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.
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.
The sd_journal_query_unique(),
                sd_journal_enumerate_unique() and
                sd_journal_restart_unique()
                interfaces are available as shared library, which can
                be compiled and linked to with the
                libsystemd-journal
                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;
        }
        SD_JOURNAL_FOREACH_UNIQUE(j, d, l)
                printf("%.*s\n", (int) l, (const char*) d);
        sd_journal_close(j);
        return 0;
}