NAME

       sd_id128_to_string, SD_ID128_TO_STRING, SD_ID128_STRING_MAX, sd_id128_to_uuid_string,
       SD_ID128_TO_UUID_STRING, SD_ID128_UUID_STRING_MAX, sd_id128_from_string - Format or parse 128-bit IDs as
       strings

SYNOPSIS

       #include <systemd/sd-id128.h>

       #define SD_ID128_STRING_MAX 33U

       #define SD_ID128_UUID_STRING_MAX 37U

       #define SD_ID128_TO_STRING(id) ...

       #define SD_ID128_TO_UUID_STRING(id) ...

       char *sd_id128_to_string(sd_id128_t id, char s[static SD_ID128_STRING_MAX]);

       char *sd_id128_uuid_string(sd_id128_t id, char s[static SD_ID128_UUID_STRING_MAX]);

       int sd_id128_from_string(const char *s, sd_id128_t *ret);

DESCRIPTION

       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_to_uuid_string() and SD_ID128_TO_UUID_STRING() are similar to these two functions/macros, but
       format the 128-bit values as RFC4122 UUIDs, i.e. a series of 36 lowercase hexadeciaml digits and dashes,
       terminated by a NUL byte.

       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 formatting and parsing 36 character UUIDs this is done strictly in Big Endian byte order,
       i.e. according to RFC4122[1] 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).

RETURN VALUE

       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.

NOTES

       Functions described here are available as a shared library, which can be compiled against and linked to
       with the libsystemd pkg-config(1) file.

       The code described here uses getenv(3), which is declared to be not multi-thread-safe. This means that
       the code calling the functions described here must not call setenv(3) from a parallel thread. It is
       recommended to only do calls to setenv() from an early phase of the program when no other threads have
       been started.

HISTORY

       sd_id128_to_string() and sd_id128_from_string() were added in version 187.

       sd_id128_uuid_string() was added in version 251.

SEE ALSO

       systemd(1), sd-id128(3), printf(3)

NOTES

        1. RFC4122
           https://tools.ietf.org/html/rfc4122