gethostbyname2_r - Linux

Overview

gethostbyname2_r is a low-level function in the GNU C Library (glibc) that resolves a hostname to its corresponding IPv4 or IPv6 address. It performs this task in a thread-safe manner, which is particularly useful in multithreaded applications.

Syntax

int gethostbyname2_r(const char *name,
                     int af,
                     struct hostent *result_buf,
                     char *buf,
                     size_t buflen,
                     struct hostent **result,
                     int *h_errnop);

Options/Flags

  • name: The hostname to resolve.
  • af: The address family to resolve the hostname for. Valid values are AF_INET for IPv4 and AF_INET6 for IPv6.
  • result_buf: A pointer to a user-allocated buffer where the results of the resolution will be stored.
  • buf: A pointer to a user-allocated buffer for intermediate storage.
  • buflen: The size of the buf buffer.
  • result: A pointer to a variable that will receive a pointer to the hostent structure containing the resolution results.
  • h_errnop: A pointer to an integer variable that will receive an error code if the resolution fails.

Examples

Resolving an IPv4 hostname:

#include <netdb.h>
#include <stdio.h>

int main() {
    struct hostent *result;
    int h_errno;
    char buf[4096];
    result = gethostbyname2_r("example.com", AF_INET, NULL, buf, sizeof(buf), &result, &h_errno);
    if (result) {
        printf("IPv4 address: %s\n", result->h_addr_list[0]);
    } else {
        printf("Error resolving hostname: %s\n", hstrerror(h_errno));
    }
    return 0;
}

Resolving an IPv6 hostname:

#include <netdb.h>
#include <stdio.h>

int main() {
    struct hostent *result;
    int h_errno;
    char buf[4096];
    result = gethostbyname2_r("example.com", AF_INET6, NULL, buf, sizeof(buf), &result, &h_errno);
    if (result) {
        printf("IPv6 address: %s\n", result->h_addr_list[0]);
    } else {
        printf("Error resolving hostname: %s\n", hstrerror(h_errno));
    }
    return 0;
}

Common Issues

  • h_errno is set to HOST_NOT_FOUND: This indicates that the specified hostname does not exist.
  • h_errno is set to TRY_AGAIN: This indicates that the DNS server is temporarily unavailable. Retry the request later.
  • h_errno is set to NO_RECOVERY: This indicates that the DNS lookup failed and cannot be recovered.

Integration

gethostbyname2_r can be used in conjunction with other networking-related commands and functions, such as getaddrinfo() and socket(), to establish network connections and resolve hostnames programmatically.

Related Commands

  • gethostbyname(): Resolves a hostname to its corresponding IPv4 address.
  • gethostbyname2(): Resolves a hostname to its corresponding IPv4 or IPv6 address.
  • getaddrinfo(): Resolves a hostname or service name to a list of corresponding IP addresses and port numbers.