# sd_journal_enumerate_fields

## NAME



           sd_journal_enumerate_fields, sd_journal_restart_fields,
           SD_JOURNAL_FOREACH_FIELD - Read used field names from the journal
    


## SYNOPSIS



           **#include <systemd/sd-journal.h>**
    
           **int sd_journal_enumerate_fields(sd_journal ***_j_**,**
                                           **const char ****_field_**);**
    
           **void sd_journal_restart_fields(sd_journal ***_j_**);**
    
           **SD_JOURNAL_FOREACH_FIELD(sd_journal ***_j_**, const char ***_field_**);**
    


## DESCRIPTION



           **sd_journal_enumerate_fields()** may be used to iterate through all
           field names used in the opened journal files. On each invocation
           the next field name is returned. The order of the returned field
           names is not defined. It takes two arguments: the journal context
           object, plus a pointer to a constant string pointer where the
           field name is stored in. The returned data is in a read-only
           memory map and is only valid until the next invocation of
           **sd_journal_enumerate_fields()**. Note that this call is subject to
           the data field size threshold as controlled by
           **sd_journal_set_data_threshold()**.
    
           **sd_journal_restart_fields()** resets the field name enumeration
           index to the beginning of the list. The next invocation of
           **sd_journal_enumerate_fields()** will return the first field name
           again.
    
           The **SD_JOURNAL_FOREACH_FIELD()** macro may be used as a handy
           wrapper around **sd_journal_restart_fields()** and
           **sd_journal_enumerate_fields()**.
    
           These functions currently are not influenced by matches set with
           **sd_journal_add_match()** but this might change in a later version of
           this software.
    
           To retrieve the possible values a specific field can take use
           sd_journal_query_unique(3).
    


## RETURN VALUE



           **sd_journal_enumerate_fields()** returns a positive integer if the
           next field name has been read, 0 when no more field names are
           known, or a negative errno-style error code.
           **sd_journal_restart_fields()** returns nothing.
    


## 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



           Use the **SD_JOURNAL_FOREACH_FIELD()** macro to iterate through all
           field names in use in the current journal.
    
               /* SPDX-License-Identifier: MIT-0 */
    
               #include <errno.h>
               #include <stdio.h>
               #include <systemd/sd-journal.h>
    
               int main(int argc, char *argv[]) {
                 sd_journal *j;
                 const char *field;
                 int r;
    
                 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_FIELD(j, field)
                   printf("%s\n", field);
                 sd_journal_close(j);
                 return 0;
               }
    


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