#include <netdb.h>
extern int h_errno;
struct hostent *gethostbyname(const char *name);
#include <sys/socket.h> /* for AF_INET */
struct hostent *gethostbyaddr(const void *addr,
socklen_t len, int type);
void sethostent(int stayopen);
void endhostent(void);
void herror(const char *s);
const char *hstrerror(int err);
/* System V/POSIX extension */
struct hostent *gethostent(void);
/* GNU extensions */
struct hostent *gethostbyname2(const char *name, int af);
int gethostent_r(
struct hostent *ret, char *buf, size_t buflen,
struct hostent **result, int *h_errnop);
int gethostbyaddr_r(const void *addr, socklen_t len, int type,
struct hostent *ret, char *buf, size_t buflen,
struct hostent **result, int *h_errnop);
int gethostbyname_r(const char *name,
struct hostent *ret, char *buf, size_t buflen,
struct hostent **result, int *h_errnop);
int gethostbyname2_r(const char *name, int af,
struct hostent *ret, char *buf, size_t buflen,
struct hostent **result, int *h_errnop);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
gethostbyname2(), gethostent_r(), gethostbyaddr_r(), gethostbyname_r(), gethostbyname2_r(): _BSD_SOURCE || _SVID_SOURCE
The gethostbyname() function returns a structure of type hostent for the given host name. Here name is either a hostname, or an IPv4 address in standard dot notation (as for inet_addr(3)), or an IPv6 address in colon (and possibly dot) notation. (See RFC 1884 for the description of IPv6 addresses.) If name is an IPv4 or IPv6 address, no lookup is performed and gethostbyname() simply copies name into the h_name field and its struct in_addr equivalent into the h_addr_list[0] field of the returned hostent structure. If name doesn't end in a dot and the environment variable HOSTALIASES is set, the alias file pointed to by HOSTALIASES will first be searched for name (see hostname(7) for the file format). The current domain and its parents are searched unless name ends in a dot.
The gethostbyaddr() function returns a structure of type hostent for the given host address addr of length len and address type type. Valid address types are AF_INET and AF_INET6. The host address argument is a pointer to a struct of a type depending on the address type, for example a struct in_addr * (probably obtained via a call to inet_addr(3)) for address type AF_INET.
The sethostent() function specifies, if stayopen is true (1), that a connected TCP socket should be used for the name server queries and that the connection should remain open during successive queries. Otherwise, name server queries will use UDP datagrams.
The endhostent() function ends the use of a TCP connection for name server queries.
The (obsolete) herror() function prints the error message associated with the current value of h_errno on stderr.
The (obsolete) hstrerror() function takes an error number (typically h_errno) and returns the corresponding message string.
The domain name queries carried out by gethostbyname() and gethostbyaddr() use a combination of any or all of the name server named(8), a broken out line from /etc/hosts, and the Network Information Service (NIS or YP), depending upon the contents of the order line in /etc/host.conf. The default action is to query named(8), followed by /etc/hosts.
The hostent structure is defined in <netdb.h> as follows:
struct hostent {
char *h_name; /* official name of host */
char **h_aliases; /* alias list */
int h_addrtype; /* host address type */
int h_length; /* length of address */
char **h_addr_list; /* list of addresses */
}
#define h_addr h_addr_list[0] /* for backward compatibility */
The members of the hostent structure are:
In the original BSD implementation the len argument of gethostbyname() was an int. The SUSv2 standard is buggy and declares the len argument of gethostbyaddr() to be of type size_t. (That is wrong, because it has to be int, and size_t is not. POSIX.1-2001 makes it socklen_t, which is OK.) See also accept(2).
The BSD prototype for gethostbyaddr() uses const char * for the first argument.
Glibc2 also has reentrant versions gethostent_r(), gethostbyaddr_r(), gethostbyname_r() and gethostbyname2_r(). The caller supplies a hostent structure ret which will be filled in on success, and a temporary work buffer buf of size buflen. After the call, result will point to the result on success. In case of an error or if no entry is found result will be NULL. The functions return 0 on success and a non-zero error number on failure. In addition to the errors returned by the non-reentrant versions of these functions, if buf is too small, the functions will return ERANGE, and the call should be retried with a larger buffer. The global variable h_errno is not modified, but the address of a variable in which to store error numbers is passed in h_errnop.