g_h_b_n.3

操作系统设计与实现源码

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.\"	@(#)gethostbyname.3	6.12 (Berkeley) 6/23/90
.\"
.TH GETHOSTBYNAME 3 "June 23, 1990"
.UC 5
.SH NAME
g_h_b_n, gethostbyname, gethostbyaddr, gethostent, sethostent, endhostent, herror \- get network host entry
.SH SYNOPSIS
.B "#include <net/gen/netdb.h>
.PP
.B "extern int h_errno;
.PP
.B "struct hostent *gethostbyname(name)
.br
.B "char *name;
.PP
.B "struct hostent *gethostbyaddr(addr, len, type)
.br
.B "char *addr; int len, type;
.PP
.B "struct hostent *gethostent()
.PP
.B "sethostent(stayopen)
.br
.B "int stayopen;
.PP
.B "endhostent()
.PP
.B "herror(string)
.br
.B "char *string;
.PP
.SH DESCRIPTION
.I Gethostbyname
and
.I gethostbyaddr
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 the information obtained from the name server,
.IR named (8),
or broken-out fields from a line in 
.IR /etc/hosts .
If the local name server is not running these routines do a lookup in
.IR /etc/hosts .
.RS
.PP
.nf
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 */
.ft R
.ad
.fi
.RE
.PP
The members of this structure are:
.TP \w'h_addr_list'u+2n
h_name
Official name of the host.
.TP \w'h_addr_list'u+2n
h_aliases
A zero terminated array of alternate names for the host.
.TP \w'h_addr_list'u+2n
h_addrtype
The type of address being returned; currently always AF_INET.
.TP \w'h_addr_list'u+2n
h_length
The length, in bytes, of the address.
.TP \w'h_addr_list'u+2n
h_addr_list
A zero terminated array of network addresses for the host.
Host addresses are returned in network byte order.
.TP \w'h_addr_list'u+2n
h_addr
The first address in h_addr_list; this is for backward compatiblity.
.PP
When using the nameserver,
.I gethostbyname
will search for the named host in the current domain and its parents
unless the name ends in a dot.
If the name contains no dot, and if the environment variable ``HOSTALAIASES''
contains the name of an alias file, the alias file will first be searched
for an alias matching the input name.
See
.IR hostname (7)
for the domain search procedure and the alias file format.
.PP
.I Sethostent
may be used to request the use of a connected TCP socket for queries.
If the
.I stayopen
flag is non-zero,
this sets the option to send all queries to the name server using TCP
and to retain the connection after each call to 
.I gethostbyname
or
.IR gethostbyaddr .
Otherwise, queries are performed using UDP datagrams.
.PP
.I Endhostent
closes the TCP connection.
.SH DIAGNOSTICS
.PP
Error return status from 
.I gethostbyname
and
.I gethostbyaddr
is indicated by return of a null pointer.
The external integer
.IR h_errno
may then be checked to see whether this is a temporary failure
or an invalid or unknown host.
The routine
.I herror
can be used to print an error message describing the failure.
If its argument
.I string
is non-NULL, it is printed, followed by a colon and a space.
The error message is printed with a trailing newline.
.PP
.IR h_errno
can have the following values:
.RS
.IP HOST_NOT_FOUND \w'HOST_NOT_FOUND'u+2n
No such host is known.
.IP TRY_AGAIN \w'HOST_NOT_FOUND'u+2n
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.
.IP NO_RECOVERY \w'HOST_NOT_FOUND'u+2n
Some unexpected server failure was encountered.
This is a non-recoverable error.
.IP NO_DATA \w'HOST_NOT_FOUND'u+2n
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.
.RE
.SH FILES
/etc/hosts
.SH "SEE ALSO"
resolver(3), hosts(5), hostname(7), named(8)
.SH CAVEAT
.PP
.I Gethostent
is defined, and
.I sethostent
and
.I endhostent
are redefined,
when
.IR libc
is built to use only the routines to lookup in
.IR /etc/hosts 
and not the name server.
.PP
.I Gethostent
reads the next line of
.IR /etc/hosts ,
opening the file if necessary.
.PP
.I Sethostent 
is redefined to open and rewind the file.  If the
.I stayopen
argument is non-zero,
the hosts data base will not be closed after each call to
.I gethostbyname
or
.IR gethostbyaddr .
.I Endhostent
is redefined to close the file.
.SH BUGS
All information
is contained in a static area
so it must be copied if it is
to be saved.  Only the Internet
address format is currently understood.