sd_id128_to_string, SD_ID128_TO_STRING, sd_id128_from_string, SD_ID128_STRING_MAX - Format or parse 128-bit IDs as strings
#define SD_ID128_STRING_MAX 33U
#define SD_ID128_TO_STRING(id) ...
char *sd_id128_to_string(sd_id128_t id, char s[static SD_ID128_STRING_MAX]);
int sd_id128_from_string(const char *s, sd_id128_t *ret);
sd_id128_to_string() formats a 128-bit ID as a character string. It expects the ID and a string array capable of storing 33 characters (SD_ID128_STRING_MAX). The ID will be formatted as 32 lowercase hexadecimal digits and be terminated by a NUL byte.
SD_ID128_TO_STRING() is a macro that wraps sd_id128_to_string() and passes an appropriately sized buffer as second argument, allocated as C99 compound literal. Each use will thus implicitly acquire a suitable buffer on the stack which remains valid until the end of the current code block. This is usually the simplest way to acquire a string representation of a 128-bit ID in a buffer that is valid in the current code block.
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).
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.