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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>6. Validating callback functions</title><link rel="stylesheet" href="tutorial.css" type="text/css" /><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="libConfuse tutorial" /><link rel="up" href="index.html" title="libConfuse tutorial" /><link rel="prev" href="ar01s05.html" title="5. Parsing from internal buffers" /><link rel="next" href="ar01s07.html" title="7. Value parsing callback" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">6. Validating callback functions</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ar01s05.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="ar01s07.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="6. Validating callback functions"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id442168"></a>6. Validating callback functions</h2></div></div></div><p>
Remember the problem about a negative or too large "repeat" value
in <a class="xref" href="ar01s02.html#negative-repeat-problem">Section 2, “Other types of options”</a>? The code that prints
the greeting has those lines:
</p><pre class="programlisting">
...
repeat = cfg_getint(cfg_greet, "repeat");
while(repeat--)
...
</pre><p>
The repeat variable is defined as an int, a signed integer. If the user
specified a negative repeat value in the configuration file, this code
would continue to decrease the repeat variable until it eventually
underflowed.
</p><p>
We'll fix this by not allowing a negative value in the configuration
file. Of course we could first just check if the value is negative
and then abort, using <code class="function">cfg_getint()</code> and a test.
But we will use a validating callback function instead. This way
<code class="function">cfg_parse()</code> will return an error directly when
parsing the file, additionally indicating on which line the error
is.
</p><p>
A validating callback function is defined as:
</p><pre class="programlisting">
typedef int (*cfg_validate_callback_t)(cfg_t *cfg, cfg_opt_t *opt);
</pre><p>
This function takes two arguments: the section and the option. It
should return 0 on success (ie, the value validated ok). All other
values indicates an error, and the parsing is aborted. The callback
function should notify the error itself, for example by calling
<code class="function">cfg_error()</code>.
</p><p>
Here is the code for the callback function:
</p><a id="listing7"></a><pre class="programlisting">
1 int validate_unsigned_int(cfg_t *cfg, cfg_opt_t *opt)
2 {
3 int value = cfg_opt_getnint(opt, cfg_opt_size(opt) - 1);
4 if(value < 0)
5 {
6 cfg_error(cfg, "integer option '%s' must be positive in section '%s'",
7 opt->name, cfg->name);
8 return -1;
9 }
10 return 0;
11 }
12
</pre><p>
Only the last value is validated, because libConfuse will call this
function once for every value corresponding to the option. Since
the "repeat" option is not a list, we could instead have used
<code class="function">cfg_opt_getint(opt)</code> to retrieve the only
value. However, if we later want to use this callback to validate
an integer list, it is already lists-aware.
</p><div class="sect2" title="6.1. Installing the callback"><div class="titlepage"><div><div><h3 class="title"><a id="id442390"></a>6.1. Installing the callback</h3></div></div></div><p>
The validating callback is installed with
<code class="function">cfg_set_validate_func()</code>. It is called with
a string specifying which option is affected, and a pointer to
the callback function. To specify an option in a subsection,
the section and the option must be separated with a vertical
bar ("|").
</p><p>
We're now also looking at the return code from
<code class="function">cfg_parse()</code> to verify that the parsing was
successful. The complete program is now:
</p><a id="listing8"></a><pre class="programlisting">
1 #include <stdio.h>
2 #include <confuse.h>
3
4 int validate_unsigned_int(cfg_t *cfg, cfg_opt_t *opt)
5 {
6 int value = cfg_opt_getnint(opt, cfg_opt_size(opt) - 1);
7 if(value < 0)
8 {
9 cfg_error(cfg, "integer option '%s' must be positive in section '%s'",
10 opt->name, cfg->name);
11 return -1;
12 }
13 return 0;
14 }
15
16 int main(void)
17 {
18 cfg_opt_t greet_opts[] =
19 {
20 CFG_STR_LIST("targets", "{World}", CFGF_NONE),
21 CFG_INT("repeat", 1, CFGF_NONE),
22 CFG_END()
23 };
24 cfg_opt_t opts[] =
25 {
26 CFG_SEC("greeting", greet_opts, CFGF_TITLE | CFGF_MULTI),
27 CFG_END()
28 };
29 cfg_t *cfg, *cfg_greet;
30 int repeat;
31 int i, j;
32
33 cfg = cfg_init(opts, CFGF_NONE);
34 cfg_set_validate_func(cfg, "greeting|repeat", validate_unsigned_int);
35 if(cfg_parse(cfg, "hello.conf") == CFG_PARSE_ERROR)
36 return 1;
37
38 for(j = 0; j < cfg_size(cfg, "greeting"); j++)
39 {
40 cfg_greet = cfg_getnsec(cfg, "greeting", j);
41
42 repeat = cfg_getint(cfg_greet, "repeat");
43 while(repeat--)
44 {
45 printf("%s", cfg_title(cfg_greet));
46 for(i = 0; i < cfg_size(cfg_greet, "targets"); i++)
47 printf(", %s", cfg_getnstr(cfg_greet, "targets", i));
48 printf("!\n");
49 }
50 }
51
52 cfg_free(cfg);
53 return 0;
54 }
55
</pre></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ar01s05.html">Prev</a> </td><td width="20%" align="center"> </td><td width="40%" align="right"> <a accesskey="n" href="ar01s07.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">5. Parsing from internal buffers </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 7. Value parsing callback</td></tr></table></div></body></html>
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