rb_econv_putback  255 include/ruby/encoding.h void rb_econv_putback(rb_econv_t *ec, unsigned char *p, int n);
rb_econv_putback 1560 io.c                       rb_econv_putback(fptr->readconv, (unsigned char *)fptr->rbuf + fptr->rbuf_off - putbackable, putbackable);
rb_econv_putback 3786 transcode.c        rb_econv_putback(ec, (unsigned char *)RSTRING_PTR(str), n);