systemd sd-journal 3 sd-journal APIs for submitting and querying log entries to and from the journal #include <systemd/sd-journal.h> pkg-config --cflags --libs libsystemd sd-journal.h is part of libsystemd3 and provides APIs to submit and query log entries. The APIs exposed act both as client for the systemd-journald.service8 journal service and as parser for the journal files on disk. See sd_journal_print3, sd_journal_stream_fd3, sd_journal_open3, sd_journal_next3, sd_journal_get_realtime_usec3, sd_journal_add_match3, sd_journal_seek_head3, sd_journal_enumerate_fields3, sd_journal_get_cursor3, sd_journal_get_cutoff_realtime_usec3, sd_journal_get_cutoff_monotonic_usec3, sd_journal_get_usage3, sd_journal_get_catalog3, sd_journal_get_fd3, sd_journal_has_runtime_files3 and sd_journal_has_persistent_files3 for more information about the functions implemented. Command line access for submitting entries to the journal is available with the systemd-cat1 tool. Command line access for querying entries from the journal is available with the journalctl1 tool. Functions that operate on sd_journal objects are thread agnostic — given sd_journal pointer may only be used from one specific thread at all times (and it has to be the very same one during the entire lifetime of the object), but multiple, independent threads may use multiple, independent objects safely. Other functions — those that are used to send entries to the journal, like sd_journal_print3 and similar, or those that are used to retrieve global information like sd_journal_stream_fd3 and sd_journal_get_catalog_for_message_id3 — are fully thread-safe and may be called from multiple threads in parallel. Depending on which build-time options are enabled, functions that operate on sd_journal objects might cause optional shared libraries to be dynamically loaded via dlopen3, such as decompression libraries (xz, lz4, zstd) or cryptographic libraries (gcrypt). systemd1 sd_journal_print3 sd_journal_stream_fd3 sd_journal_open3 sd_journal_next3 sd_journal_get_data3 sd_journal_get_realtime_usec3 sd_journal_add_match3 sd_journal_seek_head3 sd_journal_enumerate_fields3 sd_journal_get_cursor3 sd_journal_get_cutoff_realtime_usec3 sd_journal_get_cutoff_monotonic_usec3 sd_journal_get_usage3 sd_journal_get_fd3 sd_journal_query_unique3 sd_journal_get_catalog3 sd_journal_has_runtime_files3 sd_journal_has_persistent_files3 journalctl1 sd-id1283 pkg-config1