GETHOSTBYNAME(3NSL) Networking Services Library Functions GETHOSTBYNAME(3NSL)


gethostbyname, gethostbyname_r, gethostbyaddr, gethostbyaddr_r,
gethostent, gethostent_r, sethostent, endhostent - get network host entry


cc [ flag... ] file... -lnsl [ library... ]
#include <netdb.h>

struct hostent *gethostbyname(const char *name);

struct hostent *gethostbyname_r(const char *name,
struct hostent *result, char *buffer, int buflen,
int *h_errnop);

struct hostent *gethostbyaddr(const char *addr, int len,
int type);

struct hostent *gethostbyaddr_r(const char *addr, int length,
int type, struct hostent *result, char *buffer,
int buflen, int *h_errnop);

struct hostent *gethostent(void);

struct hostent *gethostent_r(struct hostent *result,
char *buffer, int buflen, int *h_errnop);

int sethostent(int stayopen);

int endhostent(void);


These functions are used to obtain entries describing hosts. An entry can
come from any of the sources for hosts specified in the
/etc/nsswitch.conf file. See nsswitch.conf(4). These functions have been
superseded by getipnodebyname(3SOCKET), getipnodebyaddr(3SOCKET), and
getaddrinfo(3SOCKET), which provide greater portability to applications
when multithreading is performed or technologies such as IPv6 are used.
For example, the functions described in the following cannot be used with
applications targeted to work with IPv6.

The gethostbyname() function searches for information for a host with the
hostname specified by the character-string parameter name.

The gethostbyaddr() function searches for information for a host with a
given host address. The parameter type specifies the family of the
address. This should be one of the address families defined in
<sys/socket.h>. See the NOTES section for more information. Also see the
EXAMPLES section for information on how to convert an Internet IP address
notation that is separated by periods (.) into an addr parameter. The
parameter len specifies the length of the buffer indicated by addr.

All addresses are returned in network order. In order to interpret the
addresses, byteorder(3SOCKET) must be used for byte order conversion.

The sethostent(), gethostent(), and endhostent() functions are used to
enumerate host entries from the database.

The sethostent() function sets or resets the enumeration to the beginning
of the set of host entries. This function should be called before the
first call to gethostent(). Calls to gethostbyname() and gethostbyaddr()
leave the enumeration position in an indeterminate state. If the
stayopen flag is non-zero, the system can keep allocated resources such
as open file descriptors until a subsequent call to endhostent().

Successive calls to the gethostent() function return either successive
entries or NULL, indicating the end of the enumeration.

The endhostent() function can be called to indicate that the caller
expects to do no further host entry retrieval operations; the system can
then deallocate resources it was using. It is still allowed, but possibly
less efficient, for the process to call more host retrieval functions
after calling endhostent().

Reentrant Interfaces

The gethostbyname(), gethostbyaddr(), and gethostent() functions use
static storage that is reused in each call, making these functions unsafe
for use in multithreaded applications.

The gethostbyname_r(), gethostbyaddr_r(), and gethostent_r() functions
provide reentrant interfaces for these operations.

Each reentrant interface performs the same operation as its non-reentrant
counterpart, named by removing the _r suffix. The reentrant interfaces,
however, use buffers supplied by the caller to store returned results and
the interfaces are safe for use in both single-threaded and multithreaded

Each reentrant interface takes the same parameters as its non-reentrant
counterpart, as well as the following additional parameters. The
parameter result must be a pointer to a struct hostent structure
allocated by the caller. On successful completion, the function returns
the host entry in this structure. The parameter buffer must be a pointer
to a buffer supplied by the caller. This buffer is used as storage space
for the host data. All of the pointers within the returned struct
hostent result point to data stored within this buffer. See the RETURN
VALUES section for more information. The buffer must be large enough to
hold all of the data associated with the host entry. The parameter buflen
should give the size in bytes of the buffer indicated by buffer. The
parameter h_errnop should be a pointer to an integer. An integer error
status value is stored there on certain error conditions. See the ERRORS
section for more information.

For enumeration in multithreaded applications, the position within the
enumeration is a process-wide property shared by all threads. The
sethostent() function can be used in a multithreaded application but
resets the enumeration position for all threads. If multiple threads
interleave calls to gethostent_r(), the threads will enumerate disjoint
subsets of the host database.

Like their non-reentrant counterparts, gethostbyname_r() and
gethostbyaddr_r() leave the enumeration position in an indeterminate


Host entries are represented by the struct hostent structure defined in

struct hostent {
char *h_name; /* canonical 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 */

See the EXAMPLES section for information about how to retrieve a ``.''
separated Internet IP address string from the h_addr_list field of struct

The gethostbyname(), gethostbyname_r(), gethostbyaddr(), and
gethostbyaddr_r() functions each return a pointer to a struct hostent if
they successfully locate the requested entry; otherwise they return NULL.

The gethostent() and gethostent_r() functions each return a pointer to a
struct hostent if they successfully enumerate an entry; otherwise they
return NULL, indicating the end of the enumeration.

The gethostbyname(), gethostbyaddr(), and gethostent() functions use
static storage, so returned data must be copied before a subsequent call
to any of these functions if the data is to be saved.

When the pointer returned by the reentrant functions gethostbyname_r(),
gethostbyaddr_r(), and gethostent_r() is not NULL, it is always equal to
the result pointer that was supplied by the caller.

The sethostent() and endhostent() functions return 0 on success.


The reentrant functions gethostbyname_r(), gethostbyaddr_r(), and
gethostent_r() will return NULL and set errno to ERANGE if the length of
the buffer supplied by caller is not large enough to store the result.
See Intro(2) for the proper usage and interpretation of errno in
multithreaded applications.

The reentrant functions gethostbyname_r() and gethostbyaddr_r() set the
integer pointed to by h_errnop to one of these values in case of error.

On failures, the non-reentrant functions gethostbyname() and
gethostbyaddr() set a global integer h_errno to indicate one of these
error codes (defined in <netdb.h>): HOST_NOT_FOUND, TRY_AGAIN,

If a resolver is provided with a malformed address, or if any other error
occurs before gethostbyname() is resolved, then gethostbyname() returns
an internal error with a value of -1.

The gethostbyname() function will set h_errno to NETDB_INTERNAL when it
returns a NULL value.


Example 1: Using gethostbyaddr()

Here is a sample program that gets the canonical name, aliases, and ``.''
separated Internet IP addresses for a given ``.'' separated IP address:

#include <stdio.h>
#include <stdlib.h
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
int main(int argc, const char **argv)
in_addr_t addr;
struct hostent *hp;
char **p;
if (argc != 2) {
(void) printf("usage: %s IP-address\n", argv[0]);
exit (1);
if ((int)(addr = inet_addr(argv[1])) == -1) {
(void) printf("IP-address must be of the form a.b.c.d\n");
exit (2);
hp = gethostbyaddr((char *)&addr, 4, AF_INET);
if (hp == NULL) {
(void) printf("host information for %s not found\n", argv[1]);
exit (3);
for (p = hp->h_addr_list; *p != 0; p++) {
struct in_addr in;
char **q;
(void) memcpy(&in.s_addr, *p, sizeof (in.s_addr));
(void) printf("%s\t%s", inet_ntoa(in), hp->h_name);
for (q = hp->h_aliases; *q != 0; q++)
(void) printf(" %s", *q);
(void) putchar('\n');
exit (0);

Note that the preceding sample program is unsafe for use in multithreaded


hosts file that associates the names of hosts with
their Internet Protocol (IP) addresses

network configuration database

configuration file for the name service switch


See attributes(5) for descriptions of the following attributes:

|MT-Level | See Reentrant Interfaces in the DESCRIPTION section. |


Intro(2), Intro(3), byteorder(3SOCKET), inet(3SOCKET), netdb.h(3HEAD),
netdir(3NSL), hosts(4), netconfig(4), nss(4), nsswitch.conf(4),


The reentrant interfaces gethostbyname_r(), gethostbyaddr_r(), and
gethostent_r() are included in this release on an uncommitted basis only
and are subject to change or removal in future minor releases.


To ensure that they all return consistent results, gethostbyname(),
gethostbyname_r(), and netdir_getbyname() are implemented in terms of the
same internal library function. This function obtains the system-wide
source lookup policy based on the inet family entries in netconfig(4) and
the hosts: entry in nsswitch.conf(4). Similarly, gethostbyaddr(),
gethostbyaddr_r(), and netdir_getbyaddr() are implemented in terms of the
same internal library function. If the inet family entries in
netconfig(4) have a ``-'' in the last column for nametoaddr libraries,
then the entry for hosts in nsswitch.conf will be used; nametoaddr
libraries in that column will be used, and nsswitch.conf will not be

There is no analogue of gethostent() and gethostent_r() in the netdir
functions, so these enumeration functions go straight to the hosts entry
in nsswitch.conf. Thus enumeration can return results from a different
source than that used by gethostbyname(), gethostbyname_r(),
gethostbyaddr(), and gethostbyaddr_r().

All the functions that return a struct hostent must always return the
canonical name in the h_name field. This name, by definition, is the
well-known and official hostname shared between all aliases and all
addresses. The underlying source that satisfies the request determines
the mapping of the input name or address into the set of names and
addresses in hostent. Different sources might do that in different ways.
If there is more than one alias and more than one address in hostent, no
pairing is implied between them.

The system attempts to put those addresses that are on the same subnet as
the caller before addresses that are on different subnets. However, if
address sorting is disabled by setting SORT_ADDRS to FALSE in the
/etc/default/nss file, the system does not put the local subnet addresses
first. See nss(4) for more information.

When compiling multithreaded applications, see Intro(3), MULTITHREADED
APPLICATIONS, for information about the use of the _REENTRANT flag.

Use of the enumeration interfaces gethostent() and gethostent_r() is
discouraged; enumeration might not be supported for all database sources.
The semantics of enumeration are discussed further in nsswitch.conf(4).

The current implementations of these functions only return or accept
addresses for the Internet address family (type AF_INET).

The form for an address of type AF_INET is a struct in_addr defined in
<netinet/in.h>. The functions described in inet(3SOCKET), and illustrated
in the EXAMPLES section, are helpful in constructing and manipulating
addresses in this form.

September 10, 2013 GETHOSTBYNAME(3NSL)