Description¶

sd_journal_get_fd() returns a file descriptor that may be asynchronously polled in an external event loop and is signaled 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 poll(2). Use sd_journal_get_events() for an events mask to watch for. 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 for changes to be noticed immediately. In particular network files systems do not generate suitable file change events in all cases. Cases like this can be detected with sd_journal_reliable_fd(), below. sd_journal_get_timeout() will ensure in these cases that wake-ups happen frequently enough for changes to be noticed, although with a certain latency.

sd_journal_get_events() will return the poll() mask to wait for. This function will return a combination of POLLIN and POLLOUT and similar to fill into the ".events" field of struct pollfd.

sd_journal_get_timeout() will return a timeout value for usage in poll(). This returns a value in microseconds since the epoch of CLOCK_MONOTONIC for timing out poll() in timeout_usec. See clock_gettime(2) for details about CLOCK_MONOTONIC. If there is no timeout to wait for, this will fill in (uint64_t) -1 instead. Note that poll() takes a relative timeout in milliseconds rather than an absolute timeout in microseconds. To convert the absolute 'us' timeout into relative 'ms', use code like the following:

uint64_t t;
int msec;
sd_journal_get_timeout(m, &t);
if (t == (uint64_t) -1)
  msec = -1;
else {
  struct timespec ts;
  uint64_t n;
  clock_gettime(CLOCK_MONOTONIC, &ts);
  n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
  msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
}

The code above does not do any error checking for brevity's sake. The calculated msec integer can be passed directly as poll()'s timeout parameter.

After each poll() wake-up sd_journal_process() needs to be called to process events. 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_get_events(), sd_journal_get_timeout() and sd_journal_process() is sd_journal_wait(). It will synchronously wait until the journal gets changed. 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_get_events(), sd_journal_get_timeout(), poll() and sd_journal_process() into one.

sd_journal_reliable_fd() may be used to check whether the wake-up events from the file descriptor returned by sd_journal_get_fd() are known to be quickly triggered. On certain file systems where file change events from the OS are not available (such as NFS) changes need to be polled for repeatedly, and hence are detected only with a considerable latency. This call will return a positive value if the journal changes are detected quickly and zero when they need to be polled for. Note that there is usually no need to invoke this function directly as sd_journal_get_timeout() will request appropriate timeouts anyway.

Note that all of the above change notification interfaces do not report changes instantly. Latencies are introduced for multiple reasons: as mentioned certain storage backends require time-based polling, in other cases wake-ups are optimized by coalescing events, and the OS introduces additional IO/CPU scheduling latencies.

Return Value¶

sd_journal_get_fd() returns a valid file descriptor on success or a negative errno-style error code.

sd_journal_get_events() returns a combination of POLLIN, POLLOUT and suchlike 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() will generate wake-ups immediately for all journal changes. Returns 0 if there might be a latency involved.

sd_journal_process() and sd_journal_wait() return a negative errno-style error code, or one of SD_JOURNAL_NOP, SD_JOURNAL_APPEND or SD_JOURNAL_INVALIDATE on success:

Signal safety¶

In general, sd_journal_get_fd(), sd_journal_get_events(), and sd_journal_get_timeout() are not "async signal safe" in the meaning of signal-safety(7). Nevertheless, only the first call to any of those three functions performs unsafe operations, so subsequent calls are safe.

sd_journal_process() and sd_journal_wait() are not safe. sd_journal_reliable_fd() is safe.

Notes¶

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.

Functions described here are available as a shared library, which can be compiled against and linked to with the libsystemd pkg-config(1) file.

Examples¶

Iterating through the journal, in a live view tracking all changes:

/* SPDX-License-Identifier: MIT-0 */

#include <errno.h>
#include <stdio.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 void *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, (const char*) d);
  }

  sd_journal_close(j);
  return 0;
}

Waiting with poll() (this example lacks all error checking for the sake of simplicity):

/* SPDX-License-Identifier: MIT-0 */

#define _GNU_SOURCE 1
#include <poll.h>
#include <time.h>
#include <systemd/sd-journal.h>

int wait_for_changes(sd_journal *j) {
  uint64_t t;
  int msec;
  struct pollfd pollfd;

  sd_journal_get_timeout(j, &t);
  if (t == (uint64_t) -1)
    msec = -1;
  else {
    struct timespec ts;
    uint64_t n;
    clock_gettime(CLOCK_MONOTONIC, &ts);
    n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
    msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
  }

  pollfd.fd = sd_journal_get_fd(j);
  pollfd.events = sd_journal_get_events(j);
  poll(&pollfd, 1, msec);
  return sd_journal_process(j);
}

History¶

sd_journal_get_fd(), sd_journal_process(), and sd_journal_wait() were added in version 187.

sd_journal_reliable_fd() was added in version 196.

sd_journal_get_events() and sd_journal_get_timeout() were added in version 201.

See Also¶

systemd(1), sd-journal(3), sd_journal_open(3), sd_journal_next(3), poll(2), clock_gettime(2)