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 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.

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)