6. Validating callback functions
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## 6. Validating callback functions

Remember the problem about a negative or too large "repeat" value in Section 2, “Other types of options”? The code that prints the greeting has those lines:

    ...
    repeat = cfg_getint(cfg_greet, "repeat");
    while(repeat--)
    ...

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.

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 `cfg_getint()` and a test. But we will use a validating callback function instead. This way `cfg_parse()` will return an error directly when parsing the file, additionally indicating on which line the error is.

A validating callback function is defined as:

    typedef int (*cfg_validate_callback_t)(cfg_t *cfg, cfg_opt_t *opt);

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 `cfg_error()`.

Here is the code for the callback function:

    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

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 `cfg_opt_getint(opt)` to retrieve the only value. However, if we later want to use this callback to validate an integer list, it is already lists-aware.

### 6.1. Installing the callback

The validating callback is installed with `cfg_set_validate_func()`. 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 ("|").

We're now also looking at the return code from `cfg_parse()` to verify that the parsing was successful. The complete program is now:

    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

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