XL Fortran Enterprise Edition for AIX : User's Guide - IBM

Table 1. Potential Problems Migrating Programs to XL Fortran Version 9 (continued). The
column on the right shows which problems you can avoid by using the xlf or f77 command.
Potential Problem Solution or Workaround xlf Avoids?
Programs that read to the end of the file
and then try to append records without first
executing a BACKSPACE statement do not
work correctly. The write requests generate
error messages.
Uninitialized variables are not necessarily
set to zero, and programs that ran before
may exceed the user stack limit. The reason
is that the default storage class is now
AUTOMATIC, rather than STATIC (an
implementation choice allowed by the
language).
Writing data to some files opened without a
POSITION= specifier overwrites the files,
instead of appending the data.
Newly compiled programs are unable to
read existing data files containing
NAMELIST data. The reason is that the
Fortran 90 and Fortran 95 standards define
a namelist format that is different from that
used on AIX in the past.
Some I/O statements and edit descriptors
accept or produce slightly different input
and output. For example, real output now
has a leading zero when appropriate.
The changes to I/O formats are intended to
be more usable and typical of industry
practice, so you should try to use the
defaults for any new data you produce.
Numeric results and I/O output are not
always exactly identical with XL Fortran
Version 2. Certain implementation details of
I/O, such as spacing in list-directed output
and the meanings of some IOSTAT values,
have changed since XL Fortran Version 2.
(This entry is similar to the previous one
except that these differences have no
backward-compatibility switches.)
Run-Time Problems
To compile existing
programs, specify the option
-qxlf77=softeof with the
xlf90 or xlf95 command, or
use the xlf command. For
new programs, add the
BACKSPACE statement
before writing past the
endfile record.
Ensure that you explicitly
initialize your variables, use
the -qsave option with the
xlf90 or xlf95 command, or
add SAVE statements where
needed in the source.
Use the option
-qposition=appendold, or
add POSITION= specifiers
where needed.
Set the environment variable
XLFRTEOPTS to the string
namelist=old.
The programs that produced
the old NAMELIST data
must be recompiled.
When you need to maintain
compatibility with existing
data files, compile with the
xlf command. If the
incompatibility is due to a
single specific I/O change,
see if the -qxlf77 option has
a suboption for backward
compatibility. If so, you can
switch to the xlf90 or xlf95
command and use the
-qxlf77 option on programs
that use the old data files.
You may need to generate
existing data files again or to
change any programs that
depend on these details.
When no
backward-compatibility
switch is provided by the
-qxlf77 compiler option or
XLFRTEOPTS run-time
options, there is no way to
get the old behavior back.
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Setting Up and Customizing XL Fortran 27