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3: <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>
4: Remember the problem about a negative or too large "repeat" value
5: in <a class="xref" href="ar01s02.html#negative-repeat-problem">Section 2, “Other types of options”</a>? The code that prints
6: the greeting has those lines:
7: </p><pre class="programlisting">
8: ...
9: repeat = cfg_getint(cfg_greet, "repeat");
10: while(repeat--)
11: ...
12: </pre><p>
13: The repeat variable is defined as an int, a signed integer. If the user
14: specified a negative repeat value in the configuration file, this code
15: would continue to decrease the repeat variable until it eventually
16: underflowed.
17: </p><p>
18: We'll fix this by not allowing a negative value in the configuration
19: file. Of course we could first just check if the value is negative
20: and then abort, using <code class="function">cfg_getint()</code> and a test.
21: But we will use a validating callback function instead. This way
22: <code class="function">cfg_parse()</code> will return an error directly when
23: parsing the file, additionally indicating on which line the error
24: is.
25: </p><p>
26: A validating callback function is defined as:
27: </p><pre class="programlisting">
28: typedef int (*cfg_validate_callback_t)(cfg_t *cfg, cfg_opt_t *opt);
29: </pre><p>
30: This function takes two arguments: the section and the option. It
31: should return 0 on success (ie, the value validated ok). All other
32: values indicates an error, and the parsing is aborted. The callback
33: function should notify the error itself, for example by calling
34: <code class="function">cfg_error()</code>.
35: </p><p>
36: Here is the code for the callback function:
37: </p><a id="listing7"></a><pre class="programlisting">
38: 1 int validate_unsigned_int(cfg_t *cfg, cfg_opt_t *opt)
39: 2 {
40: 3 int value = cfg_opt_getnint(opt, cfg_opt_size(opt) - 1);
41: 4 if(value < 0)
42: 5 {
43: 6 cfg_error(cfg, "integer option '%s' must be positive in section '%s'",
44: 7 opt->name, cfg->name);
45: 8 return -1;
46: 9 }
47: 10 return 0;
48: 11 }
49: 12
50: </pre><p>
51: Only the last value is validated, because libConfuse will call this
52: function once for every value corresponding to the option. Since
53: the "repeat" option is not a list, we could instead have used
54: <code class="function">cfg_opt_getint(opt)</code> to retrieve the only
55: value. However, if we later want to use this callback to validate
56: an integer list, it is already lists-aware.
57: </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>
58: The validating callback is installed with
59: <code class="function">cfg_set_validate_func()</code>. It is called with
60: a string specifying which option is affected, and a pointer to
61: the callback function. To specify an option in a subsection,
62: the section and the option must be separated with a vertical
63: bar ("|").
64: </p><p>
65: We're now also looking at the return code from
66: <code class="function">cfg_parse()</code> to verify that the parsing was
67: successful. The complete program is now:
68: </p><a id="listing8"></a><pre class="programlisting">
69: 1 #include <stdio.h>
70: 2 #include <confuse.h>
71: 3
72: 4 int validate_unsigned_int(cfg_t *cfg, cfg_opt_t *opt)
73: 5 {
74: 6 int value = cfg_opt_getnint(opt, cfg_opt_size(opt) - 1);
75: 7 if(value < 0)
76: 8 {
77: 9 cfg_error(cfg, "integer option '%s' must be positive in section '%s'",
78: 10 opt->name, cfg->name);
79: 11 return -1;
80: 12 }
81: 13 return 0;
82: 14 }
83: 15
84: 16 int main(void)
85: 17 {
86: 18 cfg_opt_t greet_opts[] =
87: 19 {
88: 20 CFG_STR_LIST("targets", "{World}", CFGF_NONE),
89: 21 CFG_INT("repeat", 1, CFGF_NONE),
90: 22 CFG_END()
91: 23 };
92: 24 cfg_opt_t opts[] =
93: 25 {
94: 26 CFG_SEC("greeting", greet_opts, CFGF_TITLE | CFGF_MULTI),
95: 27 CFG_END()
96: 28 };
97: 29 cfg_t *cfg, *cfg_greet;
98: 30 int repeat;
99: 31 int i, j;
100: 32
101: 33 cfg = cfg_init(opts, CFGF_NONE);
102: 34 cfg_set_validate_func(cfg, "greeting|repeat", validate_unsigned_int);
103: 35 if(cfg_parse(cfg, "hello.conf") == CFG_PARSE_ERROR)
104: 36 return 1;
105: 37
106: 38 for(j = 0; j < cfg_size(cfg, "greeting"); j++)
107: 39 {
108: 40 cfg_greet = cfg_getnsec(cfg, "greeting", j);
109: 41
110: 42 repeat = cfg_getint(cfg_greet, "repeat");
111: 43 while(repeat--)
112: 44 {
113: 45 printf("%s", cfg_title(cfg_greet));
114: 46 for(i = 0; i < cfg_size(cfg_greet, "targets"); i++)
115: 47 printf(", %s", cfg_getnstr(cfg_greet, "targets", i));
116: 48 printf("!\n");
117: 49 }
118: 50 }
119: 51
120: 52 cfg_free(cfg);
121: 53 return 0;
122: 54 }
123: 55
124: </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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