Thread: INT n instruction in C Program

Hello,

I am trying to read the IP address of a machine through a software interrupt.
Is it possible to call the Software Interrupt (INT instruction) from a C program to do a specific task (in this case find the IP) that I want? If so can someone throw me a pointer or a sample code?

Thanks in Advance

Hey thanks for replying. I am using Ubuntu 8.04. And gcc 4.2.3
I managed to write the socket() call in _asm, I am next trying to write the ioctl call in _asm. I wrote the following code for socket

Code:

 
*__asm__("sub  $12,%%esp\n" // increasing stack for Interrupt
        "movl   $2,(%%esp)\n" // AF_INET
        "movl   $1,4(%%esp)\n" // SOCK_STREAM
        "movl   $0,8(%%esp)\n"  // third parameter to socket()
        "movl   $102,%%eax\n" // INT number for socket_CALL
        "movl   $1,%%ebx\n"     // SYS_socketcall_socket
        "movl   %%esp,%%ecx\n"
        "int    $0x80\n"
        "add    $12,%%esp\n" // restoring stack 
        : "=a" (s)
       );

This works perfectly. On trying to write similar code for ioctl, I am getting a return value of -9. Can someone help me out with this.

Code:

#include <sys/ioctl.h>
#include <net/if.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <stdio.h>
int main()
{
int i, res;
int s = 0;
s = socket(AF_INET,SOCK_STREAM,0);
   struct ifreq ifr;
   struct sockaddr_in *sin = (struct sockaddr_in *) &ifr.ifr_addr;
   char * ip;
   ifr.ifr_ifindex = 1;
   res = ioctl (s, SIOCGIFNAME, &ifr);
   printf("\n first res %d", res);
 res = ioctl(s, 35088, &ifr);
   printf("\n second  result %d", res);
__asm__("sub    $12,%%esp\n"
        "movl   %%edx,(%%esp)\n"
        "movl   $35088,4(%%esp)\n"
        "movl   %%ecx,8(%%esp)\n"
        "movl   $54,%%eax\n" // IOCTL interrupt number
        "movl   %%esp,%%ebx\n" // Placing ESP for interrupt handler, is this a wrong register? 
        "int    $0x80\n"
        "add    $12,%%esp\n"
        :"=a" (res)
        :"d" (s), "c" (&ifr)
);
 printf(" \n new result was %d ", res);
return 0;
}

I am getting -9 as output. Am I passing the parameters wrong?
Thanks in advance.

I resolved it, I was clobbering the parameters, the following code worked

Code:

 
   i = 35088;
__asm__("movl   $54,%%eax\n"
        "int    $0x80\n"
       :"=a" (res)
        :"b" (s), "c" (i), "d"(&ifr)
);

You can debug that stuff pretty easily with strace.
I was just looking at it, here's the output of previos code:

Code:

ioctl(4, SIOCGIFNAME, {ifr_index=1, ifr_name="lo"}) = 0
ioctl(4, SIOCGIFNAME, {ifr_index=1, ifr_name="lo"}) = 0
ioctl(-1079534860, 0xbfa79b30, 0x4)     = -1 EBADF (Bad file descriptor)