sd_journal_foreach_unique(3) — Linux manual page



NAME         top

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

SYNOPSIS         top

       #include <systemd/sd-journal.h>

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

       int sd_journal_enumerate_available_unique(sd_journal *j,
                                                 const void **data,
                                                 size_t *length);

       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         top

       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), but any field can be specified.
       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() and
       sd_journal_enumerate_available_unique(). Invoking one of those calls
       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 object,
       plus a pair of pointers to pointer/size variables where the data
       object and its size shall be stored. 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() and only the initial part of the
       field up to the threshold is returned. An error is returned for
       fields which cannot be retrieved. See the error list below for

       sd_journal_enumerate_available_unique() is similar to
       sd_journal_enumerate_unique(), but silently skips any fields which
       may be valid, but are too large or not supported by current

       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

       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         top

       sd_journal_query_unique() returns 0 on success or a negative
       errno-style error code.  sd_journal_enumerate_unique() and and
       sd_journal_query_available_unique() return a positive integer if the
       next field data has been read, 0 when no more fields remain, or a
       negative errno-style error code.  sd_journal_restart_unique() doesn't
       return anything.

       Returned errors may indicate the following problems:

           One of the required parameters is NULL or invalid.

           The journal object was created in a different process.

           The read pointer is not positioned at a valid entry;
           sd_journal_next(3) or a related call has not been called at least

           The current entry does not include the specified field.

           A compressed entry is too large.

           The data field is too large for this computer architecture (e.g.
           above 4 GB on a 32-bit architecture).

           The journal is compressed with an unsupported method or the
           journal uses an unsupported feature.

           The journal is corrupted (possibly just the entry being iterated

           An I/O error was reported by the kernel.

NOTES         top

       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         top

       Use the SD_JOURNAL_FOREACH_UNIQUE macro to iterate through all values
       a field of the journal can take (and which can be accessed on the
       given architecture and are not compressed with an unsupported
       mechanism). 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);
             return 0;

SEE ALSO         top

       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)

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug
       report for this manual page, see
       ⟨⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨⟩ on 2020-11-01.  (At that
       time, the date of the most recent commit that was found in the repos‐
       itory was 2020-11-01.)  If you discover any rendering problems in
       this HTML version of the page, or you believe there is a better or
       more up-to-date source for the page, or you have corrections or im‐
       provements to the information in this COLOPHON (which is not part of
       the original manual page), send a mail to

systemd 247                                       SD_JOURNAL_QUERY_UNIQUE(3)