Opening a file on unit 5 or 6

Question

I have a read/write operation going on in the Fortran code snippet as follows

OPEN(5,FILE=\'WKDAT.dat\', STATUS=\'OLD\')
OPEN(6,FILE=\'WKLST.dat\', STATUS=\'         


        

Answer 1

Nothing in the standard says units 5 and 6 have any special meaning although in practice standard input and standard output are often pre-connected to 5 and 6.

Module iso_fortran_env from Fortran 2008 contains constants

INPUT_UNIT
OUTPUT_UNIT
ERROR_UNIT

with the unit numbers where standard input, standard output and standard error are connected. These are allowed to be different than 5 and 6.

Opening a file in unit that is in use causes the unit to be associated with the new file.

For example the Cray Fortran manual says:

Unit numbers 100, 101, and 102 are permanently associated with the standard input, standard output, and standard error files, respectively.

That means if you open some other file as unit 5 or 6 standard input and standard output still have some other unit where they are pre-connected and they will not be closed.