NAME
gethostbyaddr, gethostbyname - network host database functions
SYNOPSIS
#include <netdb.h>
struct hostent *gethostbyaddr(const void *addr, socklen_t len,
int type);
struct hostent *gethostbyname(const char *name);
DESCRIPTION
These functions shall retrieve information about hosts. This
information is considered to be stored in a database that can be
accessed sequentially or randomly. Implementation of this database is
unspecified.
Note: In many cases it is implemented by the Domain Name System, as
documented in RFC 1034, RFC 1035, and RFC 1886.
Entries shall be returned in hostent structures.
The gethostbyaddr() function shall return an entry containing addresses
of address family type for the host with address addr. The len argument
contains the length of the address pointed to by addr. The
gethostbyaddr() function need not be reentrant. A function that is not
required to be reentrant is not required to be thread-safe.
The gethostbyname() function shall return an entry containing addresses
of address family AF_INET for the host with name name. The
gethostbyname() function need not be reentrant. A function that is not
required to be reentrant is not required to be thread-safe.
The addr argument of gethostbyaddr() shall be an in_addr structure when
type is AF_INET. It contains a binary format (that is, not null-
terminated) address in network byte order. The gethostbyaddr() function
is not guaranteed to return addresses of address families other than
AF_INET, even when such addresses exist in the database.
If gethostbyaddr() returns successfully, then the h_addrtype field in
the result shall be the same as the type argument that was passed to
the function, and the h_addr_list field shall list a single address
that is a copy of the addr argument that was passed to the function.
The name argument of gethostbyname() shall be a node name; the behavior
of gethostbyname() when passed a numeric address string is unspecified.
For IPv4, a numeric address string shall be in the dotted-decimal
notation described in inet_addr() .
If name is not a numeric address string and is an alias for a valid
host name, then gethostbyname() shall return information about the host
name to which the alias refers, and name shall be included in the list
of aliases returned.
RETURN VALUE
Upon successful completion, these functions shall return a pointer to a
hostent structure if the requested entry was found, and a null pointer
if the end of the database was reached or the requested entry was not
found.
Upon unsuccessful completion, gethostbyaddr() and gethostbyname() shall
set h_errno to indicate the error.
ERRORS
These functions shall fail in the following cases. The gethostbyaddr()
and gethostbyname() functions shall set h_errno to the value shown in
the list below. Any changes to errno are unspecified.
HOST_NOT_FOUND
No such host is known.
NO_DATA
The server recognized the request and the name, but no address
is available. Another type of request to the name server for the
domain might return an answer.
NO_RECOVERY
An unexpected server failure occurred which cannot be recovered.
TRY_AGAIN
A temporary and possibly transient error occurred, such as a
failure of a server to respond.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
The gethostbyaddr() and gethostbyname() functions may return pointers
to static data, which may be overwritten by subsequent calls to any of
these functions.
The getaddrinfo() and getnameinfo() functions are preferred over the
gethostbyaddr() and gethostbyname() functions.
RATIONALE
None.
FUTURE DIRECTIONS
The gethostbyaddr() and gethostbyname() functions may be withdrawn in a
future version.
SEE ALSO
endhostent() , endservent() , gai_strerror() , getaddrinfo() ,
h_errno() , inet_addr() , the Base Definitions volume of
IEEE Std 1003.1-2001, <netdb.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .