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SD_HWDB_GET(3) sd_hwdb_get SD_HWDB_GET(3)


sd_hwdb_get, sd_hwdb_seek, sd_hwdb_enumerate, SD_HWDB_FOREACH_PROPERTY - Seek to a location in hwdb or access entries


#include <systemd/sd-hwdb.h>

int sd_hwdb_get(sd_hwdb *hwdb, const char *modalias, const char *key, const char **value);

int sd_hwdb_seek(sd_hwdb *hwdb, const char *modalias);

int sd_hwdb_enumerate(sd_hwdb *hwdb, const char **key, const char **value);

SD_HWDB_FOREACH_PROPERTY(hwdb, modalias, key, value);


sd_hwdb_get() queries the hwdb object created earlier with sd_hwdb_new(3) for entries matching the specified string modalias, and returns the value corresponding to the key key. The value is returned as a NUL-terminated string in value. It must not be modified by the caller and is valid as long as a reference to hwdb is kept. When multiple patterns in the database match modalias, the one with the highest priority is used. See hwdb(7) for details.

sd_hwdb_seek() selects entries matching the specified string modalias. Subsequent queries with sd_hwdb_enumerate() will access the key-value pairs for that string.

sd_hwdb_enumerate() returns (in turn) all the key-value pairs defined for the string used with sd_hwdb_seek(). Each pair is returned as NUL-terminated strings in key and value. The strings must not be modified by the caller and are valid as long as a reference to hwdb is kept. When multiple patterns in the database match modalias, the combination of all matching key-value pairs is used. See hwdb(7) for details.

The SD_HWDB_FOREACH_PROPERTY() macro combines sd_hwdb_seek() and sd_hwdb_enumerate(). No error handling is performed and iteration simply stops on error. See the example below.


On success, sd_hwdb_get() and sd_hwdb_seek() return a non-negative integer. On failure, they return a negative errno-style error code.

sd_hwdb_enumerate() returns a positive integer if another key-value pair was found or zero if all entries have already been enumerated. On failure, it returns a negative errno-style error code.


Returned errors may indicate the following problems:


A parameter is NULL.


An entry for the specified modalias was not found.


sd_hwdb_seek() was not called before sd_hwdb_enumerate().


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.


Example 1. Look up hwdb entries for a USB device

/* SPDX-License-Identifier: MIT-0 */
#include <stdio.h>
#include <stdint.h>
#include <systemd/sd-hwdb.h>
int print_usb_properties(uint16_t vid, uint16_t pid) {

char match[128];
sd_hwdb *hwdb;
const char *key, *value;
int r;
/* Match this USB vendor and product ID combination */
snprintf(match, sizeof match, "usb:v%04Xp%04X", vid, pid);
r = sd_hwdb_new(&hwdb);
if (r < 0)
return r;
SD_HWDB_FOREACH_PROPERTY(hwdb, match, key, value)
printf("%s: \"%s\" → \"%s\"\n", match, key, value);
return 0; } int main(int argc, char **argv) {
print_usb_properties(0x046D, 0xC534);
return 0; }

The effect is similar to calling systemd-hwdb query usb:v046DpC534.


systemd(1), systemd-udevd.service(8), sd-hwdb(3), systemd-hwdb(8)

systemd 254