mtbl_merger - merge multiple MTBL data sources into a single output

       Multiple MTBL data sources may be merged together using the mtbl_merger interface, which reads key-value
       entries from one or more sources and provides these entries in sorted order. The sorted entries may be
       consumed via the mtbl_source(3) and mtbl_iter(3) interfaces.

       Because the MTBL format does not allow duplicate keys, the caller must provide a function which will
       accept a key and two conflicting values for that key and return a replacement value. This function may be
       called multiple times for the same key if more than two sources are being merged.

       mtbl_merger objects are created with the mtbl_merger_init() function, which requires a non-NULL mopt
       argument which has been configured with a merge function fp.

       One or more mtbl_reader objects must be provided as input to the mtbl_merger object by calling
       mtbl_merger_add_source(). After the desired sources have been configured, mtbl_merger_source() should be
       called in order to consume the merged output via the mtbl_source(3) interface.

   Mergeroptionsmerge_func

           This option specifies a merge function callback, consisting of a function pointer fp and a pointer to
           user data clos which will be passed as the first argument to fp. The merge function callback will be
           used during iteration over the mtbl_merger object to merge entries with duplicate keys in the input
           sources.

           The remaining arguments to the merge function are:

           key — pointer to the key for which there exist duplicate values.

           len_key — length of the key.

           val0 — pointer to the first value.

           len_val_0 — length of the first value.

           val1 — pointer to the second value.

           len_val_1 — length of the second value.

           merged_val — pointer to where the callee should place its merged value.

           len_merged_val — pointer to where the callee should place the length of its merged value.

           merged_val must be allocated with the system allocator, and the mtbl_merger interface takes
           responsibility for free()ing the value once it is no longer needed.

           The callee may provide an empty value as the merged value, in which case merged_val must still
           contain an allocated, non-NULL value and len_merged_val must contain the value 0.

           The callee may indicate an error by returning NULL in the merged_val argument, which will abort
           iteration over the mtbl_merger object.

       dupsort_func

           This option provides a comparison function for multiple data with the same key. It will be used to
           sort the data during the merge process. If the merge_func option is set to or left NULL, the
           dupsort_func will order the results returned by the merge process for data with identical keys.

       mtbl_merger - merge multiple MTBL data sources into a single output

       If the merge function callback is unable to provide a merged value (that is, it fails to return a
       non-NULL value in its merged_val argument), the merge process will be aborted, and any iterators over the
       mtbl_merger object (via the mtbl_source(3) interface) will return mtbl_res_failure.

                                                   03/28/2019                                     MTBL_MERGER(3)

#include<mtbl.h>

       Merger objects:

       structmtbl_merger*mtbl_merger_init(conststructmtbl_merger_options*mopt);voidmtbl_merger_destroy(structmtbl_merger**m);voidmtbl_merger_add_source(structmtbl_merger*m,conststructmtbl_source*s);conststructmtbl_source*mtbl_merger_source(structmtbl_merger*m);

       Merger options:

       structmtbl_merger_options*mtbl_merger_options_init(void);voidmtbl_merger_options_destroy(structmtbl_merger_options**mopt);voidmtbl_merger_options_set_merge_func(structmtbl_merger_options*mopt,mtbl_merge_funcfp,void*clos);voidmtbl_merger_options_set_dupsort_func(structmtbl_merger_options*mopt,mtbl_dupsort_funcfp,void*clos);typedefvoid(*mtbl_merge_func)(void*clos,constuint8_t*key,size_tlen_key,constuint8_t*val0,size_tlen_val0,constuint8_t*val1,size_tlen_val1,uint8_t**merged_val,size_t*len_merged_val);typedefint(*mtbl_dupsort_func)(void*clos,constuint8_t*key,size_tlen_key,constuint8_t*val0,size_tlen_val0,constuint8_t*val1,size_t'len_val1);