12.5 Linking embedded applications using swipl-ld

The utility program swipl-ld (Win32: swipl-ld.exe) may be used to link a combination of C files and Prolog files into a stand-alone executable. swipl-ld automates most of what is described in the previous sections.

In normal usage, a copy is made of the default embedding template .../swipl/include/stub.c. The main() routine is modified to suit your application. PL_initialise() must be passed the program name (argv[0]) (Win32: the executing program can be obtained using GetModuleFileName()). The other elements of the command line may be modified. Next, swipl-ld is typically invoked as:

swipl-ld -o output stubfile.c [other-c-or-o-files] [plfiles]

swipl-ld will first split the options into various groups for both the C compiler and the Prolog compiler. Next, it will add various default options to the C compiler and call it to create an executable holding the user's C code and the Prolog kernel. Then, it will call the SWI-Prolog compiler to create a saved state from the provided Prolog files and finally, it will attach this saved state to the created emulator to create the requested executable.

Below, it is described how the options are split and which additional options are passed.

-help

Print brief synopsis.

-pl prolog

Select the Prolog to use. This Prolog is used for two purposes: get the home directory as well as the compiler/linker options and create a saved state of the Prolog code.

-ld linker

Linker used to link the raw executable. Default is to use the C compiler (Win32: link.exe).

-cc C compiler

Compiler for .c files found on the command line. Default is the compiler used to build SWI-Prolog accessible through the Prolog flag c_cc (Win32: cl.exe).

-c++ C++-compiler

Compiler for C++ source file (extensions .cpp, .cxx, .cc or .C) found on the command line. Default is c++ or g++ if the C compiler is gcc (Win32: cl.exe).

-nostate

Just relink the kernel, do not add any Prolog code to the new kernel. This is used to create a new kernel holding additional foreign predicates on machines that do not support the shared-library (DLL) interface, or if building the state cannot be handled by the default procedure used by swipl-ld. In the latter case the state is created separately and appended to the kernel using cat <kernel> <state> > <out> (Win32: copy /b <kernel>+<state> <out>).

-shared

Link C, C++ or object files into a shared object (DLL) that can be loaded by the load_foreign_library/1 predicate. If used with -c it sets the proper options to compile a C or C++ file ready for linking into a shared object.

-dll

Windows only. Embed SWI-Prolog into a DLL rather than an executable.

-c

Compile C or C++ source files into object files. This turns swipl-ld into a replacement for the C or C++ compiler, where proper options such as the location of the include directory are passed automatically to the compiler.

-E

Invoke the C preprocessor. Used to make swipl-ld a replacement for the C or C++ compiler.

-pl-options , ...

Additional options passed to Prolog when creating the saved state. The first character immediately following pl-options is used as separator and translated to spaces when the argument is built. Example: -pl-options,-F,xpce passes -F xpce as additional flags to Prolog.

-ld-options , ...

Passes options to the linker, similar to -pl-options.

-cc-options , ...

Passes options to the C/C++ compiler, similar to -pl-options.

-v

Select verbose operation, showing the various programs and their options.

-o outfile

Reserved to specify the final output file.

-llibrary

Specifies a library for the C compiler. By default, -lswipl (Win32: libpl.lib) and the libraries needed by the Prolog kernel are given.

-Llibrary-directory

Specifies a library directory for the C compiler. By default the directory containing the Prolog C library for the current architecture is passed.

-g | -Iinclude-directory | -Ddefinition

These options are passed to the C compiler. By default, the include directory containing SWI-Prolog.h is passed. swipl-ld adds two additional * -Ddef flags:

-D__SWI_PROLOG__

Indicates the code is to be connected to SWI-Prolog.

-D__SWI_EMBEDDED__

Indicates the creation of an embedded program.

*.o | *.c | *.C | *.cxx | *.cpp

Passed as input files to the C compiler.

*.pl | *.qlf

Passed as input files to the Prolog compiler to create the saved state.

*

All other options. These are passed as linker options to the C compiler.

12.5.1 A simple example

The following is a very simple example going through all the steps outlined above. It provides an arithmetic expression evaluator. We will call the application calc and define it in the files calc.c and calc.pl. The Prolog file is simple:

calc(Atom) :-
        term_to_atom(Expr, Atom),
        A is Expr,
        write(A),
        nl.

The C part of the application parses the command line options, initialises the Prolog engine, locates the calc/1 predicate and calls it. The coder is in figure 8.

#include <stdio.h>
#include <string.h>
#include <SWI-Prolog.h>

#define MAXLINE 1024

int
main(int argc, char **argv)
{ char expression[MAXLINE];
  char *e = expression;
  char *program = argv[0];
  char *plav[2];
  int n;

  /* combine all the arguments in a single string */

  for(n=1; n<argc; n++)
  { if ( n != 1 )
      *e++ = ' ';
    strcpy(e, argv[n]);
    e += strlen(e);
  }

  /* make the argument vector for Prolog */

  plav[0] = program;
  plav[1] = NULL;

  /* initialise Prolog */

  if ( !PL_initialise(1, plav) )
    PL_halt(1);

  /* Lookup calc/1 and make the arguments and call */

  { predicate_t pred = PL_predicate("calc", 1, "user");
    term_t h0 = PL_new_term_refs(1);
    int rval;

    PL_put_atom_chars(h0, expression);
    rval = PL_call_predicate(NULL, PL_Q_NORMAL, pred, h0);

    PL_halt(rval ? 0 : 1);
  }

  return 0;
}

The application is now created using the command line below. The option -goal true sets the Prolog initialization goal to suppress the banner. Note that the -o calc does not specify an extension. If the platform uses a file extension for executables, swipl-ld will add this (e.g., .exe on Windows).

% swipl-ld -goal true -o calc calc.c calc.pl

The created program calc is a native executable with the Prolog code attached to it. Note that the program typically depends on the shared object libswipl and, depending on the platform and configuration, on several external shared objects.

% ./calc pi/2
1.5708