Index · Directives systemd 250


sd_journal_query_unique, sd_journal_enumerate_unique, sd_journal_enumerate_available_unique, sd_journal_restart_unique, SD_JOURNAL_FOREACH_UNIQUE — Read unique data fields from the journal


#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);
 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.

Return Value

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() doesn't return anything.


Returned errors may indicate the following problems:


One of the required parameters is NULL or invalid.


The journal object was created in a different process.


The read pointer is not positioned at a valid entry; sd_journal_next(3) or a related call has not been called at least once.


The current entry does not include the specified field.


A compressed entry is too large.


The data field is too large for this computer architecture (e.g. above 4 GB on a 32-bit architecture).


The journal is compressed with an unsupported method or the journal uses an unsupported feature.


The journal is corrupted (possibly just the entry being iterated over).


An I/O error was reported by the kernel.


All functions listed here are thread-agnostic and only a single specific thread may operate on a given object during its entire lifetime. It's safe to allocate multiple independent objects and use each from a specific thread in parallel. However, it's not safe to allocate such an object in one thread, and operate or free it from any other, even if locking is used to ensure these threads don't operate on it at the very same time.

These APIs are implemented as a shared library, which can be compiled 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: CC0-1.0 */

#include <stdio.h>
#include <string.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;
    printf("%.*s\n", (int) l, (const char*) d);
  return 0;

See Also

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)