Proplib is an API/lib to create, handle and store property lists.
It was created by Apple but NetBSD also have a clean room implementation in the system, and it's mostly used for implementing sysctl interfaces. It's a somehow limited implementation to make it faster/smaller (maybe extended in the future as demands grow).
Here's a little example code to show some of the basics:

This will create a dictionary and store it as an xml file on the disk:

[[!format c """
int
main(void)
{
    prop_dictionary_t d;
    bool r;

    d = prop_dictionary_create();

    if (d == NULL)
        goto error;

    r = prop_dictionary_set_cstring(d, "name", "Adam");
    if (!r)
        goto error;
    r = prop_dictionary_set_uint32(d, "year", 1986);
    if (!r)
        goto error;

    r = prop_dictionary_externalize_to_file(d, "test.plist");
    if (!r)
        goto error;

    return EXIT_SUCCESS;

    error :
        fprintf(stderr, "error\n");
        return EXIT_FAILURE;
}
"""]]

And this will read it and display the values:

[[!format c """
int
main(void)
{
    prop_dictionary_t d;
    uint32_t year;
    char *name;
    bool r;

    d = prop_dictionary_internalize_from_file("test.plist");
    if (d == NULL)
        goto error;

    r = prop_dictionary_get_cstring(d, "name", &name);
    if (!r)
        goto error;
    r = prop_dictionary_get_uint32(d, "year", &year);
    if (!r)
        goto error;

    printf("name: %s, year: %u\n", name, year);

    return EXIT_SUCCESS;

    error :
        fprintf(stderr, "error\n");
        return EXIT_FAILURE;
}
"""]]