In encode mode, the bin7 filter converts its input into 7-bit ASCII, by clearing the 8th bit on each processed byte.
In decode mode, it operates exactly as the 8bit filter, i.e. copies its input to the output verbatim.
This filter is described in mailutils/filter.h:
extern mu_filter_record_t mu_bit7_filter;
To create this filter, use the mu_filter_stream_create function:
int rc; /* Return code */ mu_stream_t flt; /* Filter stream */ mu_stream_t input; /* Input stream */ initialize_input_stream (&stream); rc = mu_filter_stream_create (&flt, input, "bin7", MU_FILTER_DECODE, MU_STREAM_READ);