Call C

• 7.1 Using external code and variables

  https://www.freepascal.org/docs-html/prog/progse28.html

  The official fpc documentation for calling external functions and variables.

• Using C object files in Delphi

  http://rvelthuis.de/articles/articles-cobjs.html

• How to use C code in Free Pascal projects

  https://downloads.freepascal.org/fpc/docs-pdf/CinFreePascal-old.pdf

• Creating Pascal bindings for C

  https://github.com/csukuangfj/pascal-bindings-for-c/blob/master/docs/Creating%20Pascal%20bindings%20for%20C%20(v1.0).pdf

  It shows how to use xxx.o, libxxx.a and libxxx.so in free pascal It also contains source files hosted on GitHub.

• https://github.com/PascalGameDevelopment/SDL2-for-Pascal/blob/master/units/sdl2.pas

  Read it carefully. It has lots of examples!

• See also https://github.com/MagicFoundation/Alcinoe/blob/master/Source/ZLibEx.pas

• https://github.com/gidesa/ocvWrapper24/blob/main/Delphi/OPENCVWrapper.pas

  OpenCV wrapper for delphi

• https://github.com/Kagamma/whisper-pas/blob/master/src/whisper.pas

  Wrapper for whisper

• Manual conversion of C headers to Pascal

  https://wiki.lazarus.freepascal.org/Creating_bindings_for_C_libraries#Manual_conversion_of_C_headers_to_Pascal

• See the following link for how to wrap printf, which accepts unconstrained number of arguments

  https://www.freepascal.org/docs-html/prog/progsu149.html

  Note that it proposes two methods.

ctypes

ctypes.inc can be found at https://github.com/fpc/FPCSource/blob/main/rtl/unix/ctypes.inc.

It defines commonly used integer types for C/C++.

• cint32, pcint32

• cfloat, pcfloat

• cdouble, pcdouble

• csize_t, pcsize_t

linklib

To link a lib in the code:

{$L xxx}
// or
{$LinkLib xxx}

// Note xxx can be on object file, e..g, use {$L foo.o}
// or it can be a library {$L foo} to link with libfoo.so, libfoo.dylib

// On windows, we have to use {$L foo.dll}
// See https://www.freepascal.org/docs-html/prog/progsu149.html

Or at the commandline use -Flxxx

To specify the object file in the commandline, use -Fo

Example 1 on macOS

See also https://www.freepascal.org/docs-html/prog/progsu147.html

Things to take away:

1. We can link an object file in pascal

2. The c variable name is mangled by adding a leading _ by the compiler.

3. If we use cvar without a name after external, then the fpc compiler handles the name mangling by ourselves.

4. If we don't use cvar and instead use a name, then we must add a leading _.

5. If we don't use cvar and don't use name, then the variable must be named as the mangled c variable name.

extvar.c is given below:

/*
Declare a variable, allocate storage
*/
int extvar = 12;

extdemo.pas is given below:

Program ExtDemo;

{$L extvar.o}
// {$LinkLib extvar.o} // either one is ok
// By default, it tries to find extvar.o in the current directory.
// If it is not found, then an error is throw.
//
// To fix it, we can use -Fl/some/path
// where we extvar.o is inside /some/path

var
  { Case sensitive declaration }
  extvar : longint; cvar; external;
  _extvar : longint; external;
  I : longint; external name '_extvar';

begin
  { Extvar can be used case insensitively }
  Writeln ('Variable ''extvar'' has value: ',ExtVar);
  Writeln ('Variable ''_extvar'' has value: ',_ExtVar);
  Writeln ('Variable ''I''      has value: ',i);
  I := 13;
  Writeln ('Variable ''extvar'' has value: ',ExtVar);
  Writeln ('Variable ''_extvar'' has value: ',_ExtVar);
  Writeln ('Variable ''I''      has value: ',i);
end.

Build command:

gcc -c -o extvar.o extvar.c
fpc ./extdemo.pas

Output log is:

Free Pascal Compiler version 3.2.2 [2021/05/16] for x86_64
Copyright (c) 1993-2021 by Florian Klaempfl and others
Target OS: Darwin for x86_64
Compiling ./extdemo.pas
Assembling extdemo
Linking extdemo
ld: warning: object file (extvar.o) was built for newer macOS version (13.0) than being linked (10.8)
17 lines compiled, 0.6 sec

The command:

nm ./extvar.o

prints:

0000000000000000 D _extvar

And:

./extdemo

prints:

Variable 'extvar' has value: 12
Variable '_extvar' has value: 12
Variable 'I'      has value: 12
Variable 'extvar' has value: 13
Variable '_extvar' has value: 13
Variable 'I'      has value: 13