2 .TH bres 3 "1 October 1999" "Straylight/Edgeware" "mLib utilities library"
4 bres \- background name resolver
12 .B "#include <mLib/bres.h>"
14 .B "typedef struct { ...\& } bres_client;"
16 .ta \w'\fBvoid bres_byname('u
17 .BI "void bres_byname(bres_client *" rc ", const char *" name ,
18 .BI " void (*" func ")(struct hostent *" h ", void *" p ),
20 .ta \w'\fBvoid bres_byaddr('u
21 .BI "void bres_byaddr(res_client *" rc ", struct inaddr " addr ,
22 .BI " void (*" func ")(struct hostent *" h ", void *" p ),
24 .BI "void bres_abort(bres_client *" rc );
25 .BI "void bres_exec(const char *" file );
26 .BI "void bres_init(sel_state *" sel );
31 header file declares types and functions for doing translation between
32 host names and IP addresses in the background.
34 The system must be initialized before use by a call to
36 passing it the address of an I/O multiplexor (see
39 A resolver task is stored in an object of type
41 the storage for which is allocated by the caller. The object is a
42 structure, and its contents are unspecified. The object is initialized
43 by one of the name resolution functions
47 Each function is passed the following arguments:
49 .BI "bres_client *" rc
50 Pointer to the client block to initialize and store the resolver job's
53 .BR "struct in_addr " \fIaddr "" " (" bres_byaddr )
57 .BR "const char *" \fIname "" " (" bres_byname )
58 The IP address or hostname to resolve.
60 .BI "void (*" func ")(struct hostent *" h ", void *" p )
61 A handler function to call when the resolver job is complete.
64 A pointer argument to pass to the handler function.
68 block must not be discarded until either the job is complete (i.e., the
69 handler function has been called) or
73 The handler function is passed either the address of a
75 structure describing the resolved host, or a null pointer indicating
78 structure is as returned by the standard
82 functions. This isn't the most convenient format for the results, but
83 it does have the benefit of being standard. Similarly, errors are
84 reported through the global
90 cancels a running resolver job. When it returns, the client structure
93 There are two versions of
95 The standard one uses a pool of server processes. Incoming resolver
96 jobs are passed to an available server, or a new server is started if
97 all are busy. There is a maximum number of servers, and jobs are queued
98 once this limit is reached. Old servers which have been idle for a
99 period of time are killed off. Servers are also killed if they start
100 misbehaving or their jobs are aborted.
102 By default, servers are started simply by calling
104 This can cause undesirably high memory usage in large programs. The
107 requests the resolver system to
109 a small dedicated server program to perform name lookups to reduce
110 memory consumption. The argument to
112 is the full pathname of the server program, or null to accept the
113 default set at library configuration time (which is usually correct).
115 The other implementation of
119 library to do asynchronous resolution. It can cope with many more
120 simultaneous resolver jobs, and doesn't use up external processes. If
125 function does nothing at all.
127 For security reasons, when an address is resolved, the hostname received
128 is verified by performing a forward lookup. If the forward lookup fails
129 to return the expected IP address, an error is reported.
131 .BR gethostbyname (3),
132 .BR gethostbyaddr (3),
136 Mark Wooding, <mdw@distorted.org.uk>