Originální popis anglicky:
gethostbyname, gethostbyaddr, sethostent, gethostend, endhostent, herror,
hstrerror - get network host entry
Návod, kniha: Linux Programmer's Manual
#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);
/* SYSV/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);
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 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()) 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. (See
resolv+(8)). 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
- A zero-terminated array of alternative names for the
host.
- 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
- A zero-terminated array of network addresses for the host
in network byte order.
- h_addr
- The first address in h_addr_list for backward
compatibility.
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.
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.
- /etc/host.conf
- resolver configuration file
- /etc/hosts
- host database file
BSD 4.3.
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().
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 nonzero 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.
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 1003.1-2001 makes it
socklen_t, which is OK.)
The BSD prototype for
gethostbyaddr() uses
const char * for the
first argument.
POSIX 1003.1-2001 marks
gethostbyaddr() and
gethostbyname()
obsolescent. See
getaddrinfo(3),
getnameinfo(3),
gai_strerror(3).
getaddrinfo(3),
getipnodebyaddr(3),
getipnodebyname(3),
getnameinfo(3),
inet_ntop(3),
inet_pton(3),
resolver(3),
hosts(5),
hostname(7),
named(8),
resolv+(8)
