systemd Developer Lennart Poettering lennart@poettering.net sd_journal_get_fd 3 sd_journal_get_fd sd_journal_reliable_fd sd_journal_process sd_journal_wait SD_JOURNAL_NOP SD_JOURNAL_APPEND SD_JOURNAL_INVALIDATE Journal change notification interface #include <systemd/sd-journal.h> int sd_journal_get_fd sd_journal* j int sd_journal_reliable_fd sd_journal* j int sd_journal_process sd_journal* j int sd_journal_wait sd_journal* j uint64_t timeout_usec sd_journal_get_fd() returns a file descriptor that may be asynchronously polled in an external event loop and is signaled readable as soon as the journal changes, because new entries or files were added, rotation took place, or files have been deleted, and similar. The file descriptor is suitable for usage in poll2 where it will yield POLLIN on changes. The call takes one argument: the journal context object. Note that not all file systems are capable of generating the necessary events for wakeups from this file descriptor to be enirely reliable. In particular network files systems do not generate suitable file change events in all cases. In such a case an application should not rely alone on wake-ups from this file descriptor but wake up and recheck the journal in regular time intervals, for example every 2s. To detect cases where this is necessary, use sd_journal_reliable_fd(), below. sd_journal_reliable_fd() may be used to check whether the wakeup events from the file descriptor returned by sd_journal_get_fd are sufficient to track changes to the journal. If this call returns 0, it is necessary to regularly recheck for journal changes (suggestion: every 2s). If this call returns a positive integer this is not necessary, and wakeups from the file descriptor returned by sd_journal_get_fd() are sufficient as only source for wake-ups. After each POLLIN wake-up sd_journal_process() needs to be called to process events and reset the readable state of the file descriptor. This call will also indicate what kind of change has been detected (see below; note that spurious wake-ups are possible). A synchronous alternative for using sd_journal_get_fd(), sd_journal_reliable_fd() and sd_journal_process() is sd_journal_wait(). It will synchronously wait until the journal gets changed, possibly using a 2s time-out if this is necessary (see above). In either way the maximum time this call sleeps may be controlled with the timeout_usec parameter. Pass (uint64_t) -1 to wait indefinitely. Internally this call simply combines sd_journal_get_fd(), sd_journal_reliable_fd(), poll() and sd_journal_process() into one. sd_journal_get_fd() returns a valid file descriptor on success or a negative errno-style error code. sd_journal_reliable_fd() returns a positive integer if the file descriptor returned by sd_journal_get_fd() is sufficient as sole wake-up source for journal change events. Returns 0 if it is not sufficient and the journal needs to be checked manually in regular time intervals for changes. Returns a negative errno-style error code on failure. sd_journal_process() and sd_journal_wait() return one of SD_JOURNAL_NOP, SD_JOURNAL_APPEND or SD_JOURNAL_INVALIDATE on success or a negative errno-style error code. If SD_JOURNAL_NOP is returned the journal didn't change since the last invocation. If SD_JOURNAL_APPEND is returned new entries have been appended to the end of the journal. If SD_JOURNAL_INVALIDATE journal files were added or removed (possibly due to rotation). In the latter event live-view UIs should probably refresh their entire display while in the case of SD_JOURNAL_APPEND it is sufficient to simply continue reading at the previous end of the journal. The sd_journal_get_fd(), sd_journal_reliable_fd(), sd_journal_process() and sd_journal_wait() interfaces are available as shared library, which can be compiled and linked to with the libsystemd-journal pkg-config1 file. Iterating through the journal, in a live view tracking all changes: #include <stdio.h> #include <string.h> #include <systemd/sd-journal.h> int main(int argc, char *argv[]) { int r; sd_journal *j; r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY); if (r < 0) { fprintf(stderr, "Failed to open journal: %s\n", strerror(-r)); return 1; } for (;;) { const char *d; size_t l; r = sd_journal_next(j); if (r < 0) { fprintf(stderr, "Failed to iterate to next entry: %s\n", strerror(-r)); break; } if (r == 0) { /* Reached the end, let's wait for changes, and try again */ r = sd_journal_wait(j, (uint64_t) -1); if (r < 0) { fprintf(stderr, "Failed to wait for changes: %s\n", strerror(-r)); break; } continue; } r = sd_journal_get_data(j, "MESSAGE", &d, &l); if (r < 0) { fprintf(stderr, "Failed to read message field: %s\n", strerror(-r)); continue; } printf("%.*s\n", (int) l, d); } sd_journal_close(j); return 0; } Waiting with poll() (this example lacks all error checking for the sake of simplicity): #include <sys/poll.h> #include <systemd/sd-journal.h> int wait_for_changes(sd_journal *j) { struct pollfd pollfd; pollfd.fd = sd_journal_get_fd(); pollfd.events = POLLIN; poll(&pollfd, 1, sd_journal_reliable_fd() > 0 ? -1 : 2000); return sd_journal_process(j); } systemd1, sd-journal3, sd_journal_open3, sd_journal_next3, poll2