NAME

       sd_journal_enumerate_fields, sd_journal_restart_fields, SD_JOURNAL_FOREACH_FIELD - Read
       used field names from the journal

SYNOPSIS

       #include <systemd/sd-journal.h>

       int sd_journal_enumerate_fields(sd_journal *j, const char **field);

       void sd_journal_restart_fields(sd_journal *j);

       SD_JOURNAL_FOREACH_FIELD(sd_journal *j, const char *field);

DESCRIPTION

       sd_journal_enumerate_fields() may be used to iterate through all field names used in the
       opened journal files. On each invocation the next field name is returned. The order of the
       returned field names is not defined. It takes two arguments: the journal context object,
       plus a pointer to a constant string pointer where the field name is stored in. The
       returned data is in a read-only memory map and is only valid until the next invocation of
       sd_journal_enumerate_fields(). Note that this call is subject to the data field size
       threshold as controlled by sd_journal_set_data_threshold().

       sd_journal_restart_fields() resets the field name enumeration index to the beginning of
       the list. The next invocation of sd_journal_enumerate_fields() will return the first field
       name again.

       The SD_JOURNAL_FOREACH_FIELD() macro may be used as a handy wrapper around
       sd_journal_restart_fields() and sd_journal_enumerate_fields().

       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 retrieve the possible values a specific field can take use sd_journal_query_unique(3).

RETURN VALUE

       sd_journal_enumerate_fields() returns a positive integer if the next field name has been
       read, 0 when no more field names are known, or a negative errno-style error code.
       sd_journal_restart_fields() 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.

       Functions described here are available as a shared library, which can be compiled against
       and linked to with the libsystemd pkg-config(1) file.

EXAMPLES

       Use the SD_JOURNAL_FOREACH_FIELD() macro to iterate through all field names in use in the
       current journal.

           /* SPDX-License-Identifier: MIT-0 */

           #include <errno.h>
           #include <stdio.h>
           #include <systemd/sd-journal.h>

           int main(int argc, char *argv[]) {
             sd_journal *j;
             const char *field;
             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;
             }
             SD_JOURNAL_FOREACH_FIELD(j, field)
               printf("%s\n", field);
             sd_journal_close(j);
             return 0;
           }

HISTORY

       sd_journal_enumerate_fields(), sd_journal_restart_fields(), and SD_JOURNAL_FOREACH_FIELD()
       were added in version 229.

SEE ALSO

       systemd(1), systemd.journal-fields(7), sd-journal(3), sd_journal_open(3),
       sd_journal_query_unique(3), sd_journal_get_data(3), sd_journal_add_match(3)