gethostbyname, gethostbyaddr, gethostent, sethostent, endhostent, herror - get network host entry
#include <netdb.h>
extern int h_errno;
struct hostent *
gethostbyname(char *name);
struct hostent *
gethostbyname2(char *name, int af);
struct hostent *
gethostbyaddr(char *addr, int len, type);
struct hostent *
gethostent();
sethostent(int stayopen);
endhostent();
herror(char *string);
Gethostbyname(), gethostbyname2(), and gethostbyaddr() each return a pointer to a hostent structure (see below) describing an internet host referenced by name or by address, as the function names indicate. This structure contains either the information obtained from the name server, named(8) , or broken-out fields from a line in /etc/hosts. If the local name server is not running, these routines do a lookup in /etc/hosts.
struct hostent {
char *h_name; /* official name of host */
#define h_addr h_addr_list[0] /* address, for backward compatibility */
The members of this structure are:
h_addr_list A zero-terminated array of network addresses for the host. Host addresses are returned in network byte order.
When using the nameserver, gethostbyname() will search for the named host in each parent domain given in the ``search'' directive of resolv.conf(5) unless the name contains a dot (``.''). If the name contains no dot, and if the environment variable HOSTALIASES contains the name of an alias file, the alias file will first be searched for an alias matching the input name. See hostname(7) for the domain search procedure and the alias file format.
Gethostbyname2() is an evolution of gethostbyname() intended to allow lookups in address families other than AF_INET, for example, AF_INET6. Currently, the af argument must be specified as AF_INET else the function will return NULL after having set h_errno to NETDB_INTERNAL.
Sethostent() may be used to request the use of a connected TCP socket for queries. If the stayopen flag is non-zero, this sets the option to send all queries to the name server using TCP and to retain the connection after each call to gethostbyname() or gethostbyaddr(). Otherwise, queries are performed using UDP datagrams.
Endhostent() closes the TCP connection.
Error return status from gethostbyname() and gethostbyaddr() is indicated by return of a null pointer. The external integer h_errno may then be checked to see whether this is a temporary failure or an invalid or unknown host. The routine herror() can be used to print an error message describing the failure. If its argument string is non-NULL, it is printed, followed by a colon and a space. The error message is printed with a trailing newline.
h_errno can have the following values:
hosts(5) , hostname(7) , named(8) , resolver(3) , resolver(5) .
Gethostent() is defined, and sethostent() and endhostent() are redefined, when libc is built to use only the routines to lookup in /etc/hosts and not the name server:
Gethostent() reads the next line of /etc/hosts, opening the file if necessary.
Sethostent() is redefined to open and rewind the file. If the stayopen argument is non-zero, the hosts data base will not be closed after each call to gethostbyname() or gethostbyaddr().
Endhostent() is redefined to close the file.
All information is contained in a static area so it must be copied if it is to be saved. Only the Internet address format is currently understood.