sd_journal_get_data(3) — Linux manual page


SD_JOURNAL_GET_DATA(3)     sd_journal_get_data    SD_JOURNAL_GET_DATA(3)

NAME         top

       sd_journal_get_data, sd_journal_enumerate_data,
       sd_journal_enumerate_available_data, sd_journal_restart_data,
       SD_JOURNAL_FOREACH_DATA, sd_journal_set_data_threshold,
       sd_journal_get_data_threshold - Read data fields from the current
       journal entry

SYNOPSIS         top

       #include <systemd/sd-journal.h>

       int sd_journal_get_data(sd_journal *j, const char *field,
                               const void **data, size_t *length);

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

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

       void sd_journal_restart_data(sd_journal *j);

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

       int sd_journal_set_data_threshold(sd_journal *j, size_t sz);

       int sd_journal_get_data_threshold(sd_journal *j, size_t *sz);

DESCRIPTION         top

       sd_journal_get_data() gets the data object associated with a
       specific field from the current journal entry. It takes four
       arguments: the journal context object, a string with the field
       name to request, plus a pair of pointers to pointer/size
       variables where the data object and its size shall be stored in.
       The field name should be an entry field name. Well-known field
       names are listed in systemd.journal-fields(7), but any field can
       be specified. The returned data is in a read-only memory map and
       is only valid until the next invocation of sd_journal_get_data(),
       sd_journal_enumerate_available_data(), or when the read pointer
       is altered. Note that the data returned will be prefixed with the
       field name and "=". Also note that, by default, data fields
       larger than 64K might get truncated to 64K. This threshold may be
       changed and turned off with sd_journal_set_data_threshold() (see

       sd_journal_enumerate_data() may be used to iterate through all
       fields of the current entry. On each invocation the data for the
       next field is returned. The order of these fields is not defined.
       The data returned is in the same format as with
       sd_journal_get_data() and also follows the same life-time

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

       sd_journal_restart_data() resets the data enumeration index to
       the beginning of the entry. The next invocation of
       sd_journal_enumerate_data() will return the first field of the
       entry again.

       Note that the SD_JOURNAL_FOREACH_DATA() macro may be used as a
       handy wrapper around sd_journal_restart_data() and

       Note that these functions will not work before sd_journal_next(3)
       (or related call) has been called at least once, in order to
       position the read pointer at a valid entry.

       sd_journal_set_data_threshold() may be used to change the data
       field size threshold for data returned by sd_journal_get_data(),
       sd_journal_enumerate_data() and sd_journal_enumerate_unique().
       This threshold is a hint only: it indicates that the client
       program is interested only in the initial parts of the data
       fields, up to the threshold in size — but the library might still
       return larger data objects. That means applications should not
       rely exclusively on this setting to limit the size of the data
       fields returned, but need to apply an explicit size limit on the
       returned data as well. This threshold defaults to 64K by default.
       To retrieve the complete data fields this threshold should be
       turned off by setting it to 0, so that the library always returns
       the complete data objects. It is recommended to set this
       threshold as low as possible since this relieves the library from
       having to decompress large compressed data objects in full.

       sd_journal_get_data_threshold() returns the currently configured
       data field size threshold.

RETURN VALUE         top

       sd_journal_get_data() returns 0 on success or a negative
       errno-style error code.  sd_journal_enumerate_data() and
       sd_journal_enumerate_available_data() return a positive integer
       if the next field has been read, 0 when no more fields remain, or
       a negative errno-style error code.  sd_journal_restart_data()
       doesn't return anything.  sd_journal_set_data_threshold() and
       sd_journal_get_threshold() return 0 on success or a negative
       errno-style error code.

       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.

           Memory allocation failed.

           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

       See sd_journal_next(3) for a complete example how to use

       Use the SD_JOURNAL_FOREACH_DATA() macro to iterate through all
       fields of the current journal entry:

           int print_fields(sd_journal *j) {
             const void *data;
             size_t length;
             SD_JOURNAL_FOREACH_DATA(j, data, length)
               printf("%.*s\n", (int) length, data);

HISTORY         top

       sd_journal_get_data(), sd_journal_enumerate_data(),
       sd_journal_restart_data(), and SD_JOURNAL_FOREACH_DATA() were
       added in version 187.

       sd_journal_set_data_threshold() and
       sd_journal_get_data_threshold() were added in version 196.

       sd_journal_enumerate_available_data() was added in version 246.

SEE ALSO         top

       systemd(1), systemd.journal-fields(7), sd-journal(3),
       sd_journal_open(3), sd_journal_next(3),
       sd_journal_get_realtime_usec(3), sd_journal_query_unique(3)

COLOPHON         top

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       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 2024-06-14.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2024-06-13.)  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
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       (which is not part of the original manual page), send a mail to

systemd 257~devel                                 SD_JOURNAL_GET_DATA(3)

Pages that refer to this page: sd-journal(3)sd_journal_add_match(3)sd_journal_enumerate_fields(3)sd_journal_get_catalog(3)sd_journal_get_realtime_usec(3)sd_journal_get_seqnum(3)sd_journal_next(3)sd_journal_open(3)sd_journal_query_unique(3)sd_journal_seek_head(3)systemd.directives(7)systemd.index(7)