The getaddrinfo function in Linux is used to resolve hostnames and service names into network addresses. It provides a way to obtain information about an address, allowing you to create socket connections based on domain names or IP addresses.
Here’s a simple example that demonstrates how to use getaddrinfo in C:
Example: Resolving a Hostname
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>
int main() {
struct addrinfo hints, *res;
int status;
// Set up the hints structure
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_UNSPEC; // AF_INET or AF_INET6 to force version
hints.ai_socktype = SOCK_STREAM; // TCP stream sockets
// Resolve the hostname "www.example.com" using port "80"
if ((status = getaddrinfo("www.example.com", "80", &hints, &res)) != 0) {
fprintf(stderr, "getaddrinfo error: %s\n", gai_strerror(status));
return 1;
}
// Loop through all results and connect to the first we can
for (struct addrinfo *p = res; p != NULL; p = p->ai_next) {
void *addr;
char *ipver;
// Get the pointer to the address itself,
// different fields in IPv4 and IPv6:
if (p->ai_family == AF_INET) { // IPv4
struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
addr = &(ipv4->sin_addr);
ipver = "IPv4";
} else { // IPv6
struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
addr = &(ipv6->sin6_addr);
ipver = "IPv6";
}
// Convert the IP to a string and print it:
char ipstr[INET6_ADDRSTRLEN];
inet_ntop(p->ai_family, addr, ipstr, sizeof(ipstr));
printf("Detected %s: %s\n", ipver, ipstr);
// Here you would typically create a socket and connect:
/*
int sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol);
if (sockfd == -1) {
perror("socket");
continue;
}
*/
break; // For this example just take the first result.
}
freeaddrinfo(res); // Free the linked list allocated by getaddrinfo
return 0;
}
Explanation:
- Headers: The necessary headers are included at the beginning of your code.
- Hints Structure: We initialize a
struct addrinfocalledhints, where we specify our requirements:ai_family: We set this toAF_UNSPEC, meaning we want both IPv4 and IPv6 addresses.ai_socktype: We set this toSOCK_STREAMfor TCP sockets.
- Call
getaddrinfo: This function is called with parameters including the hostname ("www.example.com"), service ("80"), our hints structure, and a pointer for storing results (res). If there’s an error during resolution, it prints an error message. - Loop Through Results: We iterate through each entry returned in the linked list pointed to by
res. Depending on whether it’s an IPv4 or IPv6 address, we extract and print its value. - Clean Up: Finally, we free any memory allocated by
getaddrinfo.
Compilation
To compile this program:
gcc -o getaddrinfo_example getaddrinfo_example.c
Then run it:
./getaddrinfo_example
This will output the detected IP addresses associated with "www.example.com" along with their types (IPv4 or IPv6).
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