User's Guide

Upgrading to XL Fortran Version 6

Here is some advice to help make the transition from an earlier FORTRAN 77 or Fortran 90 compiler platform as fast and simple as possible.

Things to Note in XL Fortran Version 6

Because XL Fortran Version 6 Release 1 is highly compatible with XL Fortran Version 5, Version 4, and Version 3, most of the advice in this section applies to upgrades from Version 2 or earlier of XL Fortran.

The xlf90, xlf90_r, and xlf90_r7 commands provide Fortran 90 conformance, and the xlf95, xlf95_r, and xlf95_r7 commands provide Fortran 95 conformance. However, these commands may cause some problems with existing FORTRAN 77 programs. The xlf, xlf_r, xlf_r7, and f77 commands avoid some of these problems by keeping the old behavior wherever possible.
Fortran 90 introduced the idea of kind parameters for types. Except for the types complex and character, XL Fortran uses numeric kind parameters that correspond to the lengths of the types. For the type complex, the kind parameter is equal to the length of the real portion, which is half of the overall length. For the type character, the kind parameter is equal to the number of bytes that are required to represent each character, and this value is 1. A FORTRAN 77 declaration that is written using the * extension for length specifiers can now be rewritten with a kind parameter:
```
       INTEGER*4  X   ! F77 notation with extension.
       INTEGER(4) X   ! F90 standard notation.
       COMPLEX*8  Y   ! *n becomes (n) for all types except
       COMPLEX(4) Y   ! COMPLEX, where the value is halved.
```
This new form is the one we use consistently throughout the XL Fortran manuals.
Because the values of kind parameters may be different for different compilers, you may want to use named constants, placed in an include file or a module, to represent the kind parameters used in your programs. The SELECTED_INT_KIND and SELECTED_REAL_KIND intrinsic functions also let you determine kind values in a portable way.
Fortran 90 introduced a standardized free source form for source code, which is different from the XL Fortran Version 2 free source form. The -qfree and -k options now use the Fortran 90 free source form; the Version 2 free source form is available through the option -qfree=ibm.
The libraries that provide Fortran 90 and Fortran 95 support are libxlf90_r.a and libxlf90.a, located in /usr/lib. A libxlf.a library of stub routines is provided in /usr/lib, but it is only used for linking existing Version 1 or Version 2 object files or running existing executables. When a Version 1 or Version 2 object file calls entry points in libxlf.a, those entry points then call equivalent entry points in libxlf90.a (for single-threaded programs) or libxlf90_r.a (for multi-threaded programs). If you recompile such object files, the result could be improved I/O performance, because the entry points in libxlf90.a or libxlf90_r.a are called directly. The library libxlf90_r.a has been provided in addition to libxlf90_t.a. The library libxlf90_r.a is a superset of libxlf90_t.a. The file xlf.cfg has been set up to link to libxlf90_r.a automatically when you use the xlf90_r, xlf90_r7, xlf95_r, xlf95_r7, xlf_r, or xlf_r7 command. Note that on AIX Version 4.3, a single combined thread-safe library, libxlf90.a, is provided to support both single- and multiple-threaded applications. The libxlf90_r.a library is a symbolic link to libxlf90.a.