Unix Network Programming Episode 23

‘inet_aton’, ‘inet_addr’, and ‘inet_ntoa’ Functions

They convert Internet addresses between ASCII strings (what humans prefer to use) and network byte ordered binary values (values that are stored in socket address structures).

1.inet_aton, inet_ntoa, and inet_addr convert an IPv4 address from a dotted-decimal string (e.g., “206.168.112.96”) to its 32-bit network byte ordered binary value. You will probably encounter these functions in lots of existing code.
2.The newer functions, inet_pton and inet_ntop, handle both IPv4 and IPv6 addresses. We describe these two functions in the next section and use them throughout the text.

‘inet_aton’, ‘inet_addr’, and ‘inet_ntoa’ Functions

They convert Internet addresses between ASCII strings (what humans prefer to use) and network byte ordered binary values (values that are stored in socket address structures).
1.inet_aton, inet_ntoa, and inet_addr convert an IPv4 address from a dotted-decimal string (e.g., “206.168.112.96”) to its 32-bit network byte ordered binary value. You will probably encounter these functions in lots of existing code.
2.The newer functions, inet_pton and inet_ntop, handle both IPv4 and IPv6 addresses. We describe these two functions in the next section and use them throughout the text.

#include 

int inet_aton(const char *strptr, struct in_addr *addrptr);
in_addr_t inet_addr(const char *strptr);
char *inet_ntoa(struct in_addr inaddr);

Today, inet_addr is deprecated and any new code should use inet_aton instead. Better still is to use the newer functions described in the next section, which handle both IPv4 and IPv6.

‘inet_pton’ and ‘inet_ntop’ Functions

The letters “p” and “n” stand for presentation and numeric. The presentation format for an address is often an ASCII string and the numeric format is the binary value that goes into a socket address structure.

#include 

int inet_pton(int family, const char *strptr, void *addrptr);
const char *inet_ntop(int family, const void *addrptr, char *strptr, size_t len);

To help specify this size, the following two definitions are defined by including the header:

#define INET_ADDRSTRLEN 16 /* for IPv4 dotted-decimal */
#define INET6_ADDRSTRLEN 46 /* for IPv6 hex string */

If len is too small to hold the resulting presentation format, including the terminating null, a null pointer is returned and errno is set to ENOSPC.

The strptr argument to inet_ntop cannot be a null pointer. The caller must allocate memory for the destination and specify its size. On success, this pointer is the return value of the function.