1SD_JOURNAL_GET_FD(3)           sd_journal_get_fd          SD_JOURNAL_GET_FD(3)
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3
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NAME

6       sd_journal_get_fd, sd_journal_get_events, sd_journal_get_timeout,
7       sd_journal_process, sd_journal_wait, sd_journal_reliable_fd,
8       SD_JOURNAL_NOP, SD_JOURNAL_APPEND, SD_JOURNAL_INVALIDATE - Journal
9       change notification interface
10

SYNOPSIS

12       #include <systemd/sd-journal.h>
13
14       int sd_journal_get_fd(sd_journal *j);
15
16       int sd_journal_get_events(sd_journal *j);
17
18       int sd_journal_get_timeout(sd_journal *j, uint64_t *timeout_usec);
19
20       int sd_journal_process(sd_journal *j);
21
22       int sd_journal_wait(sd_journal *j, uint64_t timeout_usec);
23
24       int sd_journal_reliable_fd(sd_journal *j);
25

DESCRIPTION

27       sd_journal_get_fd() returns a file descriptor that may be
28       asynchronously polled in an external event loop and is signaled as soon
29       as the journal changes, because new entries or files were added,
30       rotation took place, or files have been deleted, and similar. The file
31       descriptor is suitable for usage in poll(2). Use
32       sd_journal_get_events() for an events mask to watch for. The call takes
33       one argument: the journal context object. Note that not all file
34       systems are capable of generating the necessary events for wakeups from
35       this file descriptor for changes to be noticed immediately. In
36       particular network files systems do not generate suitable file change
37       events in all cases. Cases like this can be detected with
38       sd_journal_reliable_fd(), below.  sd_journal_get_timeout() will ensure
39       in these cases that wake-ups happen frequently enough for changes to be
40       noticed, although with a certain latency.
41
42       sd_journal_get_events() will return the poll() mask to wait for. This
43       function will return a combination of POLLIN and POLLOUT and similar to
44       fill into the ".events" field of struct pollfd.
45
46       sd_journal_get_timeout() will return a timeout value for usage in
47       poll(). This returns a value in microseconds since the epoch of
48       CLOCK_MONOTONIC for timing out poll() in timeout_usec. See
49       clock_gettime(2) for details about CLOCK_MONOTONIC. If there is no
50       timeout to wait for, this will fill in (uint64_t) -1 instead. Note that
51       poll() takes a relative timeout in milliseconds rather than an absolute
52       timeout in microseconds. To convert the absolute 'us' timeout into
53       relative 'ms', use code like the following:
54
55           uint64_t t;
56           int msec;
57           sd_journal_get_timeout(m, &t);
58           if (t == (uint64_t) -1)
59             msec = -1;
60           else {
61             struct timespec ts;
62             uint64_t n;
63             clock_gettime(CLOCK_MONOTONIC, &ts);
64             n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
65             msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
66           }
67
68       The code above does not do any error checking for brevity's sake. The
69       calculated msec integer can be passed directly as poll()'s timeout
70       parameter.
71
72       After each poll() wake-up sd_journal_process() needs to be called to
73       process events. This call will also indicate what kind of change has
74       been detected (see below; note that spurious wake-ups are possible).
75
76       A synchronous alternative for using sd_journal_get_fd(),
77       sd_journal_get_events(), sd_journal_get_timeout() and
78       sd_journal_process() is sd_journal_wait(). It will synchronously wait
79       until the journal gets changed. The maximum time this call sleeps may
80       be controlled with the timeout_usec parameter. Pass (uint64_t) -1 to
81       wait indefinitely. Internally this call simply combines
82       sd_journal_get_fd(), sd_journal_get_events(), sd_journal_get_timeout(),
83       poll() and sd_journal_process() into one.
84
85       sd_journal_reliable_fd() may be used to check whether the wake-up
86       events from the file descriptor returned by sd_journal_get_fd() are
87       known to be quickly triggered. On certain file systems where file
88       change events from the OS are not available (such as NFS) changes need
89       to be polled for repeatedly, and hence are detected only with a
90       considerable latency. This call will return a positive value if the
91       journal changes are detected quickly and zero when they need to be
92       polled for. Note that there is usually no need to invoke this function
93       directly as sd_journal_get_timeout() will request appropriate timeouts
94       anyway.
95
96       Note that all of the above change notification interfaces do not report
97       changes instantly. Latencies are introduced for multiple reasons: as
98       mentioned certain storage backends require time-based polling, in other
99       cases wake-ups are optimized by coalescing events, and the OS
100       introduces additional IO/CPU scheduling latencies.
101

RETURN VALUE

103       sd_journal_get_fd() returns a valid file descriptor on success or a
104       negative errno-style error code.
105
106       sd_journal_get_events() returns a combination of POLLIN, POLLOUT and
107       suchlike on success or a negative errno-style error code.
108
109       sd_journal_reliable_fd() returns a positive integer if the file
110       descriptor returned by sd_journal_get_fd() will generate wake-ups
111       immediately for all journal changes. Returns 0 if there might be a
112       latency involved.
113
114       sd_journal_process() and sd_journal_wait() return a negative
115       errno-style error code, or one of SD_JOURNAL_NOP, SD_JOURNAL_APPEND or
116       SD_JOURNAL_INVALIDATE on success:
117
118       •   If SD_JOURNAL_NOP is returned, the journal did not change since the
119           last invocation.
120
121       •   If SD_JOURNAL_APPEND is returned, new entries have been appended to
122           the end of the journal. In this case it is sufficient to simply
123           continue reading at the previous end location of the journal, to
124           read the newly added entries.
125
126       •   If SD_JOURNAL_INVALIDATE, journal files were added to or removed
127           from the set of journal files watched (e.g. due to rotation or
128           vacuuming), and thus entries might have appeared or disappeared at
129           arbitrary places in the log stream, possibly before or after the
130           previous end of the log stream. If SD_JOURNAL_INVALIDATE is
131           returned, live-view UIs that want to reflect on screen the precise
132           state of the log data on disk should probably refresh their entire
133           display (relative to the cursor of the log entry on the top of the
134           screen). Programs only interested in a strictly sequential stream
135           of log data may treat SD_JOURNAL_INVALIDATE the same way as
136           SD_JOURNAL_APPEND, thus ignoring any changes to the log view
137           earlier than the old end of the log stream.
138

SIGNAL SAFETY

140       In general, sd_journal_get_fd(), sd_journal_get_events(), and
141       sd_journal_get_timeout() are not "async signal safe" in the meaning of
142       signal-safety(7). Nevertheless, only the first call to any of those
143       three functions performs unsafe operations, so subsequent calls are
144       safe.
145
146       sd_journal_process() and sd_journal_wait() are not safe.
147       sd_journal_reliable_fd() is safe.
148

NOTES

150       All functions listed here are thread-agnostic and only a single
151       specific thread may operate on a given object during its entire
152       lifetime. It's safe to allocate multiple independent objects and use
153       each from a specific thread in parallel. However, it's not safe to
154       allocate such an object in one thread, and operate or free it from any
155       other, even if locking is used to ensure these threads don't operate on
156       it at the very same time.
157
158       These APIs are implemented as a shared library, which can be compiled
159       and linked to with the libsystemd pkg-config(1) file.
160

EXAMPLES

162       Iterating through the journal, in a live view tracking all changes:
163
164           /* SPDX-License-Identifier: MIT-0 */
165
166           #include <errno.h>
167           #include <stdio.h>
168           #include <systemd/sd-journal.h>
169
170           int main(int argc, char *argv[]) {
171             int r;
172             sd_journal *j;
173             r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
174             if (r < 0) {
175               errno = -r;
176               fprintf(stderr, "Failed to open journal: %m\n");
177               return 1;
178             }
179             for (;;)  {
180               const void *d;
181               size_t l;
182               r = sd_journal_next(j);
183               if (r < 0) {
184                 errno = -r;
185                 fprintf(stderr, "Failed to iterate to next entry: %m\n");
186                 break;
187               }
188               if (r == 0) {
189                 /* Reached the end, let's wait for changes, and try again */
190                 r = sd_journal_wait(j, (uint64_t) -1);
191                 if (r < 0) {
192                   errno = -r;
193                   fprintf(stderr, "Failed to wait for changes: %m\n");
194                   break;
195                 }
196                 continue;
197               }
198               r = sd_journal_get_data(j, "MESSAGE", &d, &l);
199               if (r < 0) {
200                 errno = -r;
201                 fprintf(stderr, "Failed to read message field: %m\n");
202                 continue;
203               }
204               printf("%.*s\n", (int) l, (const char*) d);
205             }
206             sd_journal_close(j);
207             return 0;
208           }
209
210       Waiting with poll() (this example lacks all error checking for the sake
211       of simplicity):
212
213           /* SPDX-License-Identifier: MIT-0 */
214
215           #include <poll.h>
216           #include <time.h>
217           #include <systemd/sd-journal.h>
218
219           int wait_for_changes(sd_journal *j) {
220             uint64_t t;
221             int msec;
222             struct pollfd pollfd;
223
224             sd_journal_get_timeout(j, &t);
225             if (t == (uint64_t) -1)
226               msec = -1;
227             else {
228               struct timespec ts;
229               uint64_t n;
230               clock_gettime(CLOCK_MONOTONIC, &ts);
231               n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
232               msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
233             }
234
235             pollfd.fd = sd_journal_get_fd(j);
236             pollfd.events = sd_journal_get_events(j);
237             poll(&pollfd, 1, msec);
238             return sd_journal_process(j);
239           }
240

SEE ALSO

242       systemd(1), sd-journal(3), sd_journal_open(3), sd_journal_next(3),
243       poll(2), clock_gettime(2)
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245
246
247systemd 253                                               SD_JOURNAL_GET_FD(3)
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