Conditional

Definition

Full-featured conditional compilation (if / ifdef / else / endif) for the fpx preprocessor This module implements standard-conforming conditional compilation with support for:

• #if with arbitrary constant expressions (using evaluate_expression)
• #ifdef / #ifndef and #elifdef / #elifndef for testing macro existence
• #elif chains (multiple alternative branches)
• #else as final fallback
• Proper nesting up to MAX_COND_DEPTH levels
• Correct "first-match" semantics — once a branch is taken, later #elif/#else are skipped
• Integration with macro expansion via is_defined() and expression evaluator

The state is maintained in a global stack (cond_stack) with cond_depth tracking nesting. The function is_active() is used throughout the preprocessor to decide whether a line should be emitted or skipped.

Examples

1. Classic include guard pattern:

   #ifndef MY_HEADER_H
   #define MY_HEADER_H
    
   ! ... header content ...
    
   #endif
   ...

2. Feature selection with if and elif:

   #if DEBUG >= 2
      print *, 'Extra verbose debugging enabled'
   #elif DEBUG == 1
      print *, 'Standard debugging'
   #else
      ! Silent mode
   #endif
   !..

3. Cross-platform code selection:

   #ifdef _OPENMP
      use omp_lib
   #else
      integer, parameter :: omp_get_thread_num = 0
   #endif
   ...

4. Complex expression in if (requires evaluate_expression support):

   #if defined(USE_MPI) && (MPI_VERSION >= 3)
      use mpi_f08
   #endif
   ...

Data Types
type	cond_state
	State of a single conditional block. More...

Variables

cond_depth

integer, public cond_depth = 0

Current nesting depth of conditional directives (0 = outside any if).

Definition at line 103 of file conditional.f90.

cond_stack

type(cond_state), dimension(max_cond_depth), public cond_stack

Global stack of conditional states (depth-limited).

Definition at line 99 of file conditional.f90.

Methods

handle_elif()

subroutine, public handle_elif	(	type(context), intent(in)	ctx,
		type(macro), dimension(:), intent(in)	macros,
		character(*), intent(in)	token )

Process elif – alternative branch after if/elif Only activates if no previous branch in the group was taken.

Parameters

[in]	ctx	Context source line containing the directive
[in]	macros	Current macro table
[in]	token	Usually 'elif'

Remarks

Definition at line 232 of file conditional.f90.

handle_elifdef()

subroutine, public handle_elifdef	(	type(context), intent(in)	ctx,
		type(macro), dimension(:), intent(in)	macros,
		character(*), intent(in)	token )

Process elifdef – test if a macro is defined.

Parameters

[in]	ctx	Context source line containing the directive
[in]	macros	Current macro table
[in]	token	Usually 'elifdef'

Remarks

Definition at line 269 of file conditional.f90.

handle_elifndef()

subroutine, public handle_elifndef	(	type(context), intent(in)	ctx,
		type(macro), dimension(:), intent(in)	macros,
		character(*), intent(in)	token )

Process elifndef – test if a macro is not defined.

Parameters

[in]	ctx	Context source line containing the directive
[in]	macros	Current macro table
[in]	token	Usually 'elifndef'

Remarks

Definition at line 306 of file conditional.f90.

handle_else()

subroutine, public handle_else

(

type(context), intent(in)

ctx

)

Process else – final fallback branch Activates only if no previous if/elif branch was true.

Parameters

[in]

ctx

Context (for error messages)

Remarks

Definition at line 342 of file conditional.f90.

handle_endif()

subroutine, public handle_endif

(

type(context), intent(in)

ctx

)

Process endif – end of conditional block Pops the top state from the stack. Reports error on unmatched endif.

Parameters

[in]

ctx

Context (for error messages)

Remarks

Definition at line 371 of file conditional.f90.

handle_if()

subroutine, public handle_if	(	type(context), intent(in)	ctx,
		type(macro), dimension(:), intent(in)	macros,
		character(*), intent(in)	token )

Process a if directive with constant expression evaluation Evaluates the expression after if using evaluate_expression() and pushes a new state onto the conditional stack.

Parameters

[in]	ctx	Context source line containing the directive
[in]	macros	Current macro table
[in]	token	Usually 'if'

Remarks

Definition at line 132 of file conditional.f90.

handle_ifdef()

subroutine, public handle_ifdef	(	type(context), intent(in)	ctx,
		type(macro), dimension(:), intent(in)	macros,
		character(*), intent(in)	token )

Process ifdef – test if a macro is defined.

Parameters

[in]	ctx	Context source line containing the directive
[in]	macros	Current macro table
[in]	token	Usually 'ifdef'

Remarks

Definition at line 165 of file conditional.f90.

handle_ifndef()

subroutine, public handle_ifndef	(	type(context), intent(in)	ctx,
		type(macro), dimension(:), intent(in)	macros,
		character(*), intent(in)	token )

Process ifndef – test if a macro is NOT defined.

Parameters

[in]	ctx	Context source line containing the directive
[in]	macros	Current macro table
[in]	token	Usually 'ifndef'

Remarks

Definition at line 198 of file conditional.f90.

is_active()

logical function, public is_active

Determine if current line is inside an active conditional block.

Returns

.true. if all enclosing if/elif/else branches are active

Remarks

Definition at line 112 of file conditional.f90.