I don't understand the following C code line
I found the following thread:
Calculate broadcast address from ip and subnet mask and there the link to http://lpccomp.bc.ca/netm开发者_开发技巧ask/netmask.cCould someone please explain the following line, I don't understand:
for ( maskbits=32 ; (mask & (1L<<(32-maskbits))) == 0 ; maskbits-- )
especially mask & (1L<<(32-maskbits))
<<
is the bitwise left shift operator; it shifts the bits of a value left by the given amount. Thus 1L<<(32-maskbits)
shifts the value 1 to the left 32-maskbits
times.
&
is the bitwise AND operator.
So the loop expression mask & (1L<<(32-maskbits)) == 0
tests all the bits within the value of mask
, from lower to higher. The loop will stop on the first (lowest) nonzero bit of mask
, at which point maskbits
will contain the number of bits above (and including) that bit.
E.g.
- if
mask == 0xFFFF
mask == 0xFFFFFFFF (== binary 11111111111111111111111111111111)
, the loop will stop on the first iteration, andmaskbits
will be 32 - if
mask == 0x0001
mask == 0x00000001 (== binary 00000000000000000000000000000001)
, the loop will again stop on the first iteration, andmaskbits
will be 32 - if
mask == 0x1000
mask == 0x01000000 (== binary 00000001000000000000000000000000)
, the loop will stop on the 24th iteration, andmaskbits
will be 8
Have a look at bitwise operators, specifically left shift.
http://en.wikipedia.org/wiki/Bitwise_operation#Shifts_in_C.2C_C.2B.2B_and_Java
To see what's happening: run it.
#include <stdio.h>
#include <iostream>
using namespace std;
char *binary (unsigned int v) {
static char binstr[33] ;
int i ;
binstr[32] = '\0' ;
for (i=0; i<32; i++) {
binstr[31-i] = v & 1 ? '1' : '0' ;
v = v / 2 ;
}
return binstr ;
}
int main(void){
unsigned long maskbits,mask;
mask = 0x01000000;
cout << "MASK IS: " << binary(mask) << "\n";
cout << "32 is: " << binary(32) << "\n\n";
for ( maskbits=32 ; (mask & (1L<<(32-maskbits))) == 0 ; maskbits-- ) {
cout << "maskbits: " << binary(maskbits) << "\n";
cout << "\t(32-maskbits): " << binary((32-maskbits)) << "\n";
cout << "\t1L<<(32-maskbits): " << binary((1L<<(32-maskbits))) << "\n";
cout << "\t(mask & (1L<<(32-maskbits))): " << binary((mask & (1L<<(32-maskbits)))) << "\n\n";
}
cout << "\nFinal maskbits: " << maskbits;
return 0;
}
http://ideone.com/eB8Kp
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