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Technical Reference: Communications, Volume 2

res_init Subroutine


Searches for a default domain name and Internet address.


Standard C Library (libc.a)


#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
void res_init ( )


The res_init subroutine reads the /etc/resolv.conf file for the default domain name and the Internet address of the initial hosts running the name server.

Note: If the /etc/resolv.conf file does not exist, the res_init subroutine attempts name resolution using the local /etc/hosts file. If the system is not using a domain name server, the /etc/resolv.conf file should not exist. The /etc/hosts file should be present on the system even if the system is using a name server. In this instance, the file should contain the host IDs that the system requires to function even if the name server is not functioning.

The res_init subroutine is one of a set of subroutines that form the resolver, a set of functions that translate domain names to Internet addresses. All resolver subroutines use the /usr/include/resolv.h file, which defines the _res structure. The res_init subroutine stores domain name information in the _res structure. Three environment variables, LOCALDOMAIN, RES_TIMEOUT, and RES_RETRY, affect default values related to the _res structure.

For more information on the _res structure, see "Understanding Domain Name Resolution" in AIX 5L Version 5.1 Communications Programming Concepts.

Implementation Specifics

The res_init subroutine is part of Base Operating System (BOS) Runtime.

All applications containing the res_init subroutine must be compiled with _BSD set to a specific value. Acceptable values are 43 and 44. In addition, all socket applications must include the BSD libbsd.a library.


                          Contains the name server and domain name.
                          Contains host names and their addresses for hosts in a network. This file is used to resolve a host name into an Internet address.

Related Information

The dn_comp subroutine, dn_expand subroutine, _getlong subroutine, _getshort subroutine, putlong subroutine, putshort subroutine, res_mkquery subroutine, res_query subroutine, res_search subroutine, res_send subroutine.

Sockets Overview and Understanding Domain Name Resolution in AIX 5L Version 5.1 Communications Programming Concepts.

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