man/sd_journal_next.3

   1 '\" t
   2 .TH "SD_JOURNAL_NEXT" "3" "" "systemd 217" "sd_journal_next"
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   4 .\" * Define some portability stuff
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   6 .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   7 .\" http://bugs.debian.org/507673
   8 .\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
   9 .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  10 .ie \n(.g .ds Aq \(aq
  11 .el       .ds Aq '
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  13 .\" * set default formatting
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  15 .\" disable hyphenation
  16 .nh
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  18 .ad l
  19 .\" -----------------------------------------------------------------
  20 .\" * MAIN CONTENT STARTS HERE *
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  22 .SH "NAME"
  23 sd_journal_next, sd_journal_previous, sd_journal_next_skip, sd_journal_previous_skip, SD_JOURNAL_FOREACH, SD_JOURNAL_FOREACH_BACKWARDS \- Advance or set back the read pointer in the journal
  24 .SH "SYNOPSIS"
  25 .sp
  26 .ft B
  27 .nf
  28 #include <systemd/sd\-journal\&.h>
  29 .fi
  30 .ft
  31 .HP \w'int\ sd_journal_next('u
  32 .BI "int sd_journal_next(sd_journal\ *" "j" ");"
  33 .HP \w'int\ sd_journal_previous('u
  34 .BI "int sd_journal_previous(sd_journal\ *" "j" ");"
  35 .HP \w'int\ sd_journal_next_skip('u
  36 .BI "int sd_journal_next_skip(sd_journal\ *" "j" ", uint64_t\ " "skip" ");"
  37 .HP \w'int\ sd_journal_previous_skip('u
  38 .BI "int sd_journal_previous_skip(sd_journal\ *" "j" ", uint64_t\ " "skip" ");"
  39 .HP \w'SD_JOURNAL_FOREACH('u
  40 .BI "SD_JOURNAL_FOREACH(sd_journal\ *" "j" ");"
  41 .HP \w'SD_JOURNAL_FOREACH_BACKWARDS('u
  42 .BI "SD_JOURNAL_FOREACH_BACKWARDS(sd_journal\ *" "j" ");"
  43 .SH "DESCRIPTION"
  44 .PP
  45 \fBsd_journal_next()\fR
  46 advances the read pointer into the journal by one entry\&. The only argument taken is a journal context object as allocated via
  47 \fBsd_journal_open\fR(3)\&. After successful invocation the entry may be read with functions such as
  48 \fBsd_journal_get_data\fR(3)\&.
  49 .PP
  50 Similarly,
  51 \fBsd_journal_previous()\fR
  52 sets the read pointer back one entry\&.
  53 .PP
  54 \fBsd_journal_next_skip()\fR
  55 and
  56 \fBsd_journal_previous_skip()\fR
  57 advance/set back the read pointer by multiple entries at once, as specified in the
  58 \fIskip\fR
  59 parameter\&.
  60 .PP
  61 The journal is strictly ordered by reception time, and hence advancing to the next entry guarantees that the entry then pointing to is later in time than then previous one, or has the same timestamp\&.
  62 .PP
  63 Note that
  64 \fBsd_journal_get_data\fR(3)
  65 and related calls will fail unless
  66 \fBsd_journal_next()\fR
  67 has been invoked at least once in order to position the read pointer on a journal entry\&.
  68 .PP
  69 Note that the
  70 \fBSD_JOURNAL_FOREACH()\fR
  71 macro may be used as a wrapper around
  72 \fBsd_journal_seek_head\fR(3)
  73 and
  74 \fBsd_journal_next()\fR
  75 in order to make iterating through the journal easier\&. See below for an example\&. Similarly,
  76 \fBSD_JOURNAL_FOREACH_BACKWARDS()\fR
  77 may be used for iterating the journal in reverse order\&.
  78 .SH "RETURN VALUE"
  79 .PP
  80 The four calls return the number of entries advanced/set back on success or a negative errno\-style error code\&. When the end or beginning of the journal is reached, a number smaller than requested is returned\&. More specifically, if
  81 \fBsd_journal_next()\fR
  82 or
  83 \fBsd_journal_previous()\fR
  84 reach the end/beginning of the journal they will return 0, instead of 1 when they are successful\&. This should be considered an EOF marker\&.
  85 .SH "NOTES"
  86 .PP
  87 The
  88 \fBsd_journal_next()\fR,
  89 \fBsd_journal_previous()\fR,
  90 \fBsd_journal_next_skip()\fR
  91 and
  92 \fBsd_journal_previous_skip()\fR
  93 interfaces are available as a shared library, which can be compiled and linked to with the
  94 \fBlibsystemd\fR\ \&\fBpkg-config\fR(1)
  95 file\&.
  96 .SH "EXAMPLES"
  97 .PP
  98 Iterating through the journal:
  99 .sp
 100 .if n \{\
 101 .RS 4
 102 .\}
 103 .nf
 104 #include <stdio\&.h>
 105 #include <string\&.h>
 106 #include <systemd/sd\-journal\&.h>
 107
 108 int main(int argc, char *argv[]) {
 109         int r;
 110         sd_journal *j;
 111         r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
 112         if (r < 0) {
 113                 fprintf(stderr, "Failed to open journal: %s\en", strerror(\-r));
 114                 return 1;
 115         }
 116         SD_JOURNAL_FOREACH(j) {
 117                 const char *d;
 118                 size_t l;
 119
 120                 r = sd_journal_get_data(j, "MESSAGE", (const void **)&d, &l);
 121                 if (r < 0) {
 122                         fprintf(stderr, "Failed to read message field: %s\en", strerror(\-r));
 123                         continue;
 124                 }
 125
 126                 printf("%\&.*s\en", (int) l, d);
 127         }
 128         sd_journal_close(j);
 129         return 0;
 130 }
 131 .fi
 132 .if n \{\
 133 .RE
 134 .\}
 135 .SH "SEE ALSO"
 136 .PP
 137 \fBsystemd\fR(1),
 138 \fBsd-journal\fR(3),
 139 \fBsd_journal_open\fR(3),
 140 \fBsd_journal_get_data\fR(3),
 141 \fBsd_journal_get_realtime_usec\fR(3),
 142 \fBsd_journal_get_cursor\fR(3)