inet_top example – LinuxTips

Ever needed to convert an IP address as a string (dot – quad) to its decimal representation? Here is a quick program I wrote in C that will convert an IP address string from the command line into its decimal representation, then from that decimal representation back into its dot quad form.

#include<stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <string.h>
#include <errno.h>
 
int main(int argc, char *argv[])
{
        struct in_addr addr;
        if(argc < 2){
                fprintf(stderr, "usage: ./quad_to_byte [ip_address]\n");
                return -1;
        }
 
        char *byte_order = malloc(sizeof(argv[1]));
	if(!byte_order){
		fprintf(stderr, "Could not allocate memory for conversion.\n");
		return -1;
	}
 
	// Convert address to byte order
        if(!inet_pton(AF_INET, argv[1], &addr)){
                fprintf(stderr, "Could not convert address\n");
		free(byte_order);
                return -2;
        }
 
	// Print out network byte order
        fprintf(stdout, "Network byte order: %d\n",addr.s_addr);
 
	// Convert it back to our dot quad to verify
	if(inet_ntop(AF_INET, &addr.s_addr, byte_order, sizeof(argv[1])*2) == NULL){
                fprintf(stderr, "Could not convert byte to address\n");
                fprintf(stderr, "%s\n",strerror(errno));
		free(byte_order);
		return -3;
        }
 
	// Display our dot quad converted from the network byte order
	fprintf(stdout, "Dot quad: %s\n",byte_order);
	free(byte_order);
        return 0;
}

Then to compile it you simply run:

gcc -o quad_to_byte quad_to_byte.c

Then running the simple program:

erik@debian:$ ./quad_to_byte 10.3.4.2
Network byte order: 33817354
Dot quad: 10.3.4.2

Its really that easy. If you need to only do the conversion in one direction simply use only inet_pton to go from the IP address to decimal and in the other direction use inet_ntop.

Tag:inet_top example

Convert IP Address to Decimal in C