gethostbyname(3) - Linux manual page (original) (raw)
gethostbyname(3) Library Functions Manual gethostbyname(3)
NAME top
gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent,
h_errno, herror, hstrerror, gethostbyaddr_r, gethostbyname2,
gethostbyname2_r, gethostbyname_r, gethostent_r - get network host
entryLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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(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(struct hostent *restrict** _ret_**,**
**char** _buf_**[restrict .**_bufsize_**], size_t** _bufsize_**,**
**struct hostent restrict** _result_**,**
**int *restrict** _herrnop_**);**
**[[deprecated]]**
**int gethostbyaddr_r(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** _herrnop_**);**
**[[deprecated]]**
**int gethostbyname_r(const char *restrict** _name_**,**
**struct hostent *restrict** _ret_**,**
**char** _buf_**[restrict .**_bufsize_**], size_t** _bufsize_**,**
**struct hostent restrict** _result_**,**
**int *restrict** _herrnop_**);**
**[[deprecated]]**
**int gethostbyname2_r(const char *restrict** _name_**, int** _af,_
**struct hostent *restrict** _ret_**,**
**char** _buf_**[restrict .**_bufsize_**], size_t** _bufsize_**,**
**struct hostent restrict** _result_**,**
**int *restrict** _herrnop_**);**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:
noneDESCRIPTION top
The **gethostbyname***(), **gethostbyaddr***(), **herror**(), and **hstrerror**()
functions are obsolete. Applications should use [getaddrinfo(3)](../man3/getaddrinfo.3.html),
[getnameinfo(3)](../man3/getnameinfo.3.html), and [gai_strerror(3)](../man3/gai%5Fstrerror.3.html) 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)](../man3/inet%5Faddr.3.html)). If
_name_ is an IPv4 address, no lookup is performed and
**gethostbyname**() simply copies _name_ into the _hname_ field and its
_struct inaddr_ equivalent into the _haddrlist[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)](../man7/hostname.7.html)
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 inaddr *_ (probably obtained via a call to [inet_addr(3)](../man3/inet%5Faddr.3.html))
for address type **AF_INET**.
The (obsolete) **herror**() function prints the error message
associated with the current value of _herrno_ on _stderr_.
The (obsolete) **hstrerror**() function takes an error number
(typically _herrno_) 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)](../man5/nsswitch.conf.5.html))
configured sources or a local name server (**named**(8)). The default
action is to query the Name Service Switch ([nsswitch.conf(5)](../man5/nsswitch.conf.5.html))
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)](../man5/host.conf.5.html)).
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:
_hname_ The official name of the host.
_haliases_
An array of alternative names for the host, terminated by a
null pointer.
_haddrtype_
The type of address; always **AF_INET** or **AF_INET6** at present.
_hlength_
The size of the address in bytes.
_haddrlist_
An array of pointers to network addresses for the host (in
network byte order), terminated by a null pointer.
_haddr_ The first address in _haddrlist_ for backward
compatibility.RETURN VALUE top
The **gethostbyname**() and **gethostbyaddr**() functions return the
_hostent_ structure or a null pointer if an error occurs. On error,
the _herrno_ variable holds an error number. When non-NULL, the
return value may point at static data, see the notes below.ERRORS top
The variable _herrno_ 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.FILES top
_/etc/host.conf_
resolver configuration file
_/etc/hosts_
host database file
_/etc/nsswitch.conf_
name service switch configurationATTRIBUTES top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌────────────────────┬───────────────┬───────────────────────────┐
│ **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.STANDARDS top
**sethostent**()
**endhostent**()
**gethostent**()
POSIX.1-2008.
**gethostent_r**()
GNU.
Others:
None.HISTORY top
**sethostent**()
**endhostent**()
**gethostent**()
POSIX.1-2001.
**gethostbyname**()
**gethostbyaddr**()
_herrno_
Marked obsolescent in POSIX.1-2001. Removed in
POSIX.1-2008, recommending the use of [getaddrinfo(3)](../man3/getaddrinfo.3.html) and
[getnameinfo(3)](../man3/getnameinfo.3.html) instead.NOTES top
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
_sizet_. (That is wrong, because it has to be _int_, and _sizet_ is
not. POSIX.1-2001 makes it _socklent_, which is OK.) See also
[accept(2)](../man2/accept.2.html).
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 _herrno_ is not modified, but the
address of a variable in which to store error numbers is passed in
_herrnop_.BUGS top
**gethostbyname**() does not recognize components of a dotted IPv4
address string that are expressed in hexadecimal.SEE ALSO top
[getaddrinfo(3)](../man3/getaddrinfo.3.html), [getnameinfo(3)](../man3/getnameinfo.3.html), [inet(3)](../man3/inet.3.html), [inet_ntop(3)](../man3/inet%5Fntop.3.html),
[inet_pton(3)](../man3/inet%5Fpton.3.html), [resolver(3)](../man3/resolver.3.html), [hosts(5)](../man5/hosts.5.html), [nsswitch.conf(5)](../man5/nsswitch.conf.5.html),
[hostname(7)](../man7/hostname.7.html), **named**(8)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-12-24 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)