sd_journal_query_unique - Man Page

#include <systemd/sd-journal​.h>

int sd_journal_query_unique(sd_journal *j, const char *field);

int sd_journal_enumerate_available_unique(sd_journal *j, const void **data, size_t *length);

int sd_journal_enumerate_unique(sd_journal *j, const void **data, size_t *length);

void sd_journal_restart_unique(sd_journal *j);

SD_JOURNAL_FOREACH_UNIQUE(sd_journal *j, const void *data, size_t length);

sd_journal_query_unique() queries the journal for all unique values the specified field can take​. It takes two arguments: the journal to query and the field name to look for​. Well-known field names are listed on systemd.journal-fields(7), but any field can be specified​. Field names must be specified without a trailing "="​. After this function has been executed successfully the field values may be queried using sd_journal_enumerate_unique() and sd_journal_enumerate_available_unique()​. Invoking one of those calls will change the field name being queried and reset the enumeration index to the first field value that matches​.

sd_journal_enumerate_unique() may be used to iterate through all data fields which match the previously selected field name as set with sd_journal_query_unique()​. On each invocation the next field data matching the field name is returned​. The order of the returned data fields is not defined​. It takes three arguments: the journal object, plus a pair of pointers to pointer/size variables where the data object and its size shall be stored​. The returned data is in a read-only memory map and is only valid until the next invocation of sd_journal_enumerate_unique()​. Note that the data returned will be prefixed with the field name and "="​. Note that this call is subject to the data field size threshold as controlled by sd_journal_set_data_threshold() and only the initial part of the field up to the threshold is returned​. An error is returned for fields which cannot be retrieved​. See the error list below for details​.

sd_journal_enumerate_available_unique() is similar to sd_journal_enumerate_unique(), but silently skips any fields which may be valid, but are too large or not supported by current implementation​.

sd_journal_restart_unique() resets the data enumeration index to the beginning of the list​. The next invocation of sd_journal_enumerate_unique() will return the first field data matching the field name again​.

Note that the SD_JOURNAL_FOREACH_UNIQUE() macro may be used as a handy wrapper around sd_journal_restart_unique() and sd_journal_enumerate_available_unique()​.

Note that these functions currently are not influenced by matches set with sd_journal_add_match() but this might change in a later version of this software​.

To enumerate all field names currently in use (and thus all suitable field parameters for sd_journal_query_unique()), use the sd_journal_enumerate_fields(3) call​.

sd_journal_query_unique() returns 0 on success or a negative errno-style error code​. sd_journal_enumerate_unique() and sd_journal_query_available_unique() return a positive integer if the next field data has been read, 0 when no more fields remain, or a negative errno-style error code​. sd_journal_restart_unique() does not return anything​.

All functions listed here are thread-agnostic and only a single thread may operate on a given object at any given time​. Different threads may access the same object at different times​. Multiple independent objects may be used from different threads in parallel​.

Functions described here are available as a shared library, which can be compiled against and linked to with the libsystemd ​pkg-config(1) file​.

Use the SD_JOURNAL_FOREACH_UNIQUE() macro to iterate through all values a field of the journal can take (and which can be accessed on the given architecture and are not compressed with an unsupported mechanism)​. The following example lists all unit names referenced in the journal:

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

#include <errno​.h>
#include <stdio​.h>
#include <systemd/sd-journal​.h>

int main(int argc, char *argv[]) {
  sd_journal *j;
  const void *d;
  size_t l;
  int r;

  r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
  if (r < 0) {
    fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
    return 1;
  }
  r = sd_journal_query_unique(j, "_SYSTEMD_UNIT");
  if (r < 0) {
    fprintf(stderr, "Failed to query journal: %s\n", strerror(-r));
    return 1;
  }
  SD_JOURNAL_FOREACH_UNIQUE(j, d, l)
    printf("%​.*s\n", (int) l, (const char*) d);
  sd_journal_close(j);
  return 0;
}

sd_journal_query_unique(), sd_journal_enumerate_unique(), sd_journal_restart_unique(), and SD_JOURNAL_FOREACH_UNIQUE() were added in version 195​.

sd_journal_enumerate_available_unique() was added in version 246​.

systemd(1), systemd.journal-fields(7), sd-journal(3), sd_journal_open(3), sd_journal_enumerate_fields(3), sd_journal_get_data(3), sd_journal_add_match(3)

sd-journal(3), sd_journal_enumerate_fields(3), sd_journal_get_data(3), systemd.directives(7), systemd.index(7).

The man pages sd_journal_enumerate_available_unique(3), sd_journal_enumerate_unique(3), SD_JOURNAL_FOREACH_UNIQUE(3) and sd_journal_restart_unique(3) are aliases of sd_journal_query_unique(3).