IP Address to Integer - C

Question

Hey guys,

I previously posted how to implement a function that will convert an integer to an IP address string. So how would we go vice-versa, that is, given a string (154.111.23.23) that is an address, how can we get that integer back, using no inet functions.

Answer 1

scanf the string into four bytes and add/shift them into a 32 bit integer.

Answer 2

//No checking of the input    
unsigned int c1,c2,c3,c4;
scanf("%d.%d.%d.%d",&c1,&c2,&c3,&c4);
unsigned long ip = (unsigned long)c4+c3*256+c2*256*256+c1*256*256*256;
printf("The unsigned long integer is %lu\n",ip);

EDIT: For those how are interested on the code produced, GCC is smart enough to replace my multiplication of 256 with shifts left. (In my program I've also called exit):

0x80483d4 <main>:  lea    0x4(%esp),%ecx
0x80483d8 <main+4>:  and    $0xfffffff0,%esp
0x80483db <main+7>:  pushl  -0x4(%ecx)
0x80483de <main+10>:  push   %ebp
0x80483df <main+11>:  mov    %esp,%ebp
0x80483e1 <main+13>:  push   %ecx
0x80483e2   <main+14>:  sub    $0x34,%esp
0x80483e5   <main+17>:  lea    -0x14(%ebp),%eax
0x80483e8 <main+20>:  mov    %eax,0x10(%esp)
0x80483ec <main+24>:  lea    -0x10(%ebp),%eax
0x80483ef <main+27>:  mov    %eax,0xc(%esp)
0x80483f3 <main+31>:  lea    -0xc(%ebp),%eax
0x80483f6 <main+34>:  mov    %eax,0x8(%esp)
0x80483fa <main+38>:  lea    -0x8(%ebp),%eax
0x80483fd <main+41>:  mov    %eax,0x4(%esp)
0x8048401 <main+45>:  movl   $0x8048520,(%esp)
0x8048408 <main+52>:  call   0x8048320 <scanf@plt>
0x804840d <main+57>:  mov    -0x8(%ebp),%eax
0x8048410 <main+60>:  mov    %eax,%edx
0x8048412 <main+62>:  shl    $0x8,%edx
    0x8048415   <main+65>:  mov    -0xc(%ebp),%eax
0x8048418 <main+68>:  lea    (%edx,%eax,1),%eax
0x804841b <main+71>:  mov    %eax,%edx
0x804841d <main+73>:  shl    $0x8,%edx
0x8048420 <main+76>:  mov    -0x10(%ebp),%eax
0x8048423 <main+79>:  lea    (%edx,%eax,1),%eax
0x8048426 <main+82>:  mov    %eax,%edx
0x8048428 <main+84>:  shl    $0x8,%edx
0x804842b <main+87>:  mov    -0x14(%ebp),%eax
0x804842e <main+90>:  lea    (%edx,%eax,1),%eax
0x8048431 <main+93>:  mov    %eax,-0x18(%ebp)
0x8048434 <main+96>:  mov    -0x18(%ebp),%eax
0x8048437 <main+99>:  mov    %eax,0x4(%esp)
0x804843b <main+103>:  movl   $0x804852c,(%esp)
0x8048442 <main+110>:  call   0x8048330 <printf@plt>
0x8048447 <main+115>:  movl   $0x0,(%esp)
0x804844e <main+122>:  call   0x8048340 <exit@plt>