# sd_journal_next

## NAME



           sd_journal_next, sd_journal_previous, sd_journal_step_one,
           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
    


## SYNOPSIS



           **#include <systemd/sd-journal.h>**
    
           **int sd_journal_next(sd_journal ***_j_**);**
    
           **int sd_journal_previous(sd_journal ***_j_**);**
    
           **int sd_journal_step_one(sd_journal ***_j_**, int** _advanced_**);**
    
           **int sd_journal_next_skip(sd_journal ***_j_**, uint64_t** _skip_**);**
    
           **int sd_journal_previous_skip(sd_journal ***_j_**, uint64_t** _skip_**);**
    
           **SD_JOURNAL_FOREACH(sd_journal ***_j_**);**
    
           **SD_JOURNAL_FOREACH_BACKWARDS(sd_journal ***_j_**);**
    


## DESCRIPTION



           **sd_journal_next()** advances the read pointer into the journal by
           one entry. The only argument taken is a journal context object as
           allocated via sd_journal_open(3). After successful invocation the
           entry may be read with functions such as sd_journal_get_data(3).
    
           Similarly, **sd_journal_previous()** sets the read pointer back one
           entry.
    
           **sd_journal_step_one()** also moves the read pointer. If the current
           location is the head of the journal, e.g. when this is called
           following **sd_journal_seek_head()** , then this is equivalent to
           **sd_journal_next()** , and the argument _advanced_ will be ignored.
           Similarly, if the current location is the tail of the journal,
           e.g. when this is called following **sd_journal_seek_tail()** , then
           this is equivalent to **sd_journal_previous()** , and _advanced_ will be
           ignored. Otherwise, this is equivalent to **sd_journal_next()** when
           _advanced_ is non-zero, and **sd_journal_previous()** when _advanced_ is
           zero.
    
           **sd_journal_next_skip()** and **sd_journal_previous_skip()** advance/set
           back the read pointer by multiple entries at once, as specified in
           the _skip_ parameter. The _skip_ parameter must be less than or equal
           to 2147483647 (2³¹-1).
    
           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.
    
           Note that sd_journal_get_data(3) and related calls will fail
           unless **sd_journal_next()** has been invoked at least once in order
           to position the read pointer on a journal entry.
    
           Note that the **SD_JOURNAL_FOREACH()** macro may be used as a wrapper
           around sd_journal_seek_head(3) and **sd_journal_next()** in order to
           make iterating through the journal easier. See below for an
           example. Similarly, **SD_JOURNAL_FOREACH_BACKWARDS()** may be used for
           iterating the journal in reverse order.
    


## RETURN VALUE



           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 **sd_journal_next()** or
           **sd_journal_previous()** 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.
    


## NOTES



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


## EXAMPLES



           Iterating through the journal:
    
               /* SPDX-License-Identifier: MIT-0 */
    
               #include <errno.h>
               #include <stdio.h>
               #include <systemd/sd-journal.h>
    
               int main(int argc, char *argv[]) {
                 int r;
                 sd_journal *j;
    
                 r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
                 if (r < 0) {
                   fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
                   return 1;
                 }
                 SD_JOURNAL_FOREACH(j) {
                   const char *d;
                   size_t l;
    
                   r = sd_journal_get_data(j, "MESSAGE", (const void **)&d, &l);
                   if (r < 0) {
                     fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
                     continue;
                   }
    
                   printf("%.*s\n", (int) l, d);
                 }
                 sd_journal_close(j);
                 return 0;
               }
    


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