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NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |NOTES |EXAMPLES |HISTORY |SEE ALSO |NOTES |COLOPHON | |
SD_ID128_GET_MACHINE(3) sd_id128_get_machineSD_ID128_GET_MACHINE(3)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
#include <systemd/sd-id128.h>int sd_id128_get_machine(sd_id128_t *ret);int sd_id128_get_app_specific(sd_id128_tbase, sd_id128_tapp_id,sd_id128_t *ret);int sd_id128_get_machine_app_specific(sd_id128_tapp_id,sd_id128_t *ret);int sd_id128_get_boot(sd_id128_t *ret);int sd_id128_get_boot_app_specific(sd_id128_tapp_id,sd_id128_t *ret);int sd_id128_get_invocation(sd_id128_t *ret);int sd_id128_get_invocation_app_specific(sd_id128_tapp_id,sd_id128_t *ret);
sd_id128_get_machine()returns the machine ID of the executing host. This reads and parses themachine-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 easysd_id128_get_machine_app_specific()is provided, see below.sd_id128_get_app_specific()returns a machine ID that is a combination of thebase andapp_id parameters. Internally, this function calculates HMAC-SHA256 of theapp_id parameter keyed by thebase parameter, and truncates this result to fit in sd_id128_t and turns it into a valid Variant 1 Version 4 UUID, in accordance withRFC 4122[1]. Neither of the two input parameters can be calculated from the output parameterret.sd_id128_get_machine_app_specific()is similar tosd_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 ofsd_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 likesystemd-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 callssd_id128_get_app_specific()with the result fromsd_id128_get_machine()and theapp_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. Seerandom(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 usingsd_id128_get_boot_app_specific(), see below.sd_id128_get_boot_app_specific()is analogous tosd_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. Seesystemd.exec(5) for details. The ID is cached internally. In future a different mechanism to determine the invocation ID may be added.sd_id128_get_invocation_app_specific()derives an application-specific ID from the invocation ID. Note thatsd_id128_get_machine_app_specific(),sd_id128_get_boot(),sd_id128_get_boot_app_specific(),sd_id128_get_invocation()andsd_id128_get_invocation_app_specific 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, seemachine-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 ofsd_id128_get_machine(), as mentioned. For more information about the "sd_id128_t" type seesd-id128(3).
Those calls return 0 on success (in which caseret is filled in), or a negative errno-style error code.Errors Returned errors may indicate the following problems:-ENOENT Returned bysd_id128_get_machine()andsd_id128_get_machine_app_specific()when /etc/machine-id is missing. Added in version 242.-ENOMEDIUM Returned bysd_id128_get_machine()andsd_id128_get_machine_app_specific()when /etc/machine-id is empty or all zeros. Also returned bysd_id128_get_invocation() when the invocation ID is all zeros. Added in version 242.-ENOPKG Returned bysd_id128_get_machine()andsd_id128_get_machine_app_specific()when the content of /etc/machine-id is "uninitialized". Added in version 253.-ENOSYS Returned bysd_id128_get_boot()andsd_id128_get_boot_app_specific()when /proc/ is not mounted. Added in version 253.-ENXIO Returned bysd_id128_get_invocation()if no invocation ID is set. Also returned bysd_id128_get_app_specific(),sd_id128_get_machine_app_specific(), andsd_id128_get_boot_app_specific()when theapp_id parameter is all zeros. Added in version 242.-EUCLEAN Returned by any of the functions described here when the configured value has invalid format. Added in version 253.-EPERM Requested information could not be retrieved because of insufficient permissions. Added in version 242.
Functions described here are available as a shared library, which can be compiled against and linked to with thelibsystemd pkg-config(1) file. The code described here usesgetenv(3), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not callsetenv(3) from a parallel thread. It is recommended to only do calls tosetenv()from an early phase of the program when no other threads have been started.
Example 1. Application-specific machine ID First, generate the application ID: $ systemd-id128 -p new As string: c273277323db454ea63bb96e79b53e97 As UUID: c2732773-23db-454e-a63b-b96e79b53e97 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; }sd_id128_get_machine()andsd_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.sd_id128_get_invocation_app_specific()was added in version 256.
systemd(1),systemd-id128(1),sd-id128(3),machine-id(5),systemd.exec(5),sd_id128_randomize(3),random(4)
1. RFC 4122https://tools.ietf.org/html/rfc4122
This page is part of thesystemd (systemd system and service manager) project. Information about the project can be found at ⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug report for this manual page, see ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩. This page was obtained from the project's upstream Git repository ⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that time, the date of the most recent commit that was found in the repository was 2025-08-11.) If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up-to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgsystemd 258~rc2SD_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), systemd.network(5), systemd.directives(7), systemd.index(7), pam_systemd(8)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |