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.
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).
sd_id128_to_string() always succeeds and returns a pointer to the string array passed in. sd_id128_from_string returns 0 on success, in which case ret is filled in, or a negative errno-style error code.
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.