sd_journal_query_unique(3) — Linux manual page


SD_JOURN...RY_UNIQUE(3)  sd_journal_query_unique SD_JOURN...RY_UNIQUE(3)

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

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 details.

       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
       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.

           Added in version 246.

           The journal object was created in a different process,
           library or module instance.

           Added in version 246.

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

           Added in version 246.

           The current entry does not include the specified field.

           Added in version 246.

           A compressed entry is too large.

           Added in version 246.

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

           Added in version 246.

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

           Added in version 246.

           The journal is corrupted (possibly just the entry being
           iterated over).

           Added in version 246.

           An I/O error was reported by the kernel.

           Added in version 246.

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.

       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         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:

           /* 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 void *d;
             size_t l;
             int r;

             r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
             if (r < 0) {
               errno = -r;
               fprintf(stderr, "Failed to open journal: %m\n");
               return 1;
             r = sd_journal_query_unique(j, "_SYSTEMD_UNIT");
             if (r < 0) {
               errno = -r;
               fprintf(stderr, "Failed to query journal: %m\n");
               return 1;
             SD_JOURNAL_FOREACH_UNIQUE(j, d, l)
               printf("%.*s\n", (int) l, (const char*) d);
             return 0;

HISTORY         top

       sd_journal_query_unique(), sd_journal_enumerate_unique(),
       sd_journal_restart_unique(), and SD_JOURNAL_FOREACH_UNIQUE() were
       added in version 195.

       sd_journal_enumerate_available_unique() was added in version 246.

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

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       manager) project.  Information about the project can be found at
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       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2023-12-22.)  If you discover any rendering
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systemd 255                                      SD_JOURN...RY_UNIQUE(3)

Pages that refer to this page: sd-journal(3)sd_journal_enumerate_fields(3)sd_journal_get_data(3)systemd.directives(7)systemd.index(7)