Conditional compiling

predefined symbols

Please see https://www.freepascal.org/docs-html/3.0.0/prog/progap7.html#x333-348000G.

unix is defined for both macos and unbuntu.

darwin is defined only for macos.

windows is for both 32-bit and 64-bit windows.

user defined symbols

To pass from the commandline:

fpc -dmy_symbol

that is, use -d option. It is equivalent to:

{$define my_symbol}

in the code.

To un-define a symbol from the commandline:

fpc -umy_symbol

That is, use -u option. It is equivalent to:

{#undef my_symbol}

in the code.

if and else

There are multiple forms:

Example 1

{$ifdef my_symbol}
  {some code}
{$endif}

{$ifdef my_symbol with some comments}
  {some code}
  {everything after my_symbol is a comment}
{$endif}

{$ifdef my_symbol}
  {some code}
{$else}
  {some other code}
{$endif}

{$ifdef my_symbol}
  {some code}
{$elseif defined(foo)}
  {some code}
{$endif}

{$if defined(foo) or defined(bar)}
{$endif}

{$if defined(foo) and defined(bar)}
{$endif}


{$if defined(my_symbol)}
  {some code}
{$elseif defined(foo)}
  {some code}
{$endif}

{$if defined(my_symbol)}
  {some code}
{$elseif defined(foo) or defined(bar) or defined(foobar)}
  {some code}
{$endif}

Example2

{$ifndef foo}
  {code}
{$endif}

{$if not defined(foo)}
  {code}
{$endif}

To summarize, we can use

• ifdef, endif

• ifndef

• else, elseif

• if (we can use conditions, e.g., if defined() or defined()

• defined

• undef

Some more examples from https://github.com/fpc/FPCSource/blob/main/rtl/unix/dl.pp

{$if defined(linux) or defined(freebsd) or defined(openbsd) or defined(dragonfly)}
  {$define ELF} // ELF symbol versioning.
{$endif}

{$if defined(linux)}
    { if libc is not linked explicitly, FPC might chose the wrong startup code, as
      libdl depends on libc on linux, this does not hurt }
    {$linklib c}
{$endif}

function dladdr(Lib: pointer; info: Pdl_info): Longint; cdecl; {$if not defined(aix) and not defined(android)} external; {$endif}

const
{$ifdef BSD}   // dlopen is in libc on FreeBSD.
  LibDL = 'c';
{$else}
  {$ifdef HAIKU}
    LibDL = 'root';
  {$else}
    LibDL = 'dl';
  {$endif}
{$endif}

Some more examples from https://github.com/fpc/FPCSource/blob/main/rtl/inc/ctypes.pp

{$ifdef FPC}
  {$inline on}
  {$ifndef DISABLE_INLINE}
     {$define CTYPES_INLINE}
  {$endif}
  {$define dummy}
{$endif}