SD_JOURNAL_FOREACH_BACKWARDS(3)

1SD_JOURNAL_NEXT(3)              sd_journal_next             SD_JOURNAL_NEXT(3)
2
3
4

NAME

6       sd_journal_next, sd_journal_previous, sd_journal_step_one,
7       sd_journal_next_skip, sd_journal_previous_skip, SD_JOURNAL_FOREACH,
8       SD_JOURNAL_FOREACH_BACKWARDS - Advance or set back the read pointer in
9       the journal
10

SYNOPSIS

12       #include <systemd/sd-journal.h>
13
14       int sd_journal_next(sd_journal *j);
15
16       int sd_journal_previous(sd_journal *j);
17
18       int sd_journal_step_one(sd_journal *j, int advanced);
19
20       int sd_journal_next_skip(sd_journal *j, uint64_t skip);
21
22       int sd_journal_previous_skip(sd_journal *j, uint64_t skip);
23
24       SD_JOURNAL_FOREACH(sd_journal *j);
25
26       SD_JOURNAL_FOREACH_BACKWARDS(sd_journal *j);
27

DESCRIPTION

29       sd_journal_next() advances the read pointer into the journal by one
30       entry. The only argument taken is a journal context object as allocated
31       via sd_journal_open(3). After successful invocation the entry may be
32       read with functions such as sd_journal_get_data(3).
33
34       Similarly, sd_journal_previous() sets the read pointer back one entry.
35
36       sd_journal_step_one() also moves the read pointer. If the current
37       location is the head of the journal, e.g. when this is called following
38       sd_journal_seek_head(), then this is equivalent to sd_journal_next(),
39       and the argument advanced will be ignored. Similarly, if the current
40       location is the tail of the journal, e.g. when this is called following
41       sd_journal_seek_tail(), then this is equivalent to
42       sd_journal_previous(), and advanced will be ignored. Otherwise, this is
43       equivalent to sd_journal_next() when advanced is non-zero, and
44       sd_journal_previous() when advanced is zero.
45
46       sd_journal_next_skip() and sd_journal_previous_skip() advance/set back
47       the read pointer by multiple entries at once, as specified in the skip
48       parameter. The skip parameter must be less than or equal to 2147483647
49       (2³¹-1).
50
51       The journal is strictly ordered by reception time, and hence advancing
52       to the next entry guarantees that the entry then pointing to is later
53       in time than then previous one, or has the same timestamp.
54
55       Note that sd_journal_get_data(3) and related calls will fail unless
56       sd_journal_next() has been invoked at least once in order to position
57       the read pointer on a journal entry.
58
59       Note that the SD_JOURNAL_FOREACH() macro may be used as a wrapper
60       around sd_journal_seek_head(3) and sd_journal_next() in order to make
61       iterating through the journal easier. See below for an example.
62       Similarly, SD_JOURNAL_FOREACH_BACKWARDS() may be used for iterating the
63       journal in reverse order.
64

RETURN VALUE

66       The four calls return the number of entries advanced/set back on
67       success or a negative errno-style error code. When the end or beginning
68       of the journal is reached, a number smaller than requested is returned.
69       More specifically, if sd_journal_next() or sd_journal_previous() reach
70       the end/beginning of the journal they will return 0, instead of 1 when
71       they are successful. This should be considered an EOF marker.
72

NOTES

74       All functions listed here are thread-agnostic and only a single
75       specific thread may operate on a given object during its entire
76       lifetime. It's safe to allocate multiple independent objects and use
77       each from a specific thread in parallel. However, it's not safe to
78       allocate such an object in one thread, and operate or free it from any
79       other, even if locking is used to ensure these threads don't operate on
80       it at the very same time.
81
82       Functions described here are available as a shared library, which can
83       be compiled against and linked to with the libsystemd pkg-config(1)
84       file.
85

EXAMPLES

87       Iterating through the journal:
88
89           /* SPDX-License-Identifier: MIT-0 */
90
91           #include <errno.h>
92           #include <stdio.h>
93           #include <systemd/sd-journal.h>
94
95           int main(int argc, char *argv[]) {
96             int r;
97             sd_journal *j;
98             r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
99             if (r < 0) {
100               errno = -r;
101               fprintf(stderr, "Failed to open journal: %m\n");
102               return 1;
103             }
104             SD_JOURNAL_FOREACH(j) {
105               const char *d;
106               size_t l;
107
108               r = sd_journal_get_data(j, "MESSAGE", (const void **)&d, &l);
109               if (r < 0) {
110                 errno = -r;
111                 fprintf(stderr, "Failed to read message field: %m\n");
112                 continue;
113               }
114
115               printf("%.*s\n", (int) l, d);
116             }
117             sd_journal_close(j);
118             return 0;
119           }
120

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

NOTES

EXAMPLES

SEE ALSO