gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent, h_errno,
herror, hstrerror, gethostbyaddr_r, gethostbyname2, gethostbyname2_r,
gethostbyname_r, gethostent_r - get network host entry
#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():
- Since glibc 2.19:
- _DEFAULT_SOURCE
- Glibc versions up to and including 2.19:
- _BSD_SOURCE || _SVID_SOURCE
herror(),
hstrerror():
- Since glibc 2.19:
- _DEFAULT_SOURCE
- Glibc 2.8 to 2.19:
- _BSD_SOURCE || _SVID_SOURCE
- Before glibc 2.8:
- none
h_errno:
- Since glibc 2.19
- _DEFAULT_SOURCE || _POSIX_C_SOURCE < 200809L
- Glibc 2.12 to 2.19:
- _BSD_SOURCE || _SVID_SOURCE || _POSIX_C_SOURCE < 200809L
- Before glibc 2.12:
- none
The
gethostbyname*(),
gethostbyaddr*(),
herror(), and
hstrerror() functions are obsolete. Applications should use
getaddrinfo(3),
getnameinfo(3), and
gai_strerror(3)
instead.
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)). If
name is an IPv4 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() rely on the Name Service Switch
(
nsswitch.conf(5)) configured sources or a local name server
(
named(8)). The default action is to query the Name Service Switch
(
nsswitch.conf(5)) configured sources, failing that, a local name
server (
named(8)).
The
nsswitch.conf(5) file is the modern way of controlling the order of
host lookups.
In glibc 2.4 and earlier, the
order keyword was used to control the order
of host lookups as defined in
/etc/host.conf (
host.conf(5)).
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.
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_DATA
- The requested name is valid but does not have an IP address. Another type
of request to the name server for this domain may return an answer. The
constant NO_ADDRESS is a synonym for NO_DATA.
- NO_RECOVERY
- A nonrecoverable 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
- /etc/nsswitch.conf
- name service switch configuration
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
gethostbyname () |
Thread safety |
MT-Unsafe race:hostbyname env locale |
gethostbyaddr () |
Thread safety |
MT-Unsafe race:hostbyaddr env locale |
sethostent (), endhostent (), gethostent_r () |
Thread safety |
MT-Unsafe race:hostent env locale |
herror (), hstrerror () |
Thread safety |
MT-Safe |
gethostent () |
Thread safety |
MT-Unsafe race:hostent race:hostentbuf env locale |
gethostbyname2 () |
Thread safety |
MT-Unsafe race:hostbyname2 env locale |
gethostbyaddr_r (), gethostbyname_r (), gethostbyname2_r () |
Thread safety |
MT-Safe env locale |
In the above table,
hostent in
race:hostent signifies that if any
of the functions
sethostent(),
gethostent(),
gethostent_r(), or
endhostent() are used in parallel in
different threads of a program, then data races could occur.
POSIX.1-2001 specifies
gethostbyname(),
gethostbyaddr(),
sethostent(),
endhostent(),
gethostent(), and
h_errno;
gethostbyname(),
gethostbyaddr(), and
h_errno are marked obsolescent in that standard. POSIX.1-2008 removes
the specifications of
gethostbyname(),
gethostbyaddr(), and
h_errno, recommending the use of
getaddrinfo(3) and
getnameinfo(3) instead.
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.
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.
POSIX requires the
gethostent() call, which 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
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 nonzero
error number on failure. In addition to the errors returned by the
nonreentrant 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.
gethostbyname() does not recognize components of a dotted IPv4 address
string that are expressed in hexadecimal.
getaddrinfo(3),
getnameinfo(3),
inet(3),
inet_ntop(3),
inet_pton(3),
resolver(3),
hosts(5),
nsswitch.conf(5),
hostname(7),
named(8)