# ObjectIDs

Libbson provides a simple way to generate ObjectIDs. It can be used in a single-threaded or multi-threaded manner depending on your requirements.

The bson_oid_t structure represents an `ObjectID` in MongoDB. It is a 96-bit identifier.

## Composition

- 4 bytes : The UNIX timestamp in big-endian format.

- 5 bytes : A random number.

- 3 bytes : A 24-bit monotonic counter incrementing from `rand()` in big-endian.

## Sorting ObjectIDs

The typical way to sort in C is using `qsort()`. Therefore, Libbson provides a `qsort()` compatible callback function named bson_oid_compare(). It returns `less than 1`, `greater than 1`, or `0` depending on the equality of two bson_oid_t structures.

## Comparing Object IDs

If you simply want to compare two bson_oid_t structures for equality, use bson_oid_equal().

## Generating

To generate a bson_oid_t, you may use the following.

    bson_oid_t oid;

    bson_oid_init (&oid, NULL);

## Parsing ObjectID Strings

You can also parse a string containing a bson_oid_t. The input string MUST be 24 characters or more in length.

    bson_oid_t oid;

    bson_oid_init_from_string (&oid, "123456789012345678901234");

    bson_oid_t oid;

    bson_oid_init_from_string_unsafe (&oid, "123456789012345678901234");

## Hashing ObjectIDs

If you need to store items in a hashtable, you may want to use the bson_oid_t as the key. Libbson provides a hash function for just this purpose. It is based on DJB hash.

    unsigned hash;

    hash = bson_oid_hash (oid);

## Fetching ObjectID Creation Time

You can easily fetch the time that a bson_oid_t was generated using bson_oid_get_time_t().

    time_t t;

    t = bson_oid_get_time_t (oid);
    printf ("The OID was generated at %u\n", (unsigned) t);