2 .TH bres 3 "1 October 1999" "Straylight/Edgeware" "mLib utilities library"
4 bres \- background name resolver
12 .B "#include <mLib/bres.h>"
14 .BI "void bres_byname(bres_client *" rc ", const char *" name ,
15 .BI " void (*" func ")(struct hostent *" h ", void *" p ),
17 .BI "void bres_byaddr(bres_client *" rc ", struct inaddr " addr ,
18 .BI " void (*" func ")(struct hostent *" h ", void *" p ),
20 .BI "void bres_abort(bres_client *" rc );
21 .BI "void bres_exec(const char *" file );
22 .BI "void bres_init(sel_state *" sel );
27 header file declares types and functions for doing translation between
28 host names and IP addresses in the background.
30 The system must be initialized before use by a call to
32 passing it the address of an I/O multiplexor (see
35 A resolver task is stored in an object of type
37 the storage for which is allocated by the caller. The object is a
38 structure, and its contents are unspecified. The object is initialized
39 by one of the name resolution functions
43 Each function is passed the following arguments:
45 .BI "bres_client *" rc
46 Pointer to the client block to initialize and store the resolver job's
49 .BI "struct in_addr " addr "\fR (\fBbres_byaddr\fR)"
52 .BI "const char *" name "\fR (\fBbres_byname\fR)"
53 The IP address or hostname to resolve.
55 .BI "void (*" func ")(struct hostent *" h ", void *" p )
56 A handler function to call when the resolver job is complete.
59 A pointer argument to pass to the handler function.
63 block must not be discarded until either the job is complete (i.e., the
64 handler function has been called) or
68 The handler function is passed either the address of a
70 structure describing the resolved host, or a null pointer indicating
73 structure is as returned by the standard
77 functions. This isn't the most convenient format for the results, but
78 it does have the benefit of being standard. Similarly, errors are
79 reported through the global
85 cancels a running resolver job. When it returns, the client structure
88 There are two versions of
90 The standard one uses a pool of server processes. Incoming resolver
91 jobs are passed to an available server, or a new server is started if
92 all are busy. There is a maximum number of servers, and jobs are queued
93 once this limit is reached. Old servers which have been idle for a
94 period of time are killed off. Servers are also killed if they start
95 misbehaving or their jobs are aborted.
97 By default, servers are started simply by calling
99 This can cause undesirably high memory usage in large programs. The
102 requests the resolver system to
104 a small dedicated server program to perform name lookups to reduce
105 memory consumption. The argument to
107 is the full pathname of the server program, or null to accept the
108 default set at library configuration time (which is usually correct).
110 The other implementation of
114 library to do asynchronous resolution. It can cope with many more
115 simultaneous resolver jobs, and doesn't use up external processes. If
120 function does nothing at all.
122 For security reasons, when an address is resolved, the hostname received
123 is verified by performing a forward lookup. If the forward lookup fails
124 to return the expected IP address, an error is reported.
126 .BR gethostbyname (3),
127 .BR gethostbyaddr (3),
131 Mark Wooding, <mdw@distorted.org.uk>