Comparing strings containing IPv4 addresses in C - c

Question

I have two strings ip1 = "192.168.145.123" and ip2 = "172.167.234.120".
I can compare these two strings for equality:
strncmp(ip1,ip2) == 0
However how can I find out
if (ip1 > ip2) {
...
}
What I have tried
I can use sscanf:
sscanf(ip1,"%d.%d.%d.%d",&s1,&s2,&s3,&s4)
and store the numbers and compare.
However in 32 bit I can't store the numbers as integers due to the upper limit.
Thus I have no choice but to compare the integers as strings.

Answer 1

Is it worth mentionning that there is also inet_aton ?
You can find the man page here, below is a short description and a short synopsis.
This solution will work on most POSIX systems, but I'm sure there is some equivalent in the Windows APIs, and even some abstraction wrapper.
inet_ntoa() is specified in POSIX.1-2001. inet_aton() is not specified in POSIX.1-2001, but is available on most systems.
Linux Programmer's Manual
inet_aton() converts the Internet host address cp from the IPv4 numbers-and-dots notation into binary form (in network byte order) and stores it in the structure that inp points to.
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
int inet_aton(const char *cp, struct in_addr *inp);
char *inet_ntoa(struct in_addr in);
EXAMPLE
An example of the use of inet_aton() and inet_ntoa() is shown below. Here are some example runs:
$ ./a.out 226.000.000.037 # Last byte is in octal
226.0.0.31
$ ./a.out 0x7f.1 # First byte is in hex
127.0.0.1
Program source
#define _BSD_SOURCE
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
struct in_addr addr;
if (argc != 2) {
fprintf(stderr, "%s <dotted-address>\n", argv[0]);
exit(EXIT_FAILURE);
}
if (inet_aton(argv[1], &addr) == 0) {
fprintf(stderr, "Invalid address\n");
exit(EXIT_FAILURE);
}
printf("%s\n", inet_ntoa(addr));
exit(EXIT_SUCCESS);
}
Further informations
Byte ordering (#Jonathan Leffler)
The inet_ntoa() function converts the Internet host address in, given in network byte order, to a string in IPv4 dotted-decimal notation.
inet_aton() converts the Internet host address cp from the IPv4 numbers-and-dots notation into binary form (in network byte order) and stores it in the structure that inp points to.
Structure of in_addr (#POW)
The structure in_addr as used in inet_ntoa(), inet_makeaddr(), inet_lnaof() and inet_netof() is defined in as:
typedef uint32_t in_addr_t;
struct in_addr {
in_addr_t s_addr;
};
Compare to address independently of computer-endianness
Addresses in in_addr are in network byte order (big-endian), so as pointed by #glglgl, you have to use ntohl, whose man page is available here.
The ntohl() function converts the unsigned integer netlong from network byte order to host byte order.
uint32_t ntohl(uint32_t netlong);

Answer 2

You can try the sexy way, store all values in one unsigned integer and compare it.
const char* ip1 = "192.168.145.123";
const char* ip2 = "172.167.234.120";
unsigned char s1, s2, s3, s4;
unsigned int uip1, uip2;
sscanf(ip1,"%hhu.%hhu.%hhu.%hhu",&s1,&s2,&s3,&s4);
uip1 = (s1<<24) | (s2<<16) | (s3<<8) | s4; //store all values in 32bits unsigned int
sscanf(ip2,"%hhu.%hhu.%hhu.%hhu",&s1,&s2,&s3,&s4);
uip2 = (s1<<24) | (s2<<16) | (s3<<8) | s4;
if (uip1 > uip2)
{
printf("ip1 greater !");
}
else
{
printf("ip2 greater or equal !");
}

Answer 3

How about this :-
#include<stdio.h>
#include<conio.h>
unsigned int convIP(const char ip[]) {
unsigned char s1, s2, s3, s4;
if (sscanf(ip, "%hhu.%hhu.%hhu.%hhu", &s1, &s2, &s3, &s4) != 4)
return 0;
/* Create a 32 bit Integer using left shift & bitwise OR
MSB LSB
+-----8----+-----8------+-----8-----+----8-----+
| s1 | s2 | s3 | s4 |
+----------+------------+-----------+----------+
*/
return (s1 << 24) | (s2 << 16) | (s3 << 8) | (s4 << 0);
}
int ipComp(const char ip1[], const char ip2[]) {
unsigned int ip_addr1 = convIP(ip1);
unsigned int ip_addr2 = convIP(ip2);
return (ip_addr1 >= ip_addr2);
}
int main()
{
printf("%d\n",ipComp("192.168.145.123","172.167.234.120") ); //1
printf("%d\n", ipComp("10.0.0.1","192.168.1.1") ); //0
printf("%d\n",ipComp("192.168.145.123","192.168.145.123")); //1
}
Edit: As suggested by H2CO3:
You should generally avoid using sscanf, instead you can use strtol(), like this:
unsigned long ip2int(const char *ip)
{
const char *end = ip + strlen(ip);
unsigned long n = 0;
while (ip < end) {
n <<= 8;
n |= strtoul(ip, (char **)&ip, 10);
ip++;
}
return n;
}

Answer 4

A pedantic "after the accepted answer" answer. Error checking employed.
#include <inttypes.h>
int IPstringToUns32(const char *IPString, uint32_t *IPNumber) {
uint8_t c[4]; // LSByte in c[0]
char ch;
const char * format = "%" SCNu8 ".%" SCNu8 ".%" SCNu8 ".%" SCNu8 "%c";
if (4 != sscanf(IPString, format, &c[3], &c[2], &c[1], &c[0], &ch)) {
return 1; // parse error
}
*IPNumber = (((uint32_t) c[3]) << 24) | (((uint32_t) c[2]) << 16)
| (((uint32_t) c[1]) << 8) | ((uint32_t) c[0]);
return 0;
}
Suppose one could instead use uint_fast32_t. This solution allows for leading white space before digits.
[Edit] Added classic %c at the end of the format. Thanks to #glglgl.