OUTPUT_ERR
#define ferror(f) ((f)->flags & OUTPUT_ERR)
#define clearerr(f) ((f)->flags &= ~OUTPUT_ERR)
dest->flags |= OUTPUT_ERR;
dest->flags |= OUTPUT_ERR;
if (strout.flags & OUTPUT_ERR)
#define clr_err(file) ((file)->flags &= ~OUTPUT_ERR)
#define io_err(file) ((file)->flags & OUTPUT_ERR)