Use Configure's FILE_filbuf macro instead of a raw _filbuf.
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Version 5.003_02
FILE_cnt(fp) = cnt; /* deregisterize cnt and ptr */
FILE_ptr(fp) = ptr;
- i = _filbuf(fp); /* get more characters */
+ i = FILE_filbuf(fp); /* get more characters */
cnt = FILE_cnt(fp);
ptr = FILE_ptr(fp); /* reregisterize cnt and ptr */