This section provides compatibility information for those porting DIGITAL Fortran 77 applications from DIGITAL UNIX systems. It discusses the following topics:

• Major language features for compatibility with DIGITAL Fortran 77 for Digital UNIX systems ( Section A.2.1)
• Language differences between DIGITAL Fortran 90 and DIGITAL Fortran 77, including DIGITAL Fortran 77 extensions on Digital UNIX Systems that are not supported by this version of DIGITAL Fortran 90 on DIGITAL UNIX Systems ( Section A.2.2)
• Language features detected during compilation differently by DIGITAL Fortran 90 than DIGITAL Fortran 77 for Digital UNIX Systems ( Section A.2.3)

On Digital UNIX systems, to simplify porting applications from DIGITAL Fortran 77 to DIGITAL Fortran 90, DIGITAL Fortran 90 Version 5.n supports the following DIGITAL Fortran 77 extensions that are not part of the Fortran 90 standard:

• Record structures (STRUCTURE and RECORD statements)
• I/O statements, including PRINT, ACCEPT, TYPE, DELETE, and UNLOCK
• I/O statement specifiers, such as the INQUIRE statement specifiers CARRIAGECONTROL, CONVERT, ORGANIZATION, and RECORDTYPE
• Certain data types, including 8-byte INTEGER and LOGICAL variables and 16-byte REAL variables (available on Alpha systems)
• Size specifiers for data declaration statements, such as INTEGER*4, in addition to the KIND type parameter
• IEEE floating-point data type in memory
• The POINTER statement and its associated data type (CRAY pointers).
• The typeless PARAMETER statement
• The VOLATILE statement
• The AUTOMATIC and STATIC statements
• Built-in functions used in argument lists, such as %VAL and %LOC
• Hollerith constants
• Variable-format expressions (VFEs)
• Certain intrinsic functions
• The tab source form (resembles fixed-source form)
• I/O formatting descriptors
• USEROPEN routines for user-defined open routines
• Additional language features, including the DEFINE FILE, ENCODE, DECODE, and VIRTUAL statements

In addition to language extensions, DIGITAL Fortran 90 Version 5.n also supports the following DIGITAL Fortran 77 features:

• DIGITAL Fortran 77 compilation control statements and directives (see the DIGITAL Fortran Language Reference Manual), including:
  • INCLUDE statement forms using /LIST and /NOLIST (requires compiling with -vms )
  • OPTIONS statement to override or set compiler command-line options
  • General cDEC$ directives, including:
    • cDEC$ ALIAS
    • cDEC$ IDENT
    • cDEC$ OPTIONS
    • cDEC$ PSECT
    • cDEC$ TITLE
    • cDEC$ SUBTITLE
• A nearly identical set of command-line options and their associated features (see Section A.2.4).
• The ability to call between DIGITAL Fortran 77 and DIGITAL Fortran 90 routines and a common run-time environment. For example, a DIGITAL Fortran 77 procedure and a DIGITAL Fortran 90 procedure can perform I/O to the same unit number (see Section 11.3).
• foriosdef.for symbolic parameter definitions for use with run-time (IOSTAT) error handling (see Chapter 8).

For More Information:

On the DIGITAL Fortran 90 language, see the DIGITAL Fortran Language Reference Manual.

A.2.2 Language Features Provided Only by DIGITAL Fortran 77 for DIGITAL UNIX Systems

This section lists DIGITAL Fortran 77 extensions to the FORTRAN-77 standard that are not included in DIGITAL Fortran 90 Version 5.n for DIGITAL UNIX Systems. Where appropriate, this list indicates equivalent DIGITAL Fortran 90 language features.

DIGITAL Fortran 90 conforms to the Fortran 90 standard, which is a superset of the FORTRAN-77 standard. DIGITAL Fortran 90 provides many but not all of the FORTRAN-77 extensions provided by DIGITAL Fortran 77.

The following FORTRAN-77 extensions provided by DIGITAL Fortran 77 on Digital UNIX systems are not provided by DIGITAL Fortran 90 in Version 5.n:

• Octal notation for integer constants is not part of the DIGITAL Fortran 90 Language. DIGITAL Fortran 77 ( f77 command) only supports this feature when the -vms option is specified. For example:

  I = "0014 ! Assigns 12 to I, not supported by DIGITAL Fortran 90

• The DIGITAL Fortran 90 language does not allow field names specified in the STRUCTURE statement to be the same as intrinsic or user defined operators. For example:

  STRUCTURE /FOO/ INTEGER EQ ! Incorrect END STRUCTURE

• The DIGITAL Fortran 90 compiler discards leading zeros for "disp" in the STOP statement. For example:

  STOP 001 ! Prints 1 instead of 001

• The DIGITAL Fortran 90 language prohibits dummy arguments with nonconstant bounds from being a namelist item. For example:

  SUBROUTINE FOO(A,N) DIMENSION A(N),B(10) NAMELIST /N1/ A ! Incorrect NAMELIST /N2/ B ! Correct END SUBROUTINE

• When a single-precision constant is assigned to a double-precision variable, DIGITAL Fortran 77 evaluates the constant in double precision. The Fortran 90 standard requires that the constant be evaluated in single precision.
  When a single-precision constant is assigned to a double-precision variable with DIGITAL Fortran 90, it is evaluated in single precision. You can, however, specify the f90 -fpconstant option to request that a single-precision constant assigned to a double-precision variable be evaluated in double precision.
  In the example below, DIGITAL Fortran 77 assigns identical values to D1 and D2, whereas DIGITAL Fortran 90 obeys the standard and assigns a less precise value to D1.
  For example:

  REAL*8 D1,D2 DATA D1 /2.71828182846182/ ! Incorrect - only REAL*4 value DATA D2 /2.71828182846182D0/ ! Correct - REAL*8 value

• The names of intrinsics introduced by DIGITAL Fortran 90 may conflict with the names of existing external procedures if the procedures were not specified in an EXTERNAL declaration. For example:

  EXTERNAL SUM REAL A(10),B(10) S = SUM(A) ! Correct - invokes external function T = DOT_PRODUCT(A,B) ! Incorrect - invokes intrinsic function

• When writing namelist external records, DIGITAL Fortran 90 uses the syntax for namelist external records specified by the Fortran 90 standard, rather than the DIGITAL Fortran 77 syntax (an extension to the FORTRAN-77 and Fortran 90 standards).
  Consider the following program:

  % cat test.f INTEGER I NAMELIST /N/ I I = 5 PRINT N END

  
  When this program is compiled by the f90 command and run, the following output appears:

  % f90 test.f % a.out &N I = 5 /

  
  When this program is compiled by the f77 command and run, the following output appears:

  % f77 test.f % a.out $N I = 5 $END

  
  DIGITAL Fortran 90 accepts Fortran 90 namelist syntax and DIGITAL Fortran 77 namelist syntax for reading records.

• The DIGITAL Fortran 90 language does not include C-style escape sequences. For example:

  CHARACTER NL NL = '\n' ! Incorrect NL = CHAR(10) ! Correct

• DIGITAL Fortran 90 inserts a leading blank when doing list-directed I/O to an internal file. For example:

  CHARACTER*10 C WRITE(C,*) 'FOO' ! C = ' FOO'

• DIGITAL Fortran 77 and DIGITAL Fortran 90 produce different output a real value whose data magnitude is 0 with a G field descriptor. For example:

  X = 0.0 WRITE(*,100) X ! DIGITAL Fortran 77 prints 0.0000E+00 100 FORMAT(G12.4) ! DIGITAL Fortran 90 prints 0.000

• DIGITAL Fortran 90 does not allow certain intrinsics (such as SQRT) in constant expressions for array bounds. For example:

  REAL A(SQRT(31.5)) END

• DIGITAL Fortran 77 returns UNKNOWN while DIGITAL Fortran 90 returns UNDEFINED when the ACCESS, BLANK, and FORM characteristics can not be determined. For example:

  INQUIRE(20,ACCESS=acc,BLANK=blk,FORM=form)

• DIGITAL Fortran 90 does not allow an extraneous parenthesis in I/O lists. For example:

  write(*,*) ((i,i=1,1),(j,j=1,2))

• DIGITAL Fortran 90 does not allow control characters within quoted strings. For example, if a Ctrl/C appears in a text string:

  character*5 c c = 'ab^cef' end

• DIGITAL Fortran 90 does not recognize certain hexadecimal and octal constants in DATA statements, such as those used in the following program:

  INTEGER I, J DATA I/O20101/, J/Z20/ TYPE *, I, J END

• DIGITAL Fortran 90, like DIGITAL Fortran 77, supports the use of character literal constants (such as 'ABC' or "ABC") in numeric contexts, where they are treated as Hollerith constants.
  DIGITAL Fortran 77 also allows character PARAMETER constants (typed and untyped) and character constant expressions (using the // operator) in numeric constants as an undocumented extension.
  DIGITAL Fortran 90 does allow character PARAMETER constants in numeric contexts, but does not allow character expressions. For example, the following is valid for DIGITAL Fortran 77, but will result in an error message from DIGITAL Fortran 90:

  REAL*8 R R = 'abc' // 'def' WRITE (5,*) R END

  
  DIGITAL Fortran 90 does allow PARAMETER constants:

  PARAMETER abcdef = 'abc' // 'def' REAL*8 R R = abcdef WRITE (5,*) R END

• DIGITAL Fortran 77 namelist output formats character data delimited with apostrophes. For example, consider:

  CHARACTER CHAR4*4 NAMELIST /CN100/ CHAR4 CHAR4 = 'ABCD' WRITE(20,CN100) CLOSE (20)

  
  This produces the following output file:

  $CN100 CHAR4 = 'ABCD' $END

  
  This file is read by:

  READ (20, CN100)

  
  In contrast, DIGITAL Fortran 90 produces the following output file by default:

  &CN100 CHAR4 = ABCD /

  
  When read, this generates a syntax error in NAMELIST input error. To produce delimited strings from namelist output that can be read by namelist input, use DELIM="'" in the OPEN statement of a DIGITAL Fortran 90 program.

For More Information:

• On argument passing between DIGITAL Fortran 90 and DIGITAL Fortran 77 for DIGITAL UNIX systems, see Section 11.3.
• On compatibility between DIGITAL Fortran 90 for DIGITAL UNIX systems and DIGITAL Fortran 77 on OpenVMS systems, see Section A.4.
• About the DIGITAL Fortran 90 language, see the DIGITAL Fortran Language Reference Manual.

The following language features are detected or interpreted differently by DIGITAL Fortran 90 Version 5.n and DIGITAL Fortran 77:

• The DIGITAL Fortran 90 compiler enforces the constraint that a function cannot be the target of a CALL statement. For example:

  REAL X CALL X() ! Incorrect CALL Y() ! Correct

• The DIGITAL Fortran 90 compiler enforces the constraint that the "nlist" in an EQUIVALENCE statement must contain at least two variables. For example:

  EQUIVALENCE (X) ! Incorrect EQUIVALENCE (Y,Z) ! Correct

• The DIGITAL Fortran 90 compiler enforces the constraint that entry points in a SUBROUTINE must not be typed. For example:

  SUBROUTINE ABCXYZ(I) REAL ABC I = I + 1 RETURN ENTRY ABC ! Incorrect BAR = I + 1 RETURN ENTRY XYZ ! Correct I = I + 2 RETURN END SUBROUTINE

• The DIGITAL Fortran 90 compiler enforces the constraint that a type must appear before each list in an IMPLICIT statement. For example:

  IMPLICIT REAL (A-C), (D-H) ! Incorrect IMPLICIT REAL (O-S), REAL (T-Z) ! Correct

• The DIGITAL Fortran 90 language disallows passing mismatched actual arguments to intrinsics with corresponding integer formal arguments. For example:

  R = REAL(.TRUE.) ! Incorrect R = REAL(1) ! Correct

• The DIGITAL Fortran 90 compiler enforces the constraint that a simple list element in an I/O list must be a variable or an expression. For example:

  READ (10,100) (I,J,K) ! Incorrect READ (10,100) I,J,K ! Correct

• The DIGITAL Fortran 90 compiler enforces the constraint that if two operators are consecutive, the second operator must be a plus or a minus. For example:

  I = J -.NOT.K ! Incorrect I = J - (.NOT.K) ! Correct

• The DIGITAL Fortran 90 compiler enforces the constraint that character entities with a length greater than 1 cannot be initialized with a bit constant in a DATA statement. For example:

  CHARACTER*1 C1 CHARACTER*4 C4 DATA C1/'FF'X/ ! Correct DATA C4/'FFFFFFFF'X/ ! Incorrect

• The DIGITAL Fortran 90 compiler enforces the requirement that edit descriptors in the FORMAT statement must be followed by a comma or slash separator. For example:

  1 FORMAT (SSF4.1) ! Incorrect 2 FORMAT (SS,F4.1) ! Correct

• The DIGITAL Fortran 90 compiler enforces the constraint that the number and types of actual and formal statement function arguments must match (such as incorrect number of arguments). For example:

  CHARACTER*4 C,C4,FUNC FUNC()=C4 C=FUNC(1) ! Incorrect C=FUNC() ! Correct

• The DIGITAL Fortran 90 compiler detects the use of a format of the form Ew.dE0 at compile time. For example:

  1 format(e16.8e0) ! DIGITAL Fortran 90 detects error at compile time write(*,1) 5.0 ! DIGITAL Fortran 77 compiles but an output ! conversion error occurs at run time

• DIGITAL Fortran 90 detects passing of a statement function to a routine. For example:

  foo(x) = x * 2 call bar(foo) end

• The DIGITAL Fortran 90 compiler enforces the constraint that a branch to a statement shared by more than one DO statements must occur from within the innermost loop. For example:

  DO 10 I = 1,10 IF (L1) GO TO 10 ! Incorrect DO 10 J = 1,10 IF (L2) GO TO 10 ! Correct 10 CONTINUE

• The DIGITAL Fortran 90 compiler enforces the constraint that a file must contain at least one program unit. For example, a source file containing only comment lines results in an error at the last line (end-of-file).
  The DIGITAL Fortran 77 compiler compiles files containing less than one program unit.
• The DIGITAL Fortran 90 compiler correctly detects misspellings of the ASSOCIATEVARIABLE keyword to the OPEN statement. For example:

  OPEN(1,ASSOCIATEVARIABLE = I) ! Correct OPEN(2,ASSOCIATEDVARIABLE = J) ! Incorrect (extra D)

• The DIGITAL Fortran 90 language enforces the constraint that the result of an operation is determined by the data types of its operands. For example:

  INTEGER*8 I8 I8 = 2147483647 + 1 ! Incorrect. Produces less accurate ! INTEGER*4 result I8 = 2147483647_8 + 1_8 ! Correct

• The DIGITAL Fortran 90 compiler enforces the constraint that an object can be typed only once. DIGITAL Fortran 77 issues a warning message and uses the first type. For example:

  LOGICAL B,B ! Incorrect (B multiply declared)

• The DIGITAL Fortran 90 compiler enforces the constraint that certain intrinsic procedures defined by the Fortran 90 standard cannot be passed as actual arguments. For example, DIGITAL Fortran 77 allows most intrinsic procedures to be passed as actual arguments, but the DIGITAL Fortran 90 compiler only allows those defined by the Fortran 90 standard (issues an error message).
  Consider the following program:

  program tstifx intrinsic ifix,int,sin call a(ifix) call a(int) call a(sin) stop end subroutine a(f) external f integer f print *, f(4.9) return end

  
  The IFIX and INT intrinsic procedures cannot be passed as actual arguments (the compiler issues an error message). However, the SIN intrinsic is allowed to be passed as an actual argument by the Fortran 90 standard.

• DIGITAL Fortran 90 reports character truncation with an error-level message, not as a warning.
  The following program produces an error message during compilation with DIGITAL Fortran 90, whereas DIGITAL Fortran 77 produces a warning message.

  INIT5 = 'ABCDE' INIT4 = 'ABCD' INITLONG = 'ABCDEFGHIJKLMNOP' PRINT 10, INIT5, INIT4, INITLONG 10 FORMAT (' ALL 3 VALUES SHOULD BE THE SAME: ' 3I) END

• If your code invokes DIGITAL Fortran 90 intrinsic procedures with the wrong number of arguments or an incorrect argument type, DIGITAL Fortran 90 reports this with an error-level message, not with a warning. Possible causes include:
  • A DIGITAL Fortran 90 intrinsic has been added with the same name as a user-defined subprogram and the user-defined subprogram needs to be declared as EXTERNAL.
  • An intrinsic that is an extension to an older Fortran standard is incompatible with a newer standard-conforming intrinsic (for example, the older RAN function that accepted two arguments).
  
  The following program produces an error message during compilation with DIGITAL Fortran 90, whereas DIGITAL Fortran 77 produces a warning message.

  INTEGER ANOTHERCOUNT ICOUNT=0 100 write(6,105) (ANOTHERCOUNT(ICOUNT), INT1=1,10) 105 FORMAT(' correct if print integer values 1 through 10' /10I7) Q = 1. R = .23 S = SIN(Q,R) WRITE (6,110) S 110 FORMAT(' CORRECT = 1.23 RESULT = ',f8.2) END ! INTEGER FUNCTION ANOTHERCOUNT(ICOUNT) ICOUNT=ICOUNT+1 ANOTHERCOUNT=ICOUNT RETURN END REAL FUNCTION SIN(FIRST, SECOND) SIN = FIRST + SECOND RETURN END

• DIGITAL Fortran 90 reports missing commas in FORMAT descriptors with an error-level message, not as a warning.
  The following program produces an error message during compilation with DIGITAL Fortran 90, whereas DIGITAL Fortran 77 produces a warning message:

  LOGICAL LOG/111/ TYPE 1,LOG 1 FORMAT(' '23X,'LOG='O12) END

  
  In the preceding example, the correct coding (adding the missing comma) for the FORMAT statement is:

  1 FORMAT(' ',23X,'LOG='O12)

• DIGITAL Fortran 90 generates an error when it encounters a 1-character source line containing a Ctrl/Z character, whereas DIGITAL Fortran 77 allows such a line (which is treated as a blank line).
• DIGITAL Fortran 90 does not detect an extra comma in an I/O statement when the -std option is specified, whereas DIGITAL Fortran 77 with the -stand option identifies an extra comma as an extension. For example:

  WRITE(*,*) , P(J)

• DIGITAL Fortran 90 detects the use of a character variable within parentheses in an I/O statement. For example:

  CHARACTER*10 CH/'(I5)'/ INTEGER I READ CH,I ! Acceptable READ (CH),I ! Generates error message, interpreted as an internal READ END

• DIGITAL Fortran 90 evaluates the exponentiation operator at compile time only if the exponent has an integer data type. DIGITAL Fortran 77 evaluates the exponentiation operator even when the exponent does not have an integer data type. For example:

  PARAMETER ( X = 4.0 ** 1.1)

• DIGITAL Fortran 90 detects an error when evaluating constants expressions that result in an NaN or Infinity exceptional value, while DIGITAL Fortran 77 allows such expressions. For example:

  PARAMETER ( X = 4.0 / 0.0 )

• DIGITAL Fortran 90 reports a warning error message when the same variable is initialized more than once. DIGITAL Fortran 77 allows multiple initializations of the same variable without a warning. For example:

  integer i data i /1/ data i /2/ write (*,*) i stop end

For More Information:

• On passing arguments and returning function values between DIGITAL Fortran 90 and DIGITAL Fortran 77, see Section 11.3.
• On DIGITAL Fortran 90 procedure calling and argument passing, see Section 11.1.
• On compatibility between DIGITAL Fortran 90 for DIGITAL UNIX systems and DIGITAL Fortran 77 on OpenVMS systems, see Section A.4.
• On the DIGITAL Fortran 90 language, see the DIGITAL Fortran Language Reference Manual.