6 Foreign Language Interface

SWI-Prolog offers a powerful interface to C Kernighan & Ritchie, 1978. The main design objectives of the foreign language interface are flexibility and performance. A foreign predicate is a C-function that has the same number of arguments as the predicate represented. C-functions are provided to analyse the passed terms, convert them to basic C-types as well as to instantiate arguments using unification. Non-deterministic foreign predicates are supported, providing the foreign function with a handle to control backtracking.

C can call Prolog predicates, providing both an query interface and an interface to extract multiple solutions from an non-deterministic Prolog predicate. There is no limit to the nesting of Prolog calling C, calling Prolog, etc. It is also possible to write the `main' in C and use Prolog as an embedded logical engine.

• 6.1 Overview of the Interface
• 6.2 Linking Foreign Modules
  • 6.2.1 What linking is provided?
  • 6.2.2 What kind of loading should I be using?
• 6.3 Dynamic Linking of shared libraries
• 6.4 Using the library shlib for .DLL and .so files
  • 6.4.1 Static Linking
• 6.5 Interface Data types
  • 6.5.1 Type term_t: a reference to a Prolog term
    • 6.5.1.1 Interaction with the garbage collector and stack-shifter
  • 6.5.2 Other foreign interface types
• 6.6 The Foreign Include File
  • 6.6.1 Argument Passing and Control
    • 6.6.1.1 Non-deterministic Foreign Predicates
  • 6.6.2 Atoms and functors
    • 6.6.2.1 Atoms and atom-garbage collection
  • 6.6.3 Analysing Terms via the Foreign Interface
    • 6.6.3.1 Testing the type of a term
    • 6.6.3.2 Reading data from a term
    • 6.6.3.3 Exchanging text using length and string
    • 6.6.3.4 Reading a list
    • 6.6.3.5 An example: defining write/1 in C
  • 6.6.4 Constructing Terms
  • 6.6.5 Unifying data
  • 6.6.6 Calling Prolog from C
    • 6.6.6.1 Predicate references
    • 6.6.6.2 Initiating a query from C
  • 6.6.7 Discarding Data
  • 6.6.8 Foreign Code and Modules
  • 6.6.9 Prolog exceptions in foreign code
  • 6.6.10 Foreign code and Prolog threads
  • 6.6.11 Catching Signals (Software Interrupts)
  • 6.6.12 Miscellaneous
    • 6.6.12.1 Term Comparison
    • 6.6.12.2 Recorded database
    • 6.6.12.3 Getting file names
  • 6.6.13 Errors and warnings
  • 6.6.14 Environment Control from Foreign Code
  • 6.6.15 Querying Prolog
  • 6.6.16 Registering Foreign Predicates
  • 6.6.17 Foreign Code Hooks
  • 6.6.18 Storing foreign data
    • 6.6.18.1 Examples for storing foreign data
  • 6.6.19 Embedding SWI-Prolog in other applications
    • 6.6.19.1 Threading, Signals and embedded Prolog
• 6.7 Linking embedded applications using plld
  • 6.7.1 A simple example
• 6.8 The Prolog `home' directory
• 6.9 Example of Using the Foreign Interface
• 6.10 Notes on Using Foreign Code
  • 6.10.1 Memory Allocation
  • 6.10.2 Debugging Foreign Code
  • 6.10.3 Name Conflicts in C modules
  • 6.10.4 Compatibility of the Foreign Interface