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