From Linux Man Pages
gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent, herror, hstrerror - get network host entry
SYNOPSIS
#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, int 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 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);
DESCRIPTION
The gethostbyname() function returns a structure of type hostent for the given host name. Here name is either a
host name, or an IPv4 address in standard dot notation, 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 vari-
able HOSTALIASES is set, the alias file pointed to by HOSTALIASES will first be searched for name (see host-
name(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()) 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 mes-
sage 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), depend-
ing 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:
h_name The official name of the host.
h_aliases
An array of alternative names for the host, terminated by a NULL pointer.
h_addrtype
The type of address; always AF_INET or AF_INET6 at present.
h_length
The length of the address in bytes.
h_addr_list
An array of pointers to network addresses for the host (in network byte order), terminated by a NULL
pointer.
h_addr The first address in h_addr_list for backward compatibility.
RETURN VALUE
The gethostbyname() and gethostbyaddr() functions return the hostent structure or a NULL pointer if an error
occurs. On error, the h_errno variable holds an error number. When non-NULL, the return value may point at
static data, see the notes below.
ERRORS
The variable h_errno can have the following values:
HOST_NOT_FOUND
The specified host is unknown.
NO_ADDRESS or NO_DATA
The requested name is valid but does not have an IP address.
NO_RECOVERY
A non-recoverable name server error occurred.
TRY_AGAIN
A temporary error occurred on an authoritative name server. Try again later.
FILES
/etc/host.conf
resolver configuration file
/etc/hosts
host database file
/etc/nsswitch.conf
name service switch configuration
CONFORMING TO
4.3BSD, POSIX.1-2001.
SYSTEM V/POSIX EXTENSION
POSIX requires the gethostent() call, that should return the next entry in the host data base. When using
DNS/BIND this does not make much sense, but it may be reasonable if the host data base is a file that can be read
line by line. On many systems a routine of this name reads from the file /etc/hosts. It may be available only
when the library was built without DNS support. The glibc version will ignore ipv6 entries. This function is not
reentrant, and glibc adds a reentrant version gethostent_r().
GNU EXTENSIONS
Glibc2 also has a gethostbyname2() that works like gethostbyname(), but permits to specify the address family to
which the address must belong.
Glibc2 also has reentrant versions gethostbyname_r() and gethostbyname2_r(). These return 0 on success and non-
zero on error. The result of the call is now stored in the struct with address ret. After the call, *result will
be NULL on error or point to the result on success. Auxiliary data is stored in the buffer buf of length buflen.
(If the buffer is too small, these functions will return ERANGE.) No global variable h_errno is modified, but
the address of a variable in which to store error numbers is passed in h_errnop.
NOTES
The functions gethostbyname() and gethostbyaddr() may return pointers to static data, which may be overwritten by
later calls. Copying the struct hostent does not suffice, since it contains pointers; a deep copy is required.
The SUS-v2 standard is buggy and declares the len parameter 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.)
The BSD prototype for gethostbyaddr() uses const char * for the first argument.
POSIX.1-2001 marks gethostbyaddr() and gethostbyname() obsolescent. See getaddrinfo(3), getnameinfo(3), gai_strerror(3),
.
RELATED
getaddrinfo(3), getipnodebyaddr(3), getipnodebyname(3), getnameinfo(3), inet_ntop(3), inet_pton(3), resolver(3),
hosts(5), nsswitch.conf(5), hostname(7), named(8)
CATEGORY