sd_journal_get_usage man page
sd_journal_get_usage — Journal disk usage
int sd_journal_get_usage(sd_journal *j, uint64_t *bytes);
sd_journal_get_usage() determines the total disk space currently used by journal files (in bytes). If SD_JOURNAL_LOCAL_ONLY was passed when opening the journal, this value will only reflect the size of journal files of the local host, otherwise of all hosts.
sd_journal_get_usage() returns 0 on success or a negative errno-style error code.
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.
systemd(1), sd-journal(3), sd_journal_open(3),
sd-journal(3), systemd.directives(7), systemd.index(7).