NAME

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

SYNOPSIS

       #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);

DESCRIPTION

       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.

RETURN VALUE

       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.

NOTES

       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.

EXAMPLES

       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;
           }

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)