sd_id128_get_machine(3) — Linux manual page


SD_ID128_GET_MACHINE(3)   sd_id128_get_machine   SD_ID128_GET_MACHINE(3)

NAME         top

       sd_id128_get_machine, sd_id128_get_app_specific,
       sd_id128_get_machine_app_specific, sd_id128_get_boot,
       sd_id128_get_boot_app_specific, sd_id128_get_invocation -
       Retrieve 128-bit IDs

SYNOPSIS         top

       #include <systemd/sd-id128.h>

       int sd_id128_get_machine(sd_id128_t *ret);

       int sd_id128_get_app_specific(sd_id128_t base, sd_id128_t app_id,
                                     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);

DESCRIPTION         top

       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_app_specific() returns a machine ID that is a
       combination of the base and app_id parameters. Internally, this
       function calculates HMAC-SHA256 of the app_id parameter keyed by
       the base parameter, and truncates this result to fit in
       sd_id128_t and turns it into a valid Variant 1 Version 4 UUID, in
       accordance with RFC 4122[1]. Neither of the two input parameters
       can be calculated from the output parameter ret.

       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 calls
       sd_id128_get_app_specific() with the result from
       sd_id128_get_machine() and the app_id parameter.

       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_boot_app_specific(), see below.

       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
       tries to read and parse the following:

       •   The $INVOCATION_ID environment variable that the service
           manager sets when activating a service.

       •   An entry in the kernel keyring that the system 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 Variant 1 Version 4
       compatible IDs.  sd_id128_get_machine() will also return a UUID
       Variant 1 Version 4 compatible ID on new installations but might
       not on older. It is possible to convert the machine ID
       non-reversibly into a UUID Variant 1 Version 4 compatible one.
       For more information, see machine-id(5). It is hence guaranteed
       that these functions will never return the ID consisting of all
       zero or all one bits (SD_ID128_NULL, SD_ID128_ALLF) — with the
       possible exception of sd_id128_get_machine(), as mentioned.

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

RETURN VALUE         top

       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() and
           sd_id128_get_machine_app_specific() when /etc/machine-id is

           Added in version 242.

           Returned by sd_id128_get_machine() and
           sd_id128_get_machine_app_specific() when /etc/machine-id is
           empty or all zeros. Also returned by
           sd_id128_get_invocation() when the invocation ID is all

           Added in version 242.

           Returned by sd_id128_get_machine() and
           sd_id128_get_machine_app_specific() when the content of
           /etc/machine-id is "uninitialized".

           Added in version 253.

           Returned by sd_id128_get_boot() and
           sd_id128_get_boot_app_specific() when /proc/ is not mounted.

           Added in version 253.

           Returned by sd_id128_get_invocation() if no invocation ID is
           set. Also returned by sd_id128_get_app_specific(),
           sd_id128_get_machine_app_specific(), and
           sd_id128_get_boot_app_specific() when the app_id parameter is
           all zeros.

           Added in version 242.

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

           Added in version 253.

           Requested information could not be retrieved because of
           insufficient permissions.

           Added in version 242.

NOTES         top

       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

EXAMPLES         top

       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:

           /* SPDX-License-Identifier: MIT-0 */

           #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;

HISTORY         top

       sd_id128_get_machine() and sd_id128_get_boot() were added in
       version 187.

       sd_id128_get_invocation() was added in version 232.

       sd_id128_get_machine_app_specific() was added in version 233.

       sd_id128_get_boot_app_specific() was added in version 240.

       sd_id128_get_app_specific() was added in version 255.

SEE ALSO         top

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

NOTES         top

        1. RFC 4122

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
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       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At that
       time, the date of the most recent commit that was found in the
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       (which is not part of the original manual page), send a mail to

systemd 255                                      SD_ID128_GET_MACHINE(3)

Pages that refer to this page: systemd-id128(1)sd_bus_message_get_monotonic_usec(3)sd-id128(3)sd_id128_randomize(3)sd_journal_get_cutoff_realtime_usec(3)sd_journal_get_realtime_usec(3)machine-id(5)networkd.conf(5)