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gawk: Two-way processors Customized Two-way Processors
 A "two-way processor" combines an input parser and an output wrapper for
 two-way I/O with the '|&' operator (⇒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'.
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