[patch, Fortran] PR61933 Inquire on internal unit

[Jerry DeLisle]

Hello all,

I had to do a bit of research on this.  I hope I have this correct.  For 
background information, an "internal file" is a character variable that is 
connected to an internal unit.  Likewise, an internal unit refers to an internal 
file which is a character variable.

The subject refers to an "internal unit".  This is actually touching on the 
fringes of User Defined Derived Type I/O. In at least the draft version of F2008 
Standard I have at the moment, I find the following:

1) "The unit argument passed to a defined input/output procedure will be 
negative when the parent input/output statement specified an internal unit, or 
specified an external unit that is a NEWUNIT value. When an internal unit is 
used with the INQUIRE statement, an error condition will occur, and any variable 
specified in an IOSTAT= specifier will be assigned the value 
IOSTAT_INQUIRE_INTERNAL_UNIT from the intrinsic module ISO FORTRAN ENV"

Also regarding IOSTAT= we have:

2) "Execution of an input/output statement containing the IOSTAT= specifier 
causes the scalar-int-variable in the IOSTAT= specifier to become defined with: 
the processor-dependent positive integer value of the constant 
IOSTAT_INQUIRE_INTERNAL_UNIT from the intrinsic module ISO_FORTRAN_ENV(13.8.2) 
if a unit number in an INQUIRE statement identifies an internal file"

3) The standard lists as one of the processor dependencies - "the negative value 
of the unit argument to a defined input/output procedure if the parent data 
transfer statement accesses an internal file"

To summarize all this, a negative unit number is used to communicate to defined 
type input/output procedures that the object of their I/O actions is an internal 
file, aka a character variable rather than an external file.  To avoid 
ambiguity, the negative value should not be one created by NEWUNIT.  INQUIRE on 
an internal file is not allowed and a specific error code is defined in the 
standard to be IOSTAT_INQUIRE_INTERNAL_UNIT.  I suspect the standards committee 
recognized the need to communicate to DTIO procedures and defined this error 
code to enable user code to check for this condition and pass it back up to the 
parent?

When I first implemented NEWUNIT, I reserved the values -1 thru -9 for future 
use. Our earlier email discussions on this PR centered around the case of 
UNIT=-1.  This patch generates an error for -1 only.  We can always change this 
later if the need to use -2 thru -9 comes about.

Assuming we will implement the UDDTIO, we will then use -1 for the unit number 
to convey when an internal unit has been specified in the parent input/output 
procedure. As far as I can tell, the value used is processor defined and it 
appears that the cray COMPILER uses -1. Nothing special about that choice.

I have eliminated the previous hack* I did in trans-io.c to handle the 
-fdefault-integer-8 problem way back in PR33055, reject UNIT=-1 at compile time 
in io.c, and simplified the definition of unit existence to whether or not the 
unit is connected in inquire.c.

Finally, IOSTAT_INQUIRE_INTERNAL_UNIT was already defined for us in the 
intrinsic module ISO_FORTRAN_ENV but was never added into translate_error in 
runtime/error.c, so I have now done so and modified the test case to check it.

Regression tested on x86-64-linux. New test cases added.

OK for trunk?

Regards,

Jerry

* hey, what can I say, my work is cheap as in free beer.

2015-01-09  Jerry DeLisle  <jvdelisle@gcc.gnu.org>

	PR libgfortran/61933
	* io/inquire.c (inquire_via_unit): Set existing to true if a
	gfc_unit stucture was found for the given unit number.
	* runtime/error.c (translate_error): Add case for
	LIBERROR_INQUIRE_INTERNAL_UNIT.

2015-01-09  Jerry DeLisle  <jvdelisle@gcc.gnu.org>

	PR fortran/61933
	* io.c (gfc_match_inquire): Generate error if unit number in
	inquire statement is a constant -1.  All other values allowed.
	* trans-io.c (gfc_trans_inquire): Delete dummy iostat variable.
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