Using fpp Preprocessor Directives

Preprocessor directives for string substitution

The following fpp preprocessor directives cause substitutions in your program:

Preprocessor directive for inclusion of external files

To include external files, preprocessor directive #include can be specified in one of the following forms:

#include "filename"

#include <filename>

#include reads in the contents of the named file into the specified or default location in the source. The lines read in from the file are processed by fpp just as if they were part of the current file.

When the <filename> notation is used, filename is only searched for in the standard "include" directories. For more information, see the fpp -I and the -Y options in Using Fortran Preprocessor Options. No additional tokens are allowed on the directive line after the final '"' or ">".

For #include "filename", filenames are searched for in the following order:

• In the directory in which the source file resides

• In the directories specified by the -I or -Y option

• In the default directory

For #include <filename>, filenames are searched for in the following order:

• In the directories specified by the -I or -Y option

• In the default directory

Preprocessor directive for line control

The preprocessor directive #line-number generates line control information for the Fortran compiler. It takes the following form:

#line-number "filename"

#line-number is an integer constant that is the line number of the next line. "filename" is the name of the file containing the line. If "filename" is not provided, the current filename is assumed.

Preprocessor directive for fpp variable and macro definitions

The preprocessor directive #define can be used to define both simple string variables and more complicated macros. It can take two forms.

• Definition of an fpp variable:

  #define name token-string

  In the above, occurrences of name in the source file will be replaced by token-string.

• Definition of an fpp macro:

  #define name(argument[,argument] ... ) token-string

  In the above, occurrences of the macro name followed by the comma-separated list of actual arguments within parentheses will be replaced by token-string. Each occurrence of argument in token-string is replaced by the token sequence representing the corresponding "actual" argument in the macro call.

  An error occurs if the number of macro call arguments is not the same as the number of arguments in the corresponding macro definition. For example, consider this macro definition:

  #define INTSUB(m, n, o) call mysub(m, n, o)

  Any use of the macro INTSUB must have three arguments. In macro definitions, spaces between the macro name and the open parenthesis "(" are prohibited to prevent the directive from being interpreted as an fpp variable definition with the rest of the line beginning with the open parenthesis "(" being interpreted as its token-string.

  An fpp variable or macro definition can be of any length and is limited only by the newline symbol. It can be defined in multiple lines by continuing it to the next line with the insertion of "\". For example:

  #define long_macro_name(x,\
     y) x*y

  The occurrence of a newline without a macro-continuation signifies the end of the macro definition.

The scope of a definition begins from the #define and encloses all the source lines (and source lines from #include files) to the end of the current file, except for:

• Files included by Fortran INCLUDE statements

• fpp and Fortran comments

• Fortran IMPLICIT statements that specify a single letter

• Fortran FORMAT statements

• Numeric, typeless, and character constants

Preprocessor directive for undefining a macro

The preprocessor directive #undef takes the following form:

#undef name

This preprocessor directive removes any definition for name produced by the D options, the #define preprocessor directives, or by default. No additional tokens are permitted on the directive line after name.

If name has not been previously defined, then #undef has no effect.

Preprocessor directive for macro expansion

If, during expansion of a macro, the column width of a line exceeds column 72 (for fixed format) or column 132 (for free format), fpp inserts appropriate Fortran continuation lines.

For fixed format, there is a limit on macro expansions in label fields (positions 1-5):

• A macro call (together with possible arguments) should not extend beyond column 5.

• A macro call whose name begins with one of the Fortran comment symbols is considered to be part of a comment.

• A macro expansion may produce text extending beyond column 5. In this case, a warning will be issued.

In fixed format, when the fpp option-Xw has been specified, an ambiguity may occur if a macro call occurs in a statement position and a macro name begins or coincides with a Fortran keyword. For example, consider the following:

#define callp(x)   call f(x)
  call p(0)

fpp cannot determine how to interpret the callp token sequence above. It could be considered to be a macro name. The current implementation does the following:

• The longer identifier is chosen (callp in this case)

• From this identifier the longest macro name or keyword is extracted

• If a macro name has been extracted a macro expansion is performed. If the name begins with some keyword, fpp issues an appropriate warning

• The rest of the identifier is considered as a whole identifier

In the previous example, the macro expansion is performed and the following warning is produced:

warning: possibly incorrect substitution of macro callp

This situation appears only when preprocessing fixed-format source code and when the space symbol is not interpreted as a token delimiter.

In the following case, a macro name coincides with a beginning of a keyword:

 #define INT  INTEGER*8
              INTEGER k

The INTEGER keyword will be found earlier than the INT macro name. There will be no warning when preprocessing such a macro definition.

Preprocessor directives for conditional selection of source text

The following three preprocessor directives are conditional constructs that you can use to select source text.

• #if preprocessor directive

  When #if is specified, subsequent lines up to the matching #else, #elif, or #endif preprocessor directive appear in the output only if condition evaluates to .TRUE.. The following shows an example:

  #if condition_1
    block_1
  #elif condition_2
    block_2
  #elif ...
  #else
    block_n
  #endif
  

• #ifdef preprocessor directive

  When #ifdef is specified, subsequent lines up to the matching #else, #elif, or #endif preprocessor directive appear in the output only if name has been defined, either by a #define preprocessor directive or by the D compiler option, with no intervening #undef preprocessor directive. No additional tokens are permitted on the preprocessor directive line after name. The following shows an example:

  #ifdef name
    block_1
  #elif condition
    block_2
  #elif ...
  #else
    block_n
  #endif
  

• #ifndef preprocessor directive

  When #ifndef is specified, subsequent lines up to the matching #else, #elif, or #endif preprocessor directive appear in the output only if name has not been defined, or if its definition has been removed with an #undef preprocessor directive. No additional tokens are permitted on the directive line after name. The following shows an example:

  #ifndef name
    block_1
  #elif condition
    block_2
  #elif ...
  #else
    block_n
  #endif
  

The #else, #elif, or #endif preprocessor directives are optional. They can be used in the above preprocessor directives.

Subsequent lines up to the matching #else, #elif, or #endif appear in the output only if all of the following occur:

• The condition in the preceding #if directive evaluates to .FALSE. or the name in the preceding #ifdef directive is not defined, or the name in the preceding #ifndef directive is defined.

• The conditions in all of the preceding #elif directives evaluate to .FALSE.

• The condition in the current #elif evaluates to .TRUE.

Any condition allowed in an #if directive is allowed in an #elif directive. Any number of #elif directives may appear between an #if, #ifdef, or #ifndef directive and a matching #else or #endif directive.

Conditional expressions

condition_1, condition_2, etc. are logical expressions involving fpp constants, macros, and intrinsic functions. The following items are permitted in conditional expressions:

• C language operations: <, >, ==, !=, >=, <=, +, -, /, *, %, <<, >>, &, ~, |, &&, and ||

  They are interpreted by fpp in accordance to the C language semantics. This facility is provided for compatibility with "old" Fortran programs using cpp.

• Fortran language operations: .AND., .OR., .NEQV., .XOR., .EQV., .NOT., .GT., .LT., .LE., .GE., .NE., .EQ., ** (power).

• Fortran logical constants: .TRUE. , .FALSE.

• The fpp intrinsic function "defined": defined(name) or defined name which returns .TRUE. if name is defined as an fpp variable or a macro. It returns .FALSE. if the name is not defined.

#ifdef is shorthand for #if defined(name) and #ifndef is shorthand for #if .not. defined(name).

Only these items, integer constants, and names can be used within a constant expression. A name that has not been defined with the -D option, a #define preprocessor directive, or defined by default, has a value of 0. The C operation != (not equal) can be used in the #if or #elif preprocessor directive, but not in the #define preprocessor directive, where the symbol ! is considered to be the Fortran comment symbol by default.