The getaddrinfo and getnameinfo functions are part of the POSIX
POSIX
POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
A hostname is a label that is assigned to a device connected to a computer network and that is used to identify the device in various forms of electronic communication such as the World Wide Web, e-mail or Usenet...
s and IP addresses between their human-readable text representations and structured binary formats for the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
's networking API.
getaddrinfo and getnameinfo are inverse functions of each other.
This set of functions is fully network protocol agnostic and supports both IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
. It is the recommended interface for name resolution in building protocol independent applications and for transitioning legacy IPv4 code to the IPv6 Internet.
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
data structure used to represent addresses and hostnames within the networking API is the following:
struct addrinfo {
int ai_flags;
int ai_family;
int ai_socktype;
int ai_protocol;
size_t ai_addrlen;
struct sockaddr *ai_addr;
char *ai_canonname; /* canonical name */
struct addrinfo *ai_next; /* this struct can form a linked list */
};
In recent operating systems the type of ai_addrlen has been changed from size_t to socklen_t. Most socket functions, such as accept and getpeername, require a socklen_t* parameter and programmers often pass the address to the ai_addrlen element of the addrinfo structure. If the types are incompatible, e.g., on a big-endian 64-bit Solaris 9 system where size_t is 8 bytes and socklen_t is 4 bytes, then run-time errors may result.
getaddrinfo
getaddrinfo converts human-readable text strings representing hostname
Hostname
A hostname is a label that is assigned to a device connected to a computer network and that is used to identify the device in various forms of electronic communication such as the World Wide Web, e-mail or Usenet...
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
In computer science, a linked list is a data structure consisting of a group of nodes which together represent a sequence. Under the simplest form, each node is composed of a datum and a reference to the next node in the sequence; more complex variants add additional links...
of struct addrinfo structures. An application can deallocate these linked lists with the freeaddrinfo function.
The function prototypes for these functions are specified as follows:
getnameinfo converts the internal binary representation of an IP address in the form of a struct sockaddr pointer into text strings consisting of the hostname or, if the address cannot be resolved into a name, a textual IP address representation, as well as the service port name or number. The function prototype is specified as follows:
include
include
int getnameinfo(const struct sockaddr *sa, socklen_t salen,
char *host, size_t hostlen,
char *serv, size_t servlen,
int flags);
Example
The following example uses getaddrinfo to resolve the domain name www.example.com into its list of addresses and then calls getnameinfo on each result to return the canonical name for the address. In general, this will produce the original hostname
Hostname
A hostname is a label that is assigned to a device connected to a computer network and that is used to identify the device in various forms of electronic communication such as the World Wide Web, e-mail or Usenet...
, unless the particular address has multiple names, in which case the canonical name is returned. In this example, the domain name will be printed three times, once for each of the three results obtained.
include
include
include
include
include
ifndef NI_MAXHOST
define NI_MAXHOST 1025
endif
int main(void)
{
struct addrinfo *result;
struct addrinfo *res;
int error;
/* resolve the domain name into a list of addresses */
error = getaddrinfo("www.example.com", NULL, NULL, &result);
if (error != 0)
{
fprintf(stderr, "error in getaddrinfo: %s\n", gai_strerror(error));
return EXIT_FAILURE;
}
/* loop over all returned results and do inverse lookup */
for (res = result; res != NULL; res = res->ai_next)
{
char hostname[NI_MAXHOST] = "";
error = getnameinfo(res->ai_addr, res->ai_addrlen, hostname, NI_MAXHOST, NULL, 0, 0);
if (error != 0)
{
fprintf(stderr, "error in getnameinfo: %s\n", gai_strerror(error));
continue;
}
if (*hostname != '\0')
printf("hostname: %s\n", hostname);
}
freeaddrinfo(result);
return EXIT_SUCCESS;
}
The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.
