Valid unit numbers are all nonnegative numbers except 100 through 102. Unit numbers 0, 5, and 6 are associated with the standard error, standard input, and standard output files; any unit can also refer to a pipe. All other valid unit numbers are associated with the fort.i file, or with the file name implied in a Hollerith unit number. Use the INQUIRE statement to check the validity (existence) of any unit number prior to using it, as in the following example: