# sd_id128_to_string

## 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_to_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 seesd-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.
    


## NOTES



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


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