sd_journal_add_match, sd_journal_add_disjunction, sd_journal_add_conjunction, sd_journal_flush_matches — Add or remove entry matches
#include <systemd/sd-journal.h>
| int sd_journal_add_match( | sd_journal* j, | 
| const void* data, | |
| size_t size ); | 
| int sd_journal_add_disjunction( | sd_journal* j ); | 
| int sd_journal_add_conjunction( | sd_journal* j ); | 
| void sd_journal_flush_matches( | sd_journal* j ); | 
sd_journal_add_match() adds
                a match by which to filter the entries of the journal
                file. Matches applied with this call will filter what
                can be iterated through and read from the journal file
                via calls like
                sd_journal_next(3)
                and
                sd_journal_get_data(3). Matches
                are of the form FIELD=value, where
                the field part is a short uppercase string consisting
                only of 0-9, A-Z and the underscore. It may not begin
                with two underscores or be the empty string. The value
                part may be any value, including binary. If a match is
                applied only entries with this field set will be
                iterated. Multiple matches may be active at the same
                time: if they apply to different fields only entries
                with both fields set like this will be iterated, if
                they apply to the same fields only entries where the
                field takes one of the specified values will be
                iterated. Well known fields are documented in
                systemd.journal-fields(7). Whenever
                a new match is added the current entry position is
                reset, and
                sd_journal_next(3) (or a similar call)
                needs to be called before entries can be read
                again.
sd_journal_add_disjunction()
                may be used to insert a disjunction (i.e. logical OR)
                in the match list. If this call is invoked all
                previously added matches since the last invocation of
                sd_journal_add_disjunction() or
                sd_journal_add_conjunction() are
                combined in an OR with all matches added afterwards,
                until
                sd_journal_add_disjunction() or
                sd_journal_add_conjunction() is
                invoked again to begin the next OR or AND
                term. 
sd_journal_add_conjunction()
                may be used to insert a conjunction (i.e. logical AND)
                in the match list. If this call is invoked all
                previously added matches since the last invocation of
                sd_journal_add_conjunction() are
                combined in an AND with all matches added afterwards,
                until
                sd_journal_add_conjunction() is
                invoked again to begin the next AND term. The
                combination of
                sd_journal_add_match(),
                sd_journal_add_disjunction() and
                sd_journal_add_conjunction() may
                be used to build complex search terms, even though
                full logical expressions are not available. Note that
                sd_journal_add_conjunction()
                operates one level 'higher' than
                sd_journal_add_disjunction(). It
                is hence possible to build an expression of AND terms,
                consisting of OR terms, consisting of AND terms,
                consisting of OR terms of matches (the latter OR
                expression is implicitly created for matches with the
                same field name, see above).
sd_journal_flush_matches()
                may be used to flush all matches, disjunction and
                conjunction terms again. After this call all filtering
                is removed and all entries in the journal will be
                iterated again.
Note that filtering via matches only applies to the way the journal is read, it has no effect on storage on disk.
sd_journal_add_match(),
                sd_journal_add_disjunction() and
                sd_journal_add_conjunction()
                return 0 on success or a negative errno-style error
                code. sd_journal_flush_matches()
                returns nothing.
The sd_journal_add_match(),
                sd_journal_add_disjunction(),
                sd_journal_add_conjunction() and
                sd_journal_flush_matches()
                interfaces are available as shared library, which can
                be compiled and linked to with the
                libsystemd-journal
                pkg-config(1)
                file.
The following example adds matches to a journal context object to iterate only through messages generated by the Avahi service at the four error log levels, plus all messages of the message ID 03bb1dab98ab4ecfbf6fff2738bdd964 coming from any service (this example lacks the necessary error checking):
...
int add_matches(sd_journal *j) {
        sd_journal_add_match(j, "_SYSTEMD_UNIT=avahi-daemon.service", 0);
        sd_journal_add_match(j, "PRIORITY=0", 0);
        sd_journal_add_match(j, "PRIORITY=1", 0);
        sd_journal_add_match(j, "PRIORITY=2", 0);
        sd_journal_add_match(j, "PRIORITY=3", 0);
        sd_journal_add_disjunction(j);
        sd_journal_add_match(j, "MESSAGE_ID=03bb1dab98ab4ecfbf6fff2738bdd964", 0);
}