SimpleConf

A deceptively simple way to add a configuration files to an existing command-line application.

1. Define a configuration file property -> command-line option map
2. Include a single C file
3. Done.

The grammar is defined at runtime, so that application plugins can register whatever they want in addition to a base set of keywords.

SimpleConf is the configuration file parser used in pure-ftpd.

It is a trivial piece of code, but it is very generic and can easily be reused. Hence this standalone-version, in case it could be useful to other projects.

Quick start

The code below uses getopt_long() to parse command-line arguments, but SimpleConf is compatible with pretty much anything, as it always builds a new arguments list.

#include <getopt.h>
#include <stdio.h>

static struct option getopt_long_options[] = {
    { "name", 1, NULL, 'n' }, { "bell", 0, NULL, 'b' }, { NULL, 0, NULL, 0 }
};
static const char *getopt_options = "n:b";

void parse_options(int argc, char *argv[])
{
    int flag, index = 0;
    while ((flag = getopt_long(argc, argv, getopt_options,
                               getopt_long_options, &index)) != -1) {
        switch (flag) {
            case 'n': printf("Hello %s!\n", optarg); break;
            case 'b': putchar(7); break;
        }
    }
}

int main(int argc, char *argv[])
{
    parse_options(argc, argv);
    return 0;
}

Simple changes to the main() function in order to implement support for configuration files:

#include "simpleconf.h"

int main(int argc, char *argv[])
{
    static const SimpleConfEntry sc_defs[] = {
        { "Name (<any*>)", "--name=$0" },
        { "Bell? <bool>",  "--bell"}
    };
    sc_build_command_line_from_file("hello.conf", NULL, sc_defs, 2,
                                    argv[0], &argc, &argv);
    parse_options(argc, argv);
    return 0;
}

Done.

Our hello application can now load a hello.conf configuration file such as the following:

#################################################
##                                             ##
##     Sample configuration file for Hello     ##
##                                             ##
#################################################

# Change this to your name
Name Johnny Doe

# Change to "yes" for a (visual) bell effect
Bell no

The parser is quite tolerant and the following configuration files would work equally well:

name = Johnny Doe
bell = off

name: Johnny Doe
bell: false

Parsing rules

The grammar is defined by a vector of SimpleConfEntry values. Each value maps a property name and a pattern to a command-line option.

A pattern can be made of arbitrary characters, including white spaces:

{ "PreferredFruit banana",     "--banana" },
{ "PreferredFruit kiwi fruit", "--kiwi" }

But once again, the parser is very tolerant, and will gladly accept any number of spaces (white spaces and/or tabs) between kiwi and fruit in the configuration file.

Note that the same property name can appear multiple times: the first one that fully matches the given pattern is the one that will be translated to a command-line option.

Patterns can include character classes:

• <alpha>: matches one or more alphabetic characters
• <alnum>: matches one of more alphanumberic characters
• <digits>: matches one or more digits
• <xdigits>: matches one or more hex digits
• <nospace>: matches one or more characters that are not whitespaces; the string can optionally be quoted to allow delimiters.
• <any>: matches everything except whitespaces, as well as quoted strings that can include whitespaces and delimiters. Given the kiwi fruit input, this would only match kiwi. Given the "kiwi fruit" input, this would match kiwi fruit.
• <any*>: matches everything until the end of the line, including whitespaces, without requiring quotes.
• <bool>: matches yes, on, true, 1, no, off, false and 0.

Capturing groups

The command-line translation of a config file pattern can copy parts or all of the matching input:

{ "PreferredFruit (<alpha>)", "--fruit=$0" }

Capture groups are delimited by round brackets, and can include anything, although they are mostly useful with character classes:

{ "WidthAndHeight (<digits>x<digits>)", "--size=$0" }

Up to 10 capture groups can be present in a pattern. The first one can be referred to as $0, the second one as $1 and so on until $9.

{ "Size width:(<digits>) height:(<digits>)", "--size=$0x$1" }

The whole input can also be referred to as $*.

Boolean mapping

The addition of a command-line switch can depend on the value of a boolean condition. In order to do so, the property name should be suffixed with the ? character.

{ "EnableCrazyCoolFeature? <bool>", "--crazy-cool-feature" }

In this example, the --crazy-cool-feature switch will only be added if the value of the EnableCrazyCoolFeature property is either yes, on, true or 1.

Basic API

int sc_build_command_line_from_file(const char *file_name,
                                    const SimpleConfConfig *config,
                                    const SimpleConfEntry entries[],
                                    size_t entries_count, char *app_name,
                                    int *argc_p, char ***argv_p);

Builds a new list of command-line arguments by parsing the configuration file file_name according to the list of patterns entries of size entries_count, and puts the result into argv_p and its size into argc_p.

app_name is put into (*argv_p)[0], and can be the original value for argv[0].

config can be NULL, or set to a SimpleConfConfig when using special handlers (see below).

void sc_argv_free(int argc, char *argv[]);

Deallocates the memory allocated by sc_build_command_line_from_file().

Special handlers

Specific keywords can cause a hook being called instead of the default behavior.

This can be useful to handle options in the configuration file that have no command-line equivalent, as well as to support recursive configuration files.

Property names starting with a ! character trigger that behavior:

{ "!Include <any*>", "$*" }

In this example, instead of extending the list of command-line options, the Include keyword will cause a special handler to be called.

SimpleConfSpecialHandlerResult special_handler(void **output_p, const char *arg, void *user_data)
{
    // ... do some custom processing using the value `arg` ...
    return SC_SPECIAL_HANDLER_RESULT_NEXT;
}

Possible return values are:

• SC_SPECIAL_HANDLER_RESULT_NEXT: keep parsing the current file
• SC_SPECIAL_HANDLER_RESULT_ERROR: return an error and stop parsing the current file
• SC_SPECIAL_HANDLER_RESULT_INCLUDE: parse the content of the file whose name was stored in *output_p, and continue parsing the current file afterwards.

SC_SPECIAL_HANDLER_RESULT_INCLUDE is useful to allow users to recursively include configuration files in configuration files. A handler can sanitize file names, turn relative file names into absolute ones, and enforce rules to only allow specific files.

The bare minimum is to return a copy of the original file name:

SimpleConfSpecialHandlerResult special_handler(void **output_p, const char *arg, void *user_data)
{
    if ((*output_p = strdup(arg)) == NULL) {
        return SC_SPECIAL_HANDLER_RESULT_ERROR;
    }
    return SC_SPECIAL_HANDLER_RESULT_INCLUDE;
}

The handler should be specified via a SimpleConfConfig value when calling sc_build_command_line_from_file(). The user_data field can store an arbitrary pointer, that will be conveniently available in the handler.

Real-world examples

• pure-ftpd: SimpleConf definition and example configuration file