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## LIMITATIONS AND EXTENSIONS

ftnchek accepts ANSI standard Fortran-77 programs with some minor limitations and numerous common extensions.

Limitations:
    ftnchek uses only one line of lookahead when analyzing a program into its basic syntactic elements. If a particular statement is difficult to identify, it may be handled improperly if the ambiguity is not resolved on a single line. This limitation applies to complex constants except in DATA statements, and to situations in which a variable name might be confused with a keyword. For example, if the variable name WRITE is used for an array, then a very long statement assigning a value to some element of this array could be mistaken as a WRITE statement if the equals sign is not on the same line as the word WRITE.

    The dummy arguments in statement functions are treated like ordinary variables of the program. That is, their scope is the entire subprogram, not just the statement function definition.

    The checking of FORMAT statements is lax, tolerating missing separators (comma, etc.) between format descriptors in places where the Standard requires them, and allowing .d fields on descriptors that should not have them. It does warn under -f77=format-edit-descr about nonstandard descriptor types (like O), and supported extensions.

    If a user-supplied subprogram has the same name as one of the nonstandard intrinsic functions recognized by ftnchek, it must be declared in an EXTERNAL statement in any routine that invokes it. Otherwise it will be subject to the checking normally given to the intrinsic function. Since the nonstandard intrinsics are not standard, this EXTERNAL statement is not required by the Fortran 77 Standard. Using the -intrinsic=none setting, recognition of most nonstandard intrinsics (excepting only those needed to support the double complex data type) can be turned off. See the lists of supported nonstandard intrinsic functions under the discussion of the -intrinsic setting above.

Extensions:
    All of these extensions (except lower-case characters) will generate warnings if the relevant -f77 option is set. Some of the extensions listed below are part of the Fortran-90 Standard. These are indicated by the notation (F90).

    Tabs are permitted, and translated into equivalent blanks which correspond to tab stops every 8 columns. The standard does not recognize tabs. Note that some compilers allow tabs, but treat them differently. The treatment defined for DEC FORTRAN can be achieved using the -source=dec-tab setting.

    Strings may be delimited by either quote marks or apostrophes. A sequence of two delimiter characters is interpreted as a single embedded delimiter character. (F90)

    Strings may contain UNIX-style backslash escape sequences. They will be interpreted as such if the -source=unix-backslash setting is given. Otherwise the backslash character will be treated as a normal printing character.

    Source code can be in either Fortran 90 free format or traditional fixed format. (F90)

    A semicolon is allowed as a statement separator. (F90)

    Lower case characters are permitted, and are converted internally to uppercase except in character strings. The standard specifies upper case only, except in comments and strings. (F90)

    Hollerith constants are permitted, in accordance with the Fortran 77 Standard, appendix C. They should not be used in expressions, or confused with datatype CHARACTER.

    The letter 'D' (upper or lower case) in column 1 is treated as the beginning of a comment. There is no option to treat such lines as statements instead of comments.

    Statements may be longer than 72 columns provided that the setting -columns was used to increase the limit. According to the standard, all text from columns 73 through 80 is ignored, and no line may be longer than 80 columns.

    Variable names may be longer than six characters. The standard specifies six as the maximum. ftnchek permits names up to 31 characters long (F90).

    Variable names may contain underscores and dollar signs (or other non-alphabetic characters as specified by the -identifier-chars option). These characters are are treated the same as alphabetic letters. The default type for variables beginning with these characters is REAL. In IMPLICIT type statements specifying a range of characters, the dollar sign follows Z and is followed by underscore. (Any other user-defined characters are treated the same as the dollar sign.) Fortran 90 permits underscores in variable names.

    The UNIX version tolerates the presence of preprocessor directives, namely lines beginning with the pound sign (#). These are treated as comments, except for #line directives, which are interpreted, and are used to set the line number and source file name for warnings and error messages. Note that #include directives are not processed by ftnchek. Programs that use them for including source files should be passed through the preprocessor before being input to ftnchek. As noted below, ftnchek does process INCLUDE statements, which have a different syntax. An optional program, ftnpp(1L) (available separately) provides preprocessing that properly handles INCLUDE files.

    The Fortran 90 DO ... ENDDO control structure is permitted. The CYCLE and EXIT statements are accepted. All of these may have an optional do-construct name, but construct names are not checked for consistency. (F90)

    The Fortran 90 SELECT CASE construct is accepted. (F90)

    Construct names are also accepted on IF, THEN, ELSE, ENDIF and SELECT CASE statements. (F90)

    The ACCEPT and TYPE statements (for terminal I/O) are permitted, with the same syntax as PRINT.

    The so-called ``Cray pointer'' syntax is tolerated. It is not the same as the Fortran 90 POINTER statement. There is no real checking of the statement other than basic syntax. The form of this statement is
    POINTER (pointer, pointee) [,(pointer, pointee)]
    The pointer variables are assigned a data type of INTEGER *4. Usage checking of the pointee variables is suppressed, since in practice they are accessed indirectly via the pointers.

    Statements may have any number of continuation lines. The Fortran 77 and Fortran 90 standards allow a maximum of 19 in fixed source form. The Fortran 90 standard allows a maximum of 39 in free source form.

    Relational (comparison) operators composed of punctuation, namely: < <= == /= > >= are allowed. (F90)

    Inline comments, beginning with an exclamation mark, are permitted. (F90)

    NAMELIST I/O is supported. The syntax is the same as in Fortran 90.

    FORMAT statements can contain a dollar sign to indicate suppression of carriage-return. An integer expression enclosed in angle brackets can be used anywhere in a FORMAT statement where the Fortran 77 Standard allows an integer constant (except for the length of a Hollerith constant), to provide a run-time value for a repeat specification or field width.

    Nonstandard keywords are allowed in I/O statements, corresponding to those in VMS Fortran.

    The IMPLICIT NONE statement is supported. The meaning of this statement is that all variables must have their data types explicitly declared. Rather than flag the occurrences of such variables with syntax error messages, ftnchek waits till the end of the module, and then prints out a list of all undeclared variables, as it does for the -declare option. (F90)

    Data types INTEGER, REAL, COMPLEX, and LOGICAL are allowed to have an optional precision specification in type declarations. For instance, REAL*8 means an 8-byte floating point data type. The REAL*8 datatype is not necessarily considered equivalent to DOUBLE PRECISION, depending on the -wordsize setting. The Fortran 77 Standard allows a length specification only for CHARACTER data.

    ftnchek supports the DOUBLE COMPLEX type specification for a complex quantity whose real and imaginary parts are double precision. Mixed-mode arithmetic involving single-precision complex with double-precision real data, prohibited under the Standard, yields a double complex result.

    Combined type declarations and data-statement-like initializers are accepted. These have the form of a standard Fortran 77 type declaration, followed by a slash-delimited list of constants like that used in a DATA statement. An example of the syntax is
    INTEGER N / 100 /
    This bastard form of initializing declaration was not adopted in Fortran 90. Such declarations should be written using the standard form described below, which is accepted by ftnchek.

    There is limited support for Fortran 90 attribute-based type declarations. This style of declaration is distinguished by the use of a double colon (::) between the list of attributes and the list of declared variables. The features supported may be adequate for novice programmers, but are not yet sufficient for professional-quality Fortran 90 programs. I hope to add support for more features in future releases. I invite volunteers to assist in this task. See the ToDo file in the source code distribution for details. The attributes currently accepted, besides all the usual data types, are DIMENSION, EXTERNAL, INTRINSIC, PARAMETER, and SAVE. The new form of declaration also allows assignment of values to the variables declared. At present, the (LEN=value) form of specifying character lengths is also accepted. Kind specifications, using (KIND=value) are parsed but are not processed: all kinds are treated as default kind. Also, there is little checking of these declarations beyond basic syntax.

    Many commonly found nonstandard intrinsic functions are provided. See the discussion of -intrinsic for a list of functions and how to control which ones are recognized.

    Argument checking is not tight for those nonstandard intrinsics that take arrays or mixed argument types.

    ftnchek permits the INCLUDE statement, which causes inclusion of the text of the given file. The syntax is
    INCLUDE 'filename'
    This is compatible with Fortran 90. If the -source=vms-include option is given, ftnchek follows VMS conventions with respect to this statement: it assumes a default extension of .for if no filename extension is given, and allows the qualifier /[NO]LIST following the filename, to control the listing of the included file. There is no support for including VMS text modules.

    In diagnostic output relating to items contained in include files, the location of the error is specified by both its location in the include file and the location in the parent file where the file was included.

    ftnchek accepts PARAMETER statements which lack parentheses. These will be warned about if the -f77=param-noparen flag is given.

    ftnchek accepts PARAMETER definitions that involve intrinsic functions and exponentiation by a non-integer exponent. Both of these cases are prohibited by the Fortran 77 Standard, and will be warned about if the -f77=param-intrinsic flag is given. If an intrinsic function value is a compile-time integer constant, ftnchek will evaluate it. This allows better checking if the parameter is used in declaring array sizes. Fortran 90 allows intrinsic functions in PARAMETER definitions.

    The intrinsic functions that are evaluated are:

    ABS | IABS | DIM | IDIM | MAX
    MAX0 | MIN | MIN0 | MOD | SIGN
    ISIGN | LEN | ICHAR | INDEX |

    The functions of integer arguments are evaluated only if the arguments are integer constant expressions. (These may involve integer constants, parameters, and evaluated intrinsic functions.) The function LEN is evaluated if its argument is an expression involving only character constants and variables whose length is not adjustable. The functions ICHAR and INDEX are evaluated only if the arguments are character constants. ftnchek gives a warning if it needs the value of some intrinsic function that is not evaluated.

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