char *sd_id128_to_string(sd_id128_t id, char s);
int sd_id128_from_string(const char *s, sd_id128_t *ret);
sd_id128_from_string() implements the reverse operation: it takes a 33 character string with 32 hexadecimal digits (either lowercase or uppercase, terminated by NUL) and parses them back into a 128-bit ID returned in ret. Alternatively, this call can also parse a 37-character string with a 128-bit ID formatted as RFC UUID. If ret is passed as NULL the function will validate the passed ID string, but not actually return it in parsed form.
Note that when parsing 37 character UUIDs this is done strictly in Big Endian byte order, i.e. according to RFC4122 Variant 1 rules, even if the UUID encodes a different variant. This matches behaviour in various other Linux userspace tools. It's probably wise to avoid UUIDs of other variant types.
For more information about the "sd_id128_t" type see sd-id128(3). Note that these calls operate the same way on all architectures, i.e. the results do not depend on endianness.
When formatting a 128-bit ID into a string, it is often easier to use a format string for printf(3). This is easily done using the SD_ID128_FORMAT_STR and SD_ID128_FORMAT_VAL() macros. For more information see sd-id128(3).