sd_journal_has_persistent_files() returns a positive value if persistent journal files (present in /var/log/journal/) have been found. Otherwise returns 0.
Both sd_journal_has_runtime_files() and sd_journal_has_persistent_files() return -EINVAL if their argument is NULL.
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.