gethostbyname.3

ecos实时嵌入式操作系统

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.Dd March 13, 1997
.Dt GETHOSTBYNAME 3
.Os
.Sh NAME
.Nm gethostbyname ,
.Nm gethostbyname2 ,
.Nm gethostbyaddr ,
.Nm gethostent ,
.Nm sethostent ,
.Nm endhostent ,
.Nm hstrerror ,
.Nm herror
.Nd get network host entry
.Sh SYNOPSIS
.Fd #include <netdb.h>
.Fd extern int h_errno;
.Ft struct hostent *
.Fn gethostbyname "const char *name"
.Ft struct hostent *
.Fn gethostbyname2 "const char *name" "int af"
.Ft struct hostent *
.Fn gethostbyaddr "const char *addr" "int len" "int af"
.Ft struct hostent *
.Fn gethostent void
.Ft void
.Fn sethostent "int stayopen"
.Ft void
.Fn endhostent void
.Ft void
.Fn herror "const char *string"
.Ft const char *
.Fn hstrerror "int err"
.Sh DESCRIPTION
The
.Fn gethostbyname
and
.Fn gethostbyaddr
functions each return a pointer to an object with the following structure
describing an internet host referenced by name or by address, respectively.
This structure contains either information obtained from the name server (i.e.,
.Xr resolver 3
and
.Xr named 8 ) ,
broken-out fields from a line in
.Pa /etc/hosts ,
or database entries supplied by the
.Xr yp 8
system.
.Xr resolv.conf 5
describes how the particular database is chosen.
.Bd -literal
struct	hostent {
	char	*h_name;	/* official 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 from name server */
};
#define	h_addr  h_addr_list[0]	/* address, for backward compatibility */
.Ed
.Pp
The members of this structure are:
.Bl -tag -width h_addr_list
.It Fa h_name
Official name of the host.
.It Fa h_aliases
A zero-terminated array of alternate names for the host.
.It Fa h_addrtype
The type of address being returned.
.It Fa h_length
The length, in bytes, of the address.
.It Fa h_addr_list
A zero-terminated array of network addresses for the host.
Host addresses are returned in network byte order.
.It Fa h_addr
The first address in
.Fa h_addr_list ;
this is for backward compatibility.
.El
.Pp
The function
.Fn gethostbyname
will search for the named host in the current domain and its parents
using the search lookup semantics detailed in
.Xr resolv.conf 5
and
.Xr hostname 7 .
.Pp
.Fn gethostbyname2
is an advanced form of
.Fn gethostbyname
which allows lookups in address families other than
.Dv AF_INET ,
for example
.Dv AF_INET6 .
.Pp
The
.Fn gethostbyaddr
function will search for the specified address of length
.Fa len
in the address family
.Fa af .
The only address family currently supported is
.Dv AF_INET .
.Pp
The
.Fn sethostent
function may be used to request the use of a connected
.Tn TCP
socket for queries.
If the
.Fa stayopen
flag is non-zero,
this sets the option to send all queries to the name server using
.Tn TCP
and to retain the connection after each call to
.Fn gethostbyname
or
.Fn gethostbyaddr .
Otherwise, queries are performed using
.Tn UDP
datagrams.
.Pp
The
.Fn endhostent
function closes the
.Tn TCP
connection.
.Pp
The
.Fn herror
function prints an error message describing the failure.
If its argument
.Fa string
is non-null,
it is prepended to the message string and separated from it by a colon
.Pq Ql \&:
and a space.
The error message is printed with a trailing newline.
The contents of the error message is the same as that returned by
.Fn hstrerror
with argument
.Fa h_errno .
.Sh FILES
.Bl -tag -width /etc/resolv.conf -compact
.It Pa /etc/hosts
.It Pa /etc/resolv.conf
.El
.Sh DIAGNOSTICS
Error return status from
.Fn gethostbyname ,
.Fn gethostbyname2 ,
and
.Fn gethostbyaddr
is indicated by return of a null pointer.
The external integer
.Va h_errno
may then be checked to see whether this is a temporary failure
or an invalid or unknown host.
.Pp
The variable
.Va h_errno
can have the following values:
.Bl -tag -width HOST_NOT_FOUND
.It Dv HOST_NOT_FOUND
No such host is known.
.It Dv TRY_AGAIN
This is usually a temporary error
and means that the local server did not receive
a response from an authoritative server.
A retry at some later time may succeed.
.It Dv NO_RECOVERY
Some unexpected server failure was encountered.
This is a non-recoverable error.
.It Dv NO_DATA
The requested name is valid but does not have an IP address;
this is not a temporary error.
This means that the name is known to the name server but there is no address
associated with this name.
Another type of request to the name server using this domain name
will result in an answer;
for example, a mail-forwarder may be registered for this domain.
.El
.Sh SEE ALSO
.Xr resolver 3 ,
.Xr getaddrinfo 3 ,
.Xr getnameinfo 3 ,
.Xr hosts 5 ,
.Xr resolv.conf 5 ,
.Xr hostname 7 ,
.Xr named 8
.Sh CAVEAT
If the search routines in
.Xr resolv.conf 5
decide to read the
.Pa /etc/hosts
file,
.Fn gethostent
and other functions will
read the next line of the file,
re-opening the file if necessary.
.Pp
The
.Fn sethostent
function opens and/or rewinds the file
.Pa /etc/hosts .
If the
.Fa stayopen
argument is non-zero, the file will not be closed after each call to
.Fn gethostbyname ,
.Fn gethostbyname2 ,
or
.Fn gethostbyaddr .
.Pp
The
.Fn endhostent
function closes the file.
.Sh HISTORY
The
.Fn herror
function appeared in
.Bx 4.3 .
The
.Fn endhostent ,
.Fn gethostbyaddr ,
.Fn gethostbyname ,
.Fn gethostent ,
and
.Fn sethostent
functions appeared in
.Bx 4.2 .
.Sh BUGS
These functions use static data storage;
if the data is needed for future use, it should be
copied before any subsequent calls overwrite it.
Only the Internet
address formats are currently understood.
.Pp
YP does not support any address families other than
.Dv AF_INET
and uses
the traditional database format.