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);
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16       int sd_journal_get_events(sd_journal *j);
17
18       int sd_journal_get_timeout(sd_journal *j, uint64_t *timeout_usec);
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20       int sd_journal_process(sd_journal *j);
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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_getttime(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's
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 one of
110       SD_JOURNAL_NOP, SD_JOURNAL_APPEND or SD_JOURNAL_INVALIDATE on success
111       or a negative errno-style error code. If SD_JOURNAL_NOP is returned,
112       the journal did not change since the last invocation. If
113       SD_JOURNAL_APPEND is returned, new entries have been appended to the
114       end of the journal. If SD_JOURNAL_INVALIDATE, journal files were added
115       or removed (possibly due to rotation). In the latter event, live-view
116       UIs should probably refresh their entire display, while in the case of
117       SD_JOURNAL_APPEND, it is sufficient to simply continue reading at the
118       previous end of the journal.
119

NOTES

121       The sd_journal_get_fd(), sd_journal_get_events(),
122       sd_journal_reliable_fd(), sd_journal_process() and sd_journal_wait()
123       interfaces are available as a shared library, which can be compiled and
124       linked to with the libsystemd pkg-config(1) file.
125

EXAMPLES

127       Iterating through the journal, in a live view tracking all changes:
128
129           #include <stdio.h>
130           #include <string.h>
131           #include <systemd/sd-journal.h>
132
133           int main(int argc, char *argv[]) {
134             int r;
135             sd_journal *j;
136             r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
137             if (r < 0) {
138               fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
139               return 1;
140             }
141             for (;;)  {
142               const void *d;
143               size_t l;
144               r = sd_journal_next(j);
145               if (r < 0) {
146                 fprintf(stderr, "Failed to iterate to next entry: %s\n", strerror(-r));
147                 break;
148               }
149               if (r == 0) {
150                 /* Reached the end, let's wait for changes, and try again */
151                 r = sd_journal_wait(j, (uint64_t) -1);
152                 if (r < 0) {
153                   fprintf(stderr, "Failed to wait for changes: %s\n", strerror(-r));
154                   break;
155                 }
156                 continue;
157               }
158               r = sd_journal_get_data(j, "MESSAGE", &d, &l);
159               if (r < 0) {
160                 fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
161                 continue;
162               }
163               printf("%.*s\n", (int) l, (const char*) d);
164             }
165             sd_journal_close(j);
166             return 0;
167           }
168
169       Waiting with poll() (this example lacks all error checking for the sake
170       of simplicity):
171
172           #include <poll.h>
173           #include <systemd/sd-journal.h>
174
175           int wait_for_changes(sd_journal *j) {
176             struct pollfd pollfd;
177             int msec;
178
179             sd_journal_get_timeout(m, &t);
180             if (t == (uint64_t) -1)
181               msec = -1;
182             else {
183               struct timespec ts;
184               uint64_t n;
185               clock_getttime(CLOCK_MONOTONIC, &ts);
186               n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
187               msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
188             }
189
190             pollfd.fd = sd_journal_get_fd(j);
191             pollfd.events = sd_journal_get_events(j);
192             poll(&pollfd, 1, msec);
193             return sd_journal_process(j);
194           }
195

SEE ALSO

197       systemd(1), sd-journal(3), sd_journal_open(3), sd_journal_next(3),
198       poll(2), clock_gettime(2)
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202systemd 219                                               SD_JOURNAL_GET_FD(3)
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