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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | EXAMPLES | HISTORY | SEE ALSO | COLOPHON |
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SD_JOURNAL_GET_DATA(3) sd_journal_get_data SD_JOURNAL_GET_DATA(3)
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
#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);
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_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 below).
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
semantics.
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
implementation.
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
sd_journal_enumerate_available_data().
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.
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() does
not return anything. sd_journal_set_data_threshold() and
sd_journal_get_threshold() return 0 on success or a negative
errno-style error code.
Errors
Returned errors may indicate the following problems:
-EINVAL
One of the required parameters is NULL or invalid.
Added in version 246.
-ECHILD
The journal object was created in a different process, library
or module instance.
Added in version 246.
-EADDRNOTAVAIL
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.
-ENOENT
The current entry does not include the specified field.
Added in version 246.
-ENOMEM
Memory allocation failed.
Added in version 246.
-ENOBUFS
A compressed entry is too large.
Added in version 246.
-E2BIG
The data field is too large for this computer architecture
(e.g. above 4 GB on a 32-bit architecture).
Added in version 246.
-EPROTONOSUPPORT
The journal is compressed with an unsupported method or the
journal uses an unsupported feature.
Added in version 246.
-EBADMSG
The journal is corrupted (possibly just the entry being
iterated over).
Added in version 246.
-EIO
An I/O error was reported by the kernel.
Added in version 246.
All functions listed here are thread-agnostic and only a single
specific thread may operate on a given object during its entire
lifetime. It is safe to allocate multiple independent objects and
use each from a specific thread in parallel. However, it is 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
do not 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.
See sd_journal_next(3) for a complete example how to use
sd_journal_get_data().
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);
}
...
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.
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)
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) 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 improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 258~rc2 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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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