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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | FILES | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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gethostbyname(3) Library Functions Manual gethostbyname(3)
gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent,
h_errno, herror, hstrerror, gethostbyaddr_r, gethostbyname2,
gethostbyname2_r, gethostbyname_r, gethostent_r - get network host
entry
Standard C library (libc, -lc)
#include <netdb.h>
void sethostent(int stayopen);
void endhostent(void);
[[deprecated]] extern int h_errno;
[[deprecated]] struct hostent *gethostbyname(const char *name);
[[deprecated]] struct hostent *gethostbyaddr(socklen_t size;
const void addr[size],
socklen_t size, int type);
[[deprecated]] void herror(const char *s);
[[deprecated]] const char *hstrerror(int err);
/* System V/POSIX extension */
struct hostent *gethostent(void);
/* GNU extensions */
[[deprecated]]
struct hostent *gethostbyname2(const char *name, int af);
int gethostent_r(size_t bufsize;
struct hostent *restrict ret,
char buf[restrict bufsize], size_t bufsize,
struct hostent **restrict result,
int *restrict h_errnop);
[[deprecated]]
int gethostbyaddr_r(socklen_t size, size_t bufsize;
const void addr[restrict size], socklen_t size,
int type,
struct hostent *restrict ret,
char buf[restrict bufsize], size_t bufsize,
struct hostent **restrict result,
int *restrict h_errnop);
[[deprecated]]
int gethostbyname_r(size_t bufsize;
const char *restrict name,
struct hostent *restrict ret,
char buf[restrict bufsize], size_t bufsize,
struct hostent **restrict result,
int *restrict h_errnop);
[[deprecated]]
int gethostbyname2_r(size_t bufsize;
const char *restrict name, int af,
struct hostent *restrict ret,
char buf[restrict bufsize], size_t bufsize,
struct hostent **restrict result,
int *restrict 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 up to and including 2.19:
_BSD_SOURCE || _SVID_SOURCE
herror(), hstrerror():
Since glibc 2.19:
_DEFAULT_SOURCE
glibc 2.8 to glibc 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 glibc 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 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 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 size len and address type type.
Valid address types are AF_INET and AF_INET6 (defined in
<sys/socket.h>). 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 (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)).
Historical
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; /* size 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 size 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(), │ Thread safety │ MT-Unsafe race:hostent │
│ endhostent(), │ │ env locale │
│ gethostent_r() │ │ │
├────────────────────┼───────────────┼───────────────────────────┤
│ herror(), │ Thread safety │ MT-Safe │
│ hstrerror() │ │ │
├────────────────────┼───────────────┼───────────────────────────┤
│ gethostent() │ Thread safety │ MT-Unsafe race:hostent │
│ │ │ race:hostentbuf env │
│ │ │ locale │
├────────────────────┼───────────────┼───────────────────────────┤
│ gethostbyname2() │ Thread safety │ MT-Unsafe │
│ │ │ race:hostbyname2 env │
│ │ │ locale │
├────────────────────┼───────────────┼───────────────────────────┤
│ gethostbyaddr_r(), │ Thread safety │ MT-Safe env locale │
│ gethostbyname_r(), │ │ │
│ gethostbyname2_r() │ │ │
└────────────────────┴───────────────┴───────────────────────────┘
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.
sethostent()
endhostent()
gethostent()
POSIX.1-2008.
gethostent_r()
GNU.
Others:
None.
sethostent()
endhostent()
gethostent()
POSIX.1-2001.
gethostbyname()
gethostbyaddr()
h_errno
Marked obsolescent in POSIX.1-2001. Removed in
POSIX.1-2008, 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 size argument of
gethostbyname() was an int. The SUSv2 standard is buggy and
declares the size 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.
System V/POSIX extension
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().
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 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 bufsize. 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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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Linux man-pages 6.15 2025-06-28 gethostbyname(3)
Pages that refer to this page: getent(1), pmhostname(1), byteorder(3), getaddrinfo(3), gethostid(3), getipnodebyname(3), getnameinfo(3), inet(3), pmdatrace(3), pmgetarchivelabel(3), pmnewcontext(3), pmreconnectcontext(3), rcmd(3), resolver(3), rexec(3), setnetgrent(3), sockaddr(3type), host.conf(5), nsswitch.conf(5), resolv.conf(5), environ(7), hostname(7), ip(7), nscd(8), rpcctl(8), systemd-machined.service(8), systemd-resolved.service(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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