How to convert IPv4 address to decimal - c

Question

Is there a function to convert an IPv4 address in dotted decimal notation (e.g., "192.168.0.1") into a decimal value?
There are several questions similar to this, but all are looking for or have answers that involve writing a function to solve the answer. I'm looking for an existing function in a library.

Answer 1

The C standard library doesn't have such a function. It's simple enough that this shouldn't be a problem. Here's some example code using strtoul() for parsing:
int parseIpv4Address(uint32_t *addr, const char *addrStr)
{
const char *p = addrStr;
char *endp;
unsigned long a = strtoul(p, &endp, 10);
if (*endp != '.') return -1;
unsigned long b = strtoul(p = endp+1, &endp, 10);
if (*endp != '.') return -1;
unsigned long c = strtoul(p = endp+1, &endp, 10);
if (*endp != '.') return -1;
unsigned long d = strtoul(p = endp+1, &endp, 10);
if (*endp) return -1;
*addr = (uint32_t) ((a << 24) | (b << 16) | (c << 8) | d);
return 0;
}
as unsigned long must have at least 32 value bits, there's no special care (casting) needed for the shifts here.
It doesn't validate the range of the individual parts, if you need this, you should be able to add it without problems.

Answer 2

You could use something like inet_pton which is pretty straight forward to use as you can see from the example code below.
#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
int main(void)
{
struct in_addr result;
if(inet_pton(AF_INET,"127.0.0.1",&result))
{
printf("%08x\n",result.s_addr);
}
else
{
printf("Failed\n");
}
return(0);
}
The number is in network byte order, but if you want it in host byte order, you can use ntohl to convert it.

Answer 3

It turns out that Winsock 2 provides the inet_addr() function, which simply returns the equivalent decimal value for the address.
#include <stdio.h>
#include <winsock2.h>
int main(void)
{
char* pAddr = "192.168.0.1";
unsigned long value = inet_addr(pAddr);
if (value == INADDR_NONE)
printf("Invalid format of IP address");
else
printf("Decimal representation of %s is: %lu", pAddr, value);
return 0;
}