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sd_journal_query_unique, sd_journal_enumerate_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_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). 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(). Invoking this call a second time 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 context object, plus a pair of pointers to pointer/size variables where the data object and its size shall be stored in. 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().

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_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() returns a positive integer if the next field data has been read, 0 when no more fields are known, or a negative errno-style error code. sd_journal_restart_unique() returns nothing.


All functions listed here are thread-agnostic and only a single thread may operate on a given sd_journal object.

The sd_journal_query_unique(), sd_journal_enumerate_unique() and sd_journal_restart_unique() interfaces are available 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. The following example lists all unit names referenced in the journal:

#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;


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)
systemd 232