# sd_journal_query_unique

## NAME



           sd_journal_query_unique, sd_journal_enumerate_unique,
           sd_journal_enumerate_available_unique, sd_journal_restart_unique,
           SD_JOURNAL_FOREACH_UNIQUE - Read unique data fields from the
           journal
    


## SYNOPSIS



           **#include <systemd/sd-journal.h>**
    
           **int sd_journal_query_unique(sd_journal ***_j_**, const char ***_field_**);**
    
           **int sd_journal_enumerate_available_unique(sd_journal ***_j_**,**
                                                     **const void ****_data_**,**
                                                     **size_t ***_length_**);**
    
           **int sd_journal_enumerate_unique(sd_journal ***_j_**, const void ****_data_**,**
                                           **size_t ***_length_**);**
    
           **void sd_journal_restart_unique(sd_journal ***_j_**);**
    
           **SD_JOURNAL_FOREACH_UNIQUE(sd_journal ***_j_**, const void ***_data_**,**
                                     **size_t** _length_**);**
    


## DESCRIPTION



           **sd_journal_query_unique()** queries the journal for all unique
           values the specified field can take. It takes two arguments: the
           journal to query and the field name to look for. Well-known field
           names are listed on systemd.journal-fields(7), but any field can
           be specified. Field names must be specified without a trailing
           "=". After this function has been executed successfully the field
           values may be queried using **sd_journal_enumerate_unique()** and
           **sd_journal_enumerate_available_unique()**. Invoking one of those
           calls will change the field name being queried and reset the
           enumeration index to the first field value that matches.
    
           **sd_journal_enumerate_unique()** may be used to iterate through all
           data fields which match the previously selected field name as set
           with **sd_journal_query_unique()**. On each invocation the next field
           data matching the field name is returned. The order of the
           returned data fields is not defined. It takes three arguments: the
           journal object, plus a pair of pointers to pointer/size variables
           where the data object and its size shall be stored. The returned
           data is in a read-only memory map and is only valid until the next
           invocation of **sd_journal_enumerate_unique()**. Note that the data
           returned will be prefixed with the field name and "=". Note that
           this call is subject to the data field size threshold as
           controlled by **sd_journal_set_data_threshold()** and only the initial
           part of the field up to the threshold is returned. An error is
           returned for fields which cannot be retrieved. See the error list
           below for details.
    
           **sd_journal_enumerate_available_unique()** is similar to
           **sd_journal_enumerate_unique()** , but silently skips any fields which
           may be valid, but are too large or not supported by current
           implementation.
    
           **sd_journal_restart_unique()** resets the data enumeration index to
           the beginning of the list. The next invocation of
           **sd_journal_enumerate_unique()** will return the first field data
           matching the field name again.
    
           Note that the **SD_JOURNAL_FOREACH_UNIQUE()** macro may be used as a
           handy wrapper around **sd_journal_restart_unique()** and
           **sd_journal_enumerate_available_unique()**.
    
           Note that 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 enumerate all field names currently in use (and thus all
           suitable field parameters for **sd_journal_query_unique()**), use the
           sd_journal_enumerate_fields(3) call.
    


## RETURN VALUE



           **sd_journal_query_unique()** returns 0 on success or a negative
           errno-style error code.  **sd_journal_enumerate_unique()** and
           **sd_journal_query_available_unique()** return a positive integer if
           the next field data has been read, 0 when no more fields remain,
           or a negative errno-style error code.  **sd_journal_restart_unique()**
           does not return anything.
    
       **Errors**
           Returned errors may indicate the following problems:
    
           **-EINVAL**
               One of the required parameters is **NULL** or invalid.
    
               Added in version 246.
    
           **-ECHILD**
               The journal object was created in a different process, library
               or module instance.
    
               Added in version 246.
    
           **-EADDRNOTAVAIL**
               The read pointer is not positioned at a valid entry;
               sd_journal_next(3) or a related call has not been called at
               least once.
    
               Added in version 246.
    
           **-ENOENT**
               The current entry does not include the specified field.
    
               Added in version 246.
    
           **-ENOBUFS**
               A compressed entry is too large.
    
               Added in version 246.
    
           **-E2BIG**
               The data field is too large for this computer architecture
               (e.g. above 4 GB on a 32-bit architecture).
    
               Added in version 246.
    
           **-EPROTONOSUPPORT**
               The journal is compressed with an unsupported method or the
               journal uses an unsupported feature.
    
               Added in version 246.
    
           **-EBADMSG**
               The journal is corrupted (possibly just the entry being
               iterated over).
    
               Added in version 246.
    
           **-EIO**
               An I/O error was reported by the kernel.
    
               Added in version 246.
    


## 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_UNIQUE()** macro to iterate through all
           values a field of the journal can take (and which can be accessed
           on the given architecture and are not compressed with an
           unsupported mechanism). The following example lists all unit names
           referenced in the 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 void *d;
                 size_t l;
                 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;
                 }
                 r = sd_journal_query_unique(j, "_SYSTEMD_UNIT");
                 if (r < 0) {
                   fprintf(stderr, "Failed to query journal: %s\n", strerror(-r));
                   return 1;
                 }
                 SD_JOURNAL_FOREACH_UNIQUE(j, d, l)
                   printf("%.*s\n", (int) l, (const char*) d);
                 sd_journal_close(j);
                 return 0;
               }
    


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