# sd_id128_get_machine

## NAME



           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



           **#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_**);**
    
           **int sd_id128_get_invocation_app_specific(sd_id128_t** _app_id_**,**
                                                    **sd_id128_t ***_ret_**);**
    


## DESCRIPTION



           **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.
    
           **sd_id128_get_invocation_app_specific()** derives an
           application-specific ID from the invocation ID.
    
           Note that **sd_id128_get_machine_app_specific()** ,
           **sd_id128_get_boot()** , **sd_id128_get_boot_app_specific()** ,
           **sd_id128_get_invocation()** and **sd_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, 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



           Those calls return 0 on success (in which case _ret_ is filled in),
           or a negative errno-style error code.
    
       **Errors**
           Returned errors may indicate the following problems:
    
           **-ENOENT**
               Returned by **sd_id128_get_machine()** and
               **sd_id128_get_machine_app_specific()** when /etc/machine-id is
               missing.
    
               Added in version 242.
    
           **-ENOMEDIUM**
               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 zeros.
    
               Added in version 242.
    
           **-ENOPKG**
               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.
    
           **-ENOSYS**
               Returned by **sd_id128_get_boot()** and
               **sd_id128_get_boot_app_specific()** when /proc/ is not mounted.
    
               Added in version 253.
    
           **-ENXIO**
               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.
    
           **-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.
    


## 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.
    


## EXAMPLES



           **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;
               }
    


## NOTES



            1. RFC 4122
               https://tools.ietf.org/html/rfc4122
    


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