Index · Directives systemd 250


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);
 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.

Return Value

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.


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 once.


The current entry does not include the specified field.


Memory allocation failed.


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 over).


An I/O error was reported by the kernel.


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.


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);

See Also

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)