niceSOFT/libconfuse: Small configuration file parser library for C.

doc/tutorial.xml
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Luca Ceresoli
Fix typos in documentation
17 ноя 2016, 01:42
17 ноя 2016, 01:42d8bfe5d
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<article>
    <articleinfo>
        <title>libConfuse tutorial</title>
        <author><firstname>Martin</firstname> <surname>Hedenfalk</surname></author>
    </articleinfo>
    <sect1>
        <title>Introducing libConfuse in an existing program</title>
        <para>Consider this simple program:</para>
        &listing1;
        <para>
            Simple enough, but we want to extend the program so we can greet
            others. Maybe we don't want to greet the whole world, just our
            neighbour. We use libConfuse to let the user decide whom to greet.
        </para>
        &listing2;
        <para>
            All programs using libConfuse must first include the
            <filename>confuse.h</filename> header file.  This is done on line
            2.
        </para>
        <para>
            On line 6 - 10, the options that should be recognized are defined in an
            array of cfg_opt_t structs. This is passed to the
            <function>cfg_init</function> function on line 13. The resulting
            <structname>cfg_t</structname> context is used by
            <function>cfg_parse()</function>, which reads the configuration file
            "hello.conf". When reading the configuration file, only options defined in
            the array of options passed to <function>cfg_init()</function> are
            recognized.
        </para>
        <para>
            The friendly greeting is now replaced with a parameter read from the
            configuration file. The value of the <varname>target</varname> option is retrieved with
            <function>cfg_getstr(cfg, "target")</function>.
        </para>
        <para>
            Lets take a look at the configuration file hello.conf:
        </para>

        <programlisting>
# this is the configuration file for the hello program

target = "Neighbour"
        </programlisting>
        <para>
            Here, the target option is set to the string value "Neighbour".
            What if the configuration file was empty or didn't exist? Then the
            default value for the <varname>target</varname> option would be
            used. When we initialized our options, the second parameter to the
            <function>CFG_STR()</function> macro specified the default value.
            Thus, if no <varname>target</varname> option was specified in the
            configuration file, the hello program would have printed the
            standard greeting "Hello, World".
        </para>

        <sect2>
            <title>Environment variables in values</title>

            <para>
                What else can we do in the configuration file? We can set the value to an
                environment variable:
            </para>
            <programlisting>
target = ${USER}
            </programlisting>
            <para>
                This results in the hello program greeting the user who runs it. On some
                systems, the USER variable might not be available, so we want to specify a
                default value in those cases:
            </para>
            <programlisting>
target = ${USER:-User}
            </programlisting>
            <para>
                Now, if the USER environment variable is unset, the string "User" will be
                used instead.
            </para>
        </sect2>
    </sect1>
    <sect1>
        <title>Other types of options</title>

        <para>
            Of course, not only strings can be specified in the configuration file.
            libConfuse can parse strings, integers, booleans and floating point values.
            These are the fundamental values, and they are all also available as lists.
            We'll talk more about lists in the next chapter.
        </para>
        <para>
            The macros used to initialize a string, integer, boolean and a
            float is, respectively, <function>CFG_STR()</function>,
            <function>CFG_INT()</function>, <function>CFG_BOOL()</function>,
            <function>CFG_FLOAT()</function> and
            <function>CFG_PTR()</function>. All macros take three parameters:
            the name of the option, a default value and flags. To retrieve the
            values, use <function>cfg_getstr()</function>,
            <function>cfg_getint()</function>,
            <function>cfg_getbool()</function>,
            <function>cfg_getfloat()</function> or
            <function>cfg_getptr()</function>, respectively.
        </para>
        <para>
            Let's introduce an integer option that tells us how many times to print the
            greeting:
        </para>
        &listing3;
        <para>
            Here we have used the <function>CFG_INT()</function> macro to
            initialize an integer option named "repeat". The default value is 1
            as in the standard greeting. The value is retrieved with
            <function>cfg_getint()</function>.
        </para>
        <para id="negative-repeat-problem">
            But, wait a moment, what if the user specified a negative value for
            "repeat"?  Or a too large positive value? libConfuse can handle
            that with a so-called validating callback. We'll come back to this
            problem later on, but we will first take a look at lists.
        </para>
    </sect1>

    <sect1>
        <title>Introducing lists</title>

        <para>
            That was easy. Now let's extend the program a bit so we can greet more than one
            "target". We'd like to be able to specify a list of targets to greet.
        </para>
        <para>
            The list versions of the initialization macros are named
            <function>CFG_STR_LIST()</function>,
            <function>CFG_INT_LIST()</function>,
            <function>CFG_BOOL_LIST()</function> and
            <function>CFG_FLOAT_LIST()</function>. They take the same
            parameters as the non-list versions, except the default value must
            be a string surrounded by curly braces.
        </para>
        <para>
            The modified program is shown below:
        </para>
        &listing4;
        <para>
            Three things are a bit different here. First, the macro to
            initialize the "targets" option is
            <function>CFG_STR_LIST()</function>. This tells libConfuse that
            "targets" is a list of strings. Second, the default value in the
            second parameter is surrounded by curly braces. This is needed to
            indicate to libConfuse where the list of values ends.
        </para>
        <para>
            The third change is in the printing of the greeting. First we print
            the "Hello" string. Then we loop through all values found for the
            "targets" option. The number of values is retrieved with the
            <function>cfg_size()</function> function. The string values are
            then retrieved with <function>cfg_getnstr()</function>, which is an
            indexed version of <function>cfg_getstr()</function>. In fact,
            <function>cfg_getstr()</function> is equivalent to
            <function>cfg_getnstr()</function> with an index of zero.
        </para>
        <para>
            In the configuration file hello.conf, we can now specify a list of targets to
            greet:
        </para>
        <programlisting>
# this is the configuration file for the hello program

targets = {"Life", "Universe", "Everything"}
repeat = 1
        </programlisting>
        <para>
            The output of the hello program, run with the above configuration file, is:
            "Hello, Life, Universe, Everything!"
        </para>
        <para>
            Again, if no targets were configured, the greeting would have been the standard
            "Hello, World!".
        </para>

    </sect1>
    <sect1>
        <title>Using sections</title>

        <para>
            So far, we have only use a flat configuration file. libConfuse can also handle
            sections to build a hierarchy of options. Sections can be used to group options
            in logical blocks, and those blocks can (optionally) be specified multiple
            times.
        </para>
        <para>
            Sections are initialized with the <function>CFG_SEC()</function> macro. It also takes three
            parameters: the name of the option, an array of options allowed in the section
            and flags.
        </para>
        <para>
            We'll extend the, now rather complex, hello program so we can do other kinds of
            greetings, not just "Hello". Each greeting will have its own settings for
            targets and repeat.
        </para>
        &listing5;
        <para>
            We have renamed the option array from "opts" to "greet_opts", and introduced a
            new "opts" array that only has one option: a "greeting" section.
            The second parameter of the <function>CFG_SEC()</function> macro
            points to the old greeting options "targets" and "repeat".
        </para>
        <para>
            We have also used a couple of flags to alter the behaviour of the
            section: CFGF_TITLE means that a greeting section should have a
            title and the CFGF_MULTI flag tells libConfuse that this section
            may be specified multiple times in the configuration file. The
            title of a section is retrieved with the
            <function>cfg_title()</function> function.
        </para>
        <para>
            The outmost loop (with index j) now loops through all given
            sections in the configuration file. We retrieve a section with a
            <function>cfg_getnsec()</function> call. The value returned is a
            pointer to a cfg_t struct, the same type as returned by
            <function>cfg_init()</function>. Thus we can use the ordinary value
            retrieval functions <function>cfg_getstr()</function>,
            <function>cfg_getint()</function> and so on to retrieve values of
            options inside the section.
        </para>
        <para>
            Ok, so how does the configuration file look like for this setup?
        </para>
        <programlisting>
# this is the configuration file for the hello program

greeting Hello
{
    targets = {"Life", "Universe", "Everything"}
    repeat = 1
}

greeting Bye
{
    targets = {Adams}
    repeat = 1
}
        </programlisting>
        <para>
            The program will loop through the sections in the order specified
            in the configuration file. First it will find the "Hello" section.
            It prints the title of the section, "Hello", retrieved with
            <function>cfg_title()</function>. Then the targets are printed just
            as in the previous examples, but this time the values are retrieved
            from the cfg_greet section. Next, the section titled "Bye" is
            found, and the values are retrieved from that section.
        </para>
        <para>
            When run, the program produces the following:
        </para>
        <programlisting>
$ ./listing5
Hello, Life, Universe, Everything!
Bye, Adams!
$ 
        </programlisting>

    </sect1>
    <sect1>
        <title>Parsing from internal buffers</title>
        <para>
            So far, we have only parsed configuration data from files.
            libConfuse can also parse buffers, or in-memory character
            strings. We will use this to fix a problem in the previous code.
        </para>
        <para>
            The problem is that without a configuration file, the hello program
            will not print anything. We want it to at least print the standard
            greeting "Hello, World!" if no configuration file is available.
        </para>
        <para>
            We can't have a default value for a section that can be specified
            multiple times (ie, a section with the CFGF_MULTI flag set).
            Instead we will parse a default configuration string if no section
            has been parsed:
        </para>
        &listing6;
        <para>
            Only the changes from the previous code is shown here. We check if
            the size of the "greeting" section is zero (ie, no section has been
            defined). In that case we call <function>cfg_parse_buf()</function>
            to parse a default in-memory string "greeting Hello {}". This
            string defines a greeting section with title Hello, but without any
            sub-options. This way we rely on the default values of the
            (sub-)options "targets" and "repeat".
        </para>
        <para>
            When this program is run, it issues the well-known standard greeting
            "Hello, World!" if no configuration file is present.
        </para>
    </sect1>

    <sect1>
        <title>Validating callback functions</title>
        <para>
            Remember the problem about a negative or too large "repeat" value
            in <xref linkend="negative-repeat-problem"/>?  The code that prints
            the greeting has those lines:
        </para>
        <programlisting>
...
repeat = cfg_getint(cfg_greet, "repeat");
while(repeat--)
...
        </programlisting>
        <para>
            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.
        </para>
        <para>
            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 <function>cfg_getint()</function> and a test.
            But we will use a validating callback function instead. This way
            <function>cfg_parse()</function> will return an error directly when
            parsing the file, additionally indicating on which line the error
            is.
        </para>
        <para>
            A validating callback function is defined as:
        </para>
        <programlisting>
typedef int (*cfg_validate_callback_t)(cfg_t *cfg, cfg_opt_t *opt);
        </programlisting>
        <para>
            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
            <function>cfg_error()</function>.
        </para>
        <para>
            Here is the code for the callback function:
        </para>
        &listing7;
        <para>
            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
            <function>cfg_opt_getint(opt)</function> to retrieve the only
            value. However, if we later want to use this callback to validate
            an integer list, it is already lists-aware.
        </para>

        <sect2>
            <title>Installing the callback</title>
            <para>
                The validating callback is installed with
                <function>cfg_set_validate_func()</function>. 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 ("|").
            </para>
            <para>
                We're now also looking at the return code from
                <function>cfg_parse()</function> to verify that the parsing was
                successful. The complete program is now:
            </para>
            &listing8;
        </sect2>
    </sect1>

    <sect1>
        <title>Value parsing callback</title>
        <para>
            A value parsing callback is another kind of callback function
            available in libConfuse. This function is used to map a string into
            some other other value. One example is to extend a boolean option
            to accept the values "yes", "no" and "ask" (or perhaps "true",
            "false" and "maybe"). Those values should be mapped to the integers
            1, 2 and 3.
        </para>
        <programlisting>
typedef int (*cfg_callback_t)(cfg_t *cfg, cfg_opt_t *opt,
                              const char *value, void *result);
        </programlisting>
        <para>
            
        </para>
    </sect1>

    <sect1>
        <title>Functions</title>
        <para>
            libConfuse supports functions to parse options that does not fit
            well in the general syntax. Functions can be called with a variable
            number of arguments. No data from the function or any arguments are
            stored by libConfuse after the function has run. It is up to the caller
            to process and/or save the data.
        </para>
        <para>
            A function is defined with a <function>CFG_FUNC</function> macro.
            It takes two arguments: the name of the function and a function
            callback. The callback is defined as:
        </para>
        <programlisting>
typedef int (*cfg_func_t)(cfg_t *cfg, cfg_opt_t *opt,
                          int argc, const char **argv);
        </programlisting>
        <para>
        </para>

        <sect2>
            <title>Predefined functions</title>
            <para>
                Currently there is only one pre-defined function:
                <function>cfg_include()</function>. This function includes
                another configuration file. Configuration data is immediately
                read from the included file, and is returned to the position
                right after the include() statement upon end of file.
            </para>
            <para>
                To use this function, include a <function>CFG_FUNC()</function>
                entry in your options:
            </para>
            <programlisting>
cfg_opt_t opts[] = {
    CFG_FUNC("include", cfg_include),
    CFG_END()
};
            </programlisting>
            <para>
                In the configuration file, it is used in the following way:
            </para>
            <programlisting>
include("included.conf")
            </programlisting>
        </sect2>
    </sect1>

    <sect1>
        <title>Saving configuration files</title>
        <para>
        </para>

        <sect2>
            <title>Altering the printing of certain options</title>
            <para>
            </para>
        </sect2>
    </sect1>

</article>