())
                        break;
        }

	...
}