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.
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
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
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.
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.
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