Description¶

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.

Return Value¶

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.

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¶

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

#define _GNU_SOURCE 1
#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);

  sd_hwdb_unref(hwdb);
  return 0;
}

int main(int argc, char **argv) {
  print_usb_properties(0x046D, 0xC534);
  return 0;
}

History¶

sd_hwdb_get(), sd_hwdb_seek(), sd_hwdb_enumerate(), and SD_HWDB_FOREACH_PROPERTY() were added in version 246.

See Also¶

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