# sd_journal_get_fd

## NAME



           sd_journal_get_fd, sd_journal_get_events, sd_journal_get_timeout,
           sd_journal_process, sd_journal_wait, sd_journal_reliable_fd,
           SD_JOURNAL_NOP, SD_JOURNAL_APPEND, SD_JOURNAL_INVALIDATE - Journal
           change notification interface
    


## SYNOPSIS



           **#include <systemd/sd-journal.h>**
    
           **int sd_journal_get_fd(sd_journal ***_j_**);**
    
           **int sd_journal_get_events(sd_journal ***_j_**);**
    
           **int sd_journal_get_timeout(sd_journal ***_j_**, uint64_t ***_timeout_usec_**);**
    
           **int sd_journal_process(sd_journal ***_j_**);**
    
           **int sd_journal_wait(sd_journal ***_j_**, uint64_t** _timeout_usec_**);**
    
           **int sd_journal_reliable_fd(sd_journal ***_j_**);**
    


## DESCRIPTION



           **sd_journal_get_fd()** returns a file descriptor that may be
           asynchronously polled in an external event loop and is signaled as
           soon as the journal changes, because new entries or files were
           added, rotation took place, or files have been deleted, and
           similar. The file descriptor is suitable for usage in poll(2). Use
           **sd_journal_get_events()** for an events mask to watch for. The call
           takes one argument: the journal context object. Note that not all
           file systems are capable of generating the necessary events for
           wakeups from this file descriptor for changes to be noticed
           immediately. In particular network files systems do not generate
           suitable file change events in all cases. Cases like this can be
           detected with **sd_journal_reliable_fd()** , below.
           **sd_journal_get_timeout()** will ensure in these cases that wake-ups
           happen frequently enough for changes to be noticed, although with
           a certain latency.
    
           **sd_journal_get_events()** will return the **poll()** mask to wait for.
           This function will return a combination of **POLLIN** and **POLLOUT** and
           similar to fill into the ".events" field of _struct pollfd_.
    
           **sd_journal_get_timeout()** will return a timeout value for usage in
           **poll()**. This returns a value in microseconds since the epoch of
           **CLOCK_MONOTONIC** for timing out **poll()** in _timeout_usec_. See
           clock_gettime(2) for details about **CLOCK_MONOTONIC**. If there is no
           timeout to wait for, this will fill in **(uint64_t) -1** instead. Note
           that **poll()** takes a relative timeout in milliseconds rather than
           an absolute timeout in microseconds. To convert the absolute 'us'
           timeout into relative 'ms', use code like the following:
    
               uint64_t t;
               int msec;
               sd_journal_get_timeout(m, &t);
               if (t == (uint64_t) -1)
                 msec = -1;
               else {
                 struct timespec ts;
                 uint64_t n;
                 clock_gettime(CLOCK_MONOTONIC, &ts);
                 n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
                 msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
               }
    
           The code above does not do any error checking for brevity's sake.
           The calculated _msec_ integer can be passed directly as **poll()** 's
           timeout parameter.
    
           After each **poll()** wake-up **sd_journal_process()** needs to be called
           to process events. This call will also indicate what kind of
           change has been detected (see below; note that spurious wake-ups
           are possible).
    
           A synchronous alternative for using **sd_journal_get_fd()** ,
           **sd_journal_get_events()** , **sd_journal_get_timeout()** and
           **sd_journal_process()** is **sd_journal_wait()**. It will synchronously
           wait until the journal gets changed. The maximum time this call
           sleeps may be controlled with the _timeout_usec_ parameter. Pass
           **(uint64_t) -1** to wait indefinitely. Internally this call simply
           combines **sd_journal_get_fd()** , **sd_journal_get_events()** ,
           **sd_journal_get_timeout()** , **poll()** and **sd_journal_process()** into
           one.
    
           **sd_journal_reliable_fd()** may be used to check whether the wake-up
           events from the file descriptor returned by **sd_journal_get_fd()**
           are known to be quickly triggered. On certain file systems where
           file change events from the OS are not available (such as NFS)
           changes need to be polled for repeatedly, and hence are detected
           only with a considerable latency. This call will return a positive
           value if the journal changes are detected quickly and zero when
           they need to be polled for. Note that there is usually no need to
           invoke this function directly as **sd_journal_get_timeout()** will
           request appropriate timeouts anyway.
    
           Note that all of the above change notification interfaces do not
           report changes instantly. Latencies are introduced for multiple
           reasons: as mentioned certain storage backends require time-based
           polling, in other cases wake-ups are optimized by coalescing
           events, and the OS introduces additional IO/CPU scheduling
           latencies.
    


## RETURN VALUE



           **sd_journal_get_fd()** returns a valid file descriptor on success or
           a negative errno-style error code.
    
           **sd_journal_get_events()** returns a combination of **POLLIN** , **POLLOUT**
           and suchlike on success or a negative errno-style error code.
    
           **sd_journal_reliable_fd()** returns a positive integer if the file
           descriptor returned by **sd_journal_get_fd()** will generate wake-ups
           immediately for all journal changes. Returns 0 if there might be a
           latency involved.
    
           **sd_journal_process()** and **sd_journal_wait()** return a negative
           errno-style error code, or one of **SD_JOURNAL_NOP** ,
           **SD_JOURNAL_APPEND** or **SD_JOURNAL_INVALIDATE** on success:
    
           •   If **SD_JOURNAL_NOP** is returned, the journal did not change
               since the last invocation.
    
           •   If **SD_JOURNAL_APPEND** is returned, new entries have been
               appended to the end of the journal. In this case, it is
               sufficient to simply continue reading at the previous end
               location of the journal, to read the newly added entries.
    
           •   If **SD_JOURNAL_INVALIDATE** , journal files were added to or
               removed from the set of journal files watched (e.g. due to
               rotation or vacuuming), and thus entries might have appeared
               or disappeared at arbitrary places in the log stream, possibly
               before or after the previous end of the log stream. If
               **SD_JOURNAL_INVALIDATE** is returned, live-view UIs that want to
               reflect on screen the precise state of the log data on disk
               should probably refresh their entire display (relative to the
               cursor of the log entry on the top of the screen). Programs
               only interested in a strictly sequential stream of log data
               may treat **SD_JOURNAL_INVALIDATE** the same way as
               **SD_JOURNAL_APPEND** , thus ignoring any changes to the log view
               earlier than the old end of the log stream.
    


## SIGNAL SAFETY



           In general, **sd_journal_get_fd()** , **sd_journal_get_events()** , and
           **sd_journal_get_timeout()** are _not_ "async signal safe" in the
           meaning of signal-safety(7). Nevertheless, only the first call to
           any of those three functions performs unsafe operations, so
           subsequent calls _are_ safe.
    
           **sd_journal_process()** and **sd_journal_wait()** are not safe.
           **sd_journal_reliable_fd()** is safe.
    


## 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, in a live view tracking all
           changes:
    
               /* 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;
                 }
    
                 for (;;)  {
                   const void *d;
                   size_t l;
                   r = sd_journal_next(j);
                   if (r < 0) {
                     fprintf(stderr, "Failed to iterate to next entry: %s\n", strerror(-r));
                     break;
                   }
                   if (r == 0) {
                     /* Reached the end, let's wait for changes, and try again */
                     r = sd_journal_wait(j, (uint64_t) -1);
                     if (r < 0) {
                       fprintf(stderr, "Failed to wait for changes: %s\n", strerror(-r));
                       break;
                     }
                     continue;
                   }
                   r = sd_journal_get_data(j, "MESSAGE", &d, &l);
                   if (r < 0) {
                     fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
                     continue;
                   }
                   printf("%.*s\n", (int) l, (const char*) d);
                 }
    
                 sd_journal_close(j);
                 return 0;
               }
    
           Waiting with **poll()**(this example lacks all error checking for the
           sake of simplicity):
    
               /* SPDX-License-Identifier: MIT-0 */
    
               #define _GNU_SOURCE 1
               #include <poll.h>
               #include <time.h>
               #include <systemd/sd-journal.h>
    
               int wait_for_changes(sd_journal *j) {
                 uint64_t t;
                 int msec;
                 struct pollfd pollfd;
    
                 sd_journal_get_timeout(j, &t);
                 if (t == (uint64_t) -1)
                   msec = -1;
                 else {
                   struct timespec ts;
                   uint64_t n;
                   clock_gettime(CLOCK_MONOTONIC, &ts);
                   n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
                   msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
                 }
    
                 pollfd.fd = sd_journal_get_fd(j);
                 pollfd.events = sd_journal_get_events(j);
                 poll(&pollfd, 1, msec);
                 return sd_journal_process(j);
               }
    


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