File: gawk.info,  Node: Two-way processors,  Prev: Output Wrappers,  Up: Registration Functions

17.4.6.6 Customized Two-way Processors
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A "two-way processor" combines an input parser and an output wrapper for
two-way I/O with the '|&' operator (*note Redirection::).  It makes
identical use of the 'awk_input_parser_t' and 'awk_output_buf_t'
structures as described earlier.

   A two-way processor is represented by the following structure:

     typedef struct awk_two_way_processor {
         const char *name;   /* name of the two-way processor */
         awk_bool_t (*can_take_two_way)(const char *name);
         awk_bool_t (*take_control_of)(const char *name,
                                       awk_input_buf_t *inbuf,
                                       awk_output_buf_t *outbuf);
         awk_const struct awk_two_way_processor *awk_const next;  /* for gawk */
     } awk_two_way_processor_t;

   The fields are as follows:

'const char *name;'
     The name of the two-way processor.

'awk_bool_t (*can_take_two_way)(const char *name);'
     The function pointed to by this field should return true if it
     wants to take over two-way I/O for this file name.  It should not
     change any state (variable values, etc.)  within 'gawk'.

'awk_bool_t (*take_control_of)(const char *name,'
'                              awk_input_buf_t *inbuf,'
'                              awk_output_buf_t *outbuf);'
     The function pointed to by this field should fill in the
     'awk_input_buf_t' and 'awk_output_buf_t' structures pointed to by
     'inbuf' and 'outbuf', respectively.  These structures were
     described earlier.

'awk_const struct two_way_processor *awk_const next;'
     This is for use by 'gawk'; therefore it is marked 'awk_const' so
     that the extension cannot modify it.

   As with the input parser and output processor, you provide "yes I can
take this" and "take over for this" functions, 'XXX_can_take_two_way()'
and 'XXX_take_control_of()'.

   You register your two-way processor with the following function:

'void register_two_way_processor(awk_two_way_processor_t *two_way_processor);'
     Register the two-way processor pointed to by 'two_way_processor'
     with 'gawk'.