libConfuse tutorial

Martin Hedenfalk

Table of Contents

1. Introducing libConfuse in an existing program
1.1. Environment variables in values
2. Other types of options
3. Introducing lists
4. Using sections
5. Parsing from internal buffers
6. Validating callback functions
6.1. Installing the callback
7. Value parsing callback
8. Functions
8.1. Predefined functions
9. Saving configuration files
9.1. Altering the printing of certain options

1. Introducing libConfuse in an existing program

Consider this simple program:

1	#include <stdio.h>
3	int main(void)
4	{
5	    printf("Hello, World!\n");
6	    return 0;
7	}

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.

1	#include <stdio.h>
2	#include <confuse.h>
4	int main(void)
5	{
6	    cfg_opt_t opts[] =
7	    {
8	        CFG_STR("target", "World", CFGF_NONE),
9	        CFG_END()
10	    };
11	    cfg_t *cfg;
13	    cfg = cfg_init(opts, CFGF_NONE);
14	    if(cfg_parse(cfg, "hello.conf") == CFG_PARSE_ERROR)
15	        return 1;
17	    printf("Hello, %s!\n", cfg_getstr(cfg, "target"));
19	    cfg_free(cfg);
20	    return 0;
21	}

All programs using libConfuse must first include the confuse.h header file. This is done on line 2.

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 cfg_init function on line 13. The resulting cfg_t context is used by cfg_parse(), which reads the configuration file "hello.conf". When reading the configuration file, only options defined in the array of options passed to cfg_init() are recognized.

The friendly greeting is now replaced with a parameter read from the configuration file. The value of the target option is retrieved with cfg_getstr(cfg, "target").

Lets take a look at the configuration file hello.conf:

# this is the configuration file for the hello program

target = "Neighbour"

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 target option would be used. When we initialized our options, the second parameter to the CFG_STR() macro specified the default value. Thus, if no target option was specified in the configuration file, the hello program would have printed the standard greeting "Hello, World".

1.1. Environment variables in values

What else can we do in the configuration file? We can set the value to an environment variable:

target = ${USER}

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:

target = ${USER:-User}

Now, if the USER environment variable is unset, the string "User" will be used instead.