1SD_JOURNAL_GET_FD(3)           sd_journal_get_fd          SD_JOURNAL_GET_FD(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 wakeup events
86       from the file descriptor returned by sd_journal_get_fd() are known to
87       be immediately triggered. On certain file systems where file change
88       events from the OS are not available (such as NFS) changes need to be
89       polled for repeatedly, and hence are detected only with a certain
90       latency. This call will return a positive value if the journal changes
91       are detected immediately and zero when they need to be polled for and
92       hence might be noticed only with a certain latency. Note that there is
93       usually no need to invoke this function directly as
94       sd_journal_get_timeout() on these file systems will ask for timeouts
95       explicitly anyway.
96

RETURN VALUE

98       sd_journal_get_fd() returns a valid file descriptor on success or a
99       negative errno-style error code.
100
101       sd_journal_get_events() returns a combination of POLLIN, POLLOUT and
102       suchlike on success or a negative errno-style error code.
103
104       sd_journal_reliable_fd() returns a positive integer if the file
105       descriptor returned by sd_journal_get_fd() will generate wake-ups
106       immediately for all journal changes. Returns 0 if there might be a
107       latency involved.
108
109       sd_journal_process() and sd_journal_wait() return a negative
110       errno-style error code, or one of SD_JOURNAL_NOP, SD_JOURNAL_APPEND or
111       SD_JOURNAL_INVALIDATE on success:
112
113       ·   If SD_JOURNAL_NOP is returned, the journal did not change since the
114           last invocation.
115
116       ·   If SD_JOURNAL_APPEND is returned, new entries have been appended to
117           the end of the journal. In this case it is sufficient to simply
118           continue reading at the previous end location of the journal, to
119           read the newly added entries.
120
121       ·   If SD_JOURNAL_INVALIDATE, journal files were added to or removed
122           from the set of journal files watched (e.g. due to rotation or
123           vacuuming), and thus entries might have appeared or disappeared at
124           arbitrary places in the log stream, possibly before or after the
125           previous end of the log stream. If SD_JOURNAL_INVALIDATE is
126           returned, live-view UIs that want to reflect on screen the precise
127           state of the log data on disk should probably refresh their entire
128           display (relative to the cursor of the log entry on the top of the
129           screen). Programs only interested in a strictly sequential stream
130           of log data may treat SD_JOURNAL_INVALIDATE the same way as
131           SD_JOURNAL_APPEND, thus ignoring any changes to the log view
132           earlier than the old end of the log stream.
133

SIGNAL SAFETY

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

NOTES

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

EXAMPLES

157       Iterating through the journal, in a live view tracking all changes:
158
159           #include <stdio.h>
160           #include <string.h>
161           #include <systemd/sd-journal.h>
162
163           int main(int argc, char *argv[]) {
164             int r;
165             sd_journal *j;
166             r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
167             if (r < 0) {
168               fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
169               return 1;
170             }
171             for (;;)  {
172               const void *d;
173               size_t l;
174               r = sd_journal_next(j);
175               if (r < 0) {
176                 fprintf(stderr, "Failed to iterate to next entry: %s\n", strerror(-r));
177                 break;
178               }
179               if (r == 0) {
180                 /* Reached the end, let's wait for changes, and try again */
181                 r = sd_journal_wait(j, (uint64_t) -1);
182                 if (r < 0) {
183                   fprintf(stderr, "Failed to wait for changes: %s\n", strerror(-r));
184                   break;
185                 }
186                 continue;
187               }
188               r = sd_journal_get_data(j, "MESSAGE", &d, &l);
189               if (r < 0) {
190                 fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
191                 continue;
192               }
193               printf("%.*s\n", (int) l, (const char*) d);
194             }
195             sd_journal_close(j);
196             return 0;
197           }
198
199       Waiting with poll() (this example lacks all error checking for the sake
200       of simplicity):
201
202           #include <poll.h>
203           #include <time.h>
204           #include <systemd/sd-journal.h>
205
206           int wait_for_changes(sd_journal *j) {
207             uint64_t t;
208             int msec;
209             struct pollfd pollfd;
210
211             sd_journal_get_timeout(j, &t);
212             if (t == (uint64_t) -1)
213               msec = -1;
214             else {
215               struct timespec ts;
216               uint64_t n;
217               clock_gettime(CLOCK_MONOTONIC, &ts);
218               n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
219               msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
220             }
221
222             pollfd.fd = sd_journal_get_fd(j);
223             pollfd.events = sd_journal_get_events(j);
224             poll(&pollfd, 1, msec);
225             return sd_journal_process(j);
226           }
227

SEE ALSO

229       systemd(1), sd-journal(3), sd_journal_open(3), sd_journal_next(3),
230       poll(2), clock_gettime(2)
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233
234systemd 243                                               SD_JOURNAL_GET_FD(3)
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