ulid-c

Standalone, plain C, clean-room implementation of ULID.

Installation

Using clib:

clib install aperezdc/ulid-c --save

Usage

The quickest and safest way of creating an ULID is using ulid_make_urandom():

ulid_t u;
ulid_make_urandom (&u);

It is possible to customize the timstamp and method used to obtain the random bytes of entropy using the ulid_encode() function, which takes a timestamp and a function pointer to fill an ulid_t value:

ulid_t u;
ulid_encode (&u, ulid_clock_monotonic (), ulid_entropy_rand, NULL);

In order to obtain a textual representation of the value, use ulid_string():

char s[ULID_STRING_LENGTH + 1];  // +1 for the null terminator
ulid_string (&u, s);

Encoding Functions

ulid_encode(dst, timestamp, rng, userdata) encodes an ULID value with a given timestamp, filling the entropy bytes by invoking the rng function, which gets passed the userdata value. While this allows customizing completely how an ULID is encoded, it is recommended to use the shorthand functions listed below.
ulid_string(ulid, buffer[ULID_STRING_LENGTH+1]) serializes an ulid value to a null-terminated C string buffer.

Decoding Functions

ulid_timestamp(ulid) returns the timestamp portion of the ULID, as an uint64_t value.

Shorthand Functions

ulid_encode_rand(dst, timestamp) uses ulid_entropy_rand.
ulid_encode_const(dst, timestamp, value) uses ulid_entropy_const.
ulid_make_rand(dst) uses ulid_clock_monotonic and ulid_entropy_rand.
ulid_make_const(dst, value) uses ulid_clock_monotonic and ulid_entropy_const.
ulid_make_urandom(dst) uses ulid_clock_monotonic and getrandom(2) (on Linux), arc4random(3) (on OpenBSD), or directly reads /dev/random (on other systems).

Utility Functions

ulid_equal(a, b) checks whether two ulid_t values are the same.
ulid_compare(a, b) compares two ulid_t values and returns an int value (-1, 0, 1) which can be used for sorting ULIDs.
ulid_copy(dst, src) copies an ulid_t into another.

Timestamp Functions

Unless stated otherwise, all timestamp functions have millisecond resolution. It is stronly recommended to use ulid_clock_monotonic().

ulid_time_epoch() returns seconds from the Epoch, with seconds resolution. Uses time(2) under the hood.
ulid_time_cpu_used() returns milliseconds of CPU time used by the current process. Uses clock(2) under the hood.
ulid_clock_realtime() returns the wall clock time, with milliseconds resolution. Uses clock_gettime(2) and CLOCK_REALTIME, falling back to ulid_time_epoch() on failure.
ulid_clock_monotonic() returns an ever-increasing time. Uses clock_gettime(2) and CLOCK_MONOTONIC_RAW (if available, otherwise CLOCK_MONOTONIC), falling back to ulid_clock_realtime() on failure.
ulid_clock_cpu_used() returns the amount of CPU time used by the current process. Uses clock_gettime(2) and CLOCK_PROCESS_CPUTIME_ID if available, otherwise falls back to ulid_time_cpu_used().

Entropy Functions

Those are provided to customize how the 10 entropy bytes of ULIDs are filled-in. Most of the time these do not need to be used, and in general it is strongly recommended to use the ulid_encode_urandom() or ulid_make_urandom() shorthand functions instead.

ulid_entropy_const() fills the entropy with the lower 8 bits of the constant value passed as userdata.
ulid_entropy_rand() uses the rand(3) function from the standard C library. You might want to use srand() during the initialization of your program.
ulid_entropy_fd() reads bytes from the Unix file descriptor passed as userdata, which is interpreted as an int.
ulid_entropy_file() reads bytes from the FILE* handle passed as userdata.

aperezdc / ulid-c