SD_ID128_GET_MACHINE(3)       sd_id128_get_machine       SD_ID128_GET_MACHINE(3)

       sd_id128_get_machine, sd_id128_get_machine_app_specific,
       sd_id128_get_boot, sd_id128_get_boot_app_specific,
       sd_id128_get_invocation - Retrieve 128-bit IDs

       #include <systemd/sd-id128.h>

       int sd_id128_get_machine(sd_id128_t *ret);

       int sd_id128_get_machine_app_specific(sd_id128_t app_id,
                                             sd_id128_t *ret);

       int sd_id128_get_boot(sd_id128_t *ret);

       int sd_id128_get_boot_app_specific(sd_id128_t app_id, sd_id128_t *ret);

       int sd_id128_get_invocation(sd_id128_t *ret);

       sd_id128_get_machine() returns the machine ID of the executing host. This
       reads and parses the machine-id(5) file. This function caches the machine
       ID internally to make retrieving the machine ID a cheap operation. This
       ID may be used wherever a unique identifier for the local system is
       needed. However, it is recommended to use this ID as-is only in trusted
       environments. In untrusted environments it is recommended to derive an
       application specific ID from this machine ID, in an irreversible
       (cryptographically secure) way. To make this easy
       sd_id128_get_machine_app_specific() is provided, see below.

       sd_id128_get_machine_app_specific() is similar to sd_id128_get_machine(),
       but retrieves a machine ID that is specific to the application that is
       identified by the indicated application ID. It is recommended to use this
       function instead of sd_id128_get_machine() when passing an ID to
       untrusted environments, in order to make sure that the original machine
       ID may not be determined externally. This way, the ID used by the
       application remains stable on a given machine, but cannot be easily
       correlated with IDs used in other applications on the same machine. The
       application-specific ID should be generated via a tool like systemd-id128
       new, and may be compiled into the application. This function will return
       the same application-specific ID for each combination of machine ID and
       application ID. Internally, this function calculates HMAC-SHA256 of the
       application ID, keyed by the machine ID.

       sd_id128_get_boot() returns the boot ID of the executing kernel. This
       reads and parses the /proc/sys/kernel/random/boot_id file exposed by the
       kernel. It is randomly generated early at boot and is unique for every
       running kernel instance. See random(4) for more information. This
       function also internally caches the returned ID to make this call a cheap
       operation. It is recommended to use this ID as-is only in trusted
       environments. In untrusted environments it is recommended to derive an
       application specific ID using sd_id128_get_machine_app_specific(), see

       sd_id128_get_boot_app_specific() is analogous to
       sd_id128_get_machine_app_specific() but returns an ID that changes
       between boots. Some machines may be used for a long time without
       rebooting, hence the boot ID may remain constant for a long time, and has
       properties similar to the machine ID during that time.

       sd_id128_get_invocation() returns the invocation ID of the currently
       executed service. In its current implementation, this reads and parses
       the $INVOCATION_ID environment variable that the service manager sets
       when activating a service, see systemd.exec(5) for details. The ID is
       cached internally. In future a different mechanism to determine the
       invocation ID may be added.

       Note that sd_id128_get_machine_app_specific(), sd_id128_get_boot(),
       sd_id128_get_boot_app_specific(), and sd_id128_get_invocation() always
       return UUID v4 compatible IDs.  sd_id128_get_machine() will also return a
       UUID v4-compatible ID on new installations but might not on older. It is
       possible to convert the machine ID into a UUID v4-compatible one. For
       more information, see machine-id(5).

       For more information about the "sd_id128_t" type see sd-id128(3).

       Those calls return 0 on success (in which case ret is filled in), or a
       negative errno-style error code.

       Returned errors may indicate the following problems:

           Returned by sd_id128_get_machine(),
           sd_id128_get_machine_app_specific(), and
           sd_id128_get_boot_app_specific() when /etc/machine-id is missing.

           Returned by sd_id128_get_machine(),
           sd_id128_get_machine_app_specific(), and
           sd_id128_get_boot_app_specific() when /etc/machine-id is empty or all

           Returned by sd_id128_get_invocation() if no invocation ID is set.

           Returned by any of the functions described here when the configured
           value has invalid format.

           Requested information could not be retrieved because of insufficient

       These APIs are implemented as a shared library, which can be compiled and
       linked to with the libsystemd pkg-config(1) file.

       Example 1. Application-specific machine ID

       First, generate the application ID:

           $ systemd-id128 -p new
           As string:

           As UUID:

           As man:sd-id128(3) macro:
           #define MESSAGE_XYZ SD_ID128_MAKE(c2,73,27,73,23,db,45,4e,a6,3b,b9,6e,79,b5,3e,97)

       Then use the new identifier in an example application:

           #include <stdio.h>
           #include <systemd/sd-id128.h>

           #define OUR_APPLICATION_ID SD_ID128_MAKE(c2,73,27,73,23,db,45,4e,a6,3b,b9,6e,79,b5,3e,97)

           int main(int argc, char *argv[]) {
             sd_id128_t id;
             sd_id128_get_machine_app_specific(OUR_APPLICATION_ID, &id);
             printf("Our application ID: " SD_ID128_FORMAT_STR "\n", SD_ID128_FORMAT_VAL(id));
             return 0;

       systemd(1), systemd-id128(1), sd-id128(3), machine-id(5),
       systemd.exec(5), sd_id128_randomize(3), random(4)

systemd 247                                              SD_ID128_GET_MACHINE(3)