12.8 Notes on Using Foreign Code

12.8.1 Foreign debugging functions

The functions in this section are primarily intended for debugging foreign extensions or embedded Prolog. Violating the constraints of the foreign interface often leads to crashes in a subsequent garbage collection. If this happens, the system needs to be recompiled with the cflags -DO_DEBUG. This is normally achieved by editing src/Makefile and changing the definition of COFLAGS to the value below. The -gdwarf-2 -g3 provides detailed debugging information for gcc. If you use another C compiler you may need other flags.

COFLAGS=-DO_DEBUG -gdwarf-2 -g3

After recompiling the Prolog kernel all functions listed above are available to use from the debugger (e.g. gdb) or can be placed at critical location in your code or the system code.

void PL_backtrace(int depth, int flags)

Dump a Prolog backtrace to the user_error stream. Depth is the number of frames to dump. Flags is a bitwise or of the following constants:

PL_BT_SAFE

(0x1) Do not try to print goals. Instead, just print the predicate name and arity. This reduces the likelyhood to crash if PL_backtrace() is called in a damaged environment.

PL_BT_USER

(0x2) Only show `user' frames. Default is to also show frames of hidden built-in predicates.

char * PL_backtrace_string(int depth, int flags)

As PL_backtrace(), but returns the stack as a string. The string uses UTF-8 encoding. The returned string must be freed using PL_free(). This function is was added to get stack traces from running servers where I/O is redirected or discarded. For example, using gdb, a stack trace is printed in the gdb console regardless of Prolog I/O redirection using the following command:

(gdb) printf "%s", PL_backtrace_string(25,0)

The source distribution provides the script scripts/swipl-bt that exploits gdb and PL_backtrace_string() to print stack traces in various formats for a SWI-Prolog process, given its process id.

int PL_check_data(term_t data)

Check the consistency of the term data. Returns TRUE this is actually implemented in the current version and FALSE otherwise. The actual implementation only exists if the system is compiled with the cflag -DO_DEBUG or -DO_MAINTENANCE. This is not the default.

int PL_check_stacks()

Check the consistency of the runtime stacks of the calling thread. Returns TRUE this is actually implemented in the current version and FALSE otherwise. The actual implementation only exists if the system is compiled with the cflag -DO_DEBUG or -DO_MAINTENANCE. This is not the default.

The Prolog kernel sources use the macro DEBUG(Topic, Code). These macros are disabled in the production version and must be enabled by recompiling the system as described above. Specific topics can be enabled and disabled using the predicates prolog_debug/1 and prolog_nodebug/1. In addition, they can be activated from the commandline using commandline option -d topics, where topics is a comma-separated list of debug topics to enable. For example, the code below adds many consistency checks and prints messages if the Prolog signal handler dispatches signals.

$ swipl -d chk_secure,msg_signal

prolog_debug(+Topic)

prolog_nodebug(+Topic)

Enable/disable a debug topic. Topic is an atom that identifies the desired topic. The available topics are defined in src/pl-debug.h. Please search the sources to find out what is actually printed and when. We highlight one topic here:

chk_secure(chk_secure)

dd many expensive consistency checks to the system. This should typically be used when the system crashes, notably in the garbage collector. Garbage collection crashes are in most cases caused by invalid data on the Prolog stacks. This debug topic may help locating how the invalid data was created.

12.8.2 Memory Allocation

SWI-Prolog's heap memory allocation is based on the malloc(3) library routines. SWI-Prolog provides the functions below as a wrapper around malloc(). Allocation errors in these functions trap SWI-Prolog's fatal-error handler, in which case PL_malloc() or PL_realloc() do not return.

Portable applications must use PL_free() to release strings returned by PL_get_chars() using the BUF_MALLOC argument. Portable applications may use both PL_malloc() and friends or malloc() and friends but should not mix these two sets of functions on the same memory.

void * PL_malloc(size_t bytes)

Allocate bytes of memory. On failure SWI-Prolog's fatal-error handler is called and PL_malloc() does not return. Memory allocated using these functions must use PL_realloc() and PL_free() rather than realloc() and free().

void * PL_realloc(void *mem, size_t size)

Change the size of the allocated chunk, possibly moving it. The mem argument must be obtained from a previous PL_malloc() or PL_realloc() call.

void PL_free(void *mem)

Release memory. The mem argument must be obtained from a previous PL_malloc() or PL_realloc() call.

12.8.2.1 Boehm-GC support

This section is obsolete. Although the Boehm-GC interfaces still exist, it turns out that the scalability is not good enough for SWI-Prolog. It is unlikely that SWI-Prolog will ever switch to Boehm-GC.

To accommodate future use of the Boehm garbage collector^{186http://www.hpl.hp.com/personal/Hans_Boehm/gc/} for heap memory allocation, the interface provides the functions described below. Foreign extensions that wish to use the Boehm-GC facilities can use these wrappers. Please note that if SWI-Prolog is not compiled to use Boehm-GC (default), the user is responsible for calling PL_free() to reclaim memory.

void* PL_malloc_atomic(size_t bytes)

void* PL_malloc_uncollectable(size_t bytes)

void* PL_malloc_atomic_uncollectable(size_t bytes)

If Boehm-GC is not used, these are all the same as PL_malloc(). With Boehm-GC, these map to the corresponding Boehm-GC functions. Atomic means that the content should not be scanned for pointers, and uncollectable means that the object should never be garbage collected.

void* PL_malloc_stubborn(size_t bytes)

void PL_end_stubborn_change(void *memory)

These functions allow creating objects, promising GC that the content will not change after PL_end_stubborn_change().

12.8.3 Compatibility between Prolog versions

Great care is taken to ensure binary compatibility of foreign extensions between different Prolog versions. Only the much less frequently used stream interface has been responsible for binary incompatibilities.

Source code that relies on new features of the foreign interface can use the macro PLVERSION to find the version of SWI-Prolog.h and PL_query() using the option PL_QUERY_VERSION to find the version of the attached Prolog system. Both follow the same numbering schema explained with PL_query().

12.8.4 Debugging and profiling foreign code (valgrind)

This section is only relevant for Unix users on platforms supported by valgrind. Valgrind is an excellent binary instrumentation platform. Unlike many other instrumentation platforms, valgrind can deal with code loaded through dlopen().

The callgrind tool can be used to profile foreign code loaded under SWI-Prolog. Compile the foreign library adding -g option to gcc or swipl-ld. By setting the environment variable VALGRIND to yes, SWI-Prolog will not release loaded shared objects using dlclose(). This trick is required to get source information on the loaded library. Without, valgrind claims that the shared object has no debugging information.^{187Tested using valgrind version 3.2.3 on x64.} Here is the complete sequence using bash as login shell:

% VALGRIND=yes valgrind --tool=callgrind pl <args>
<prolog interaction>
% kcachegrind callgrind.out.<pid>

12.8.5 Name Conflicts in C modules

In the current version of the system all public C functions of SWI-Prolog are in the symbol table. This can lead to name clashes with foreign code. Someday I should write a program to strip all these symbols from the symbol table (why does Unix not have that?). For now I can only suggest you give your function another name. You can do this using the C preprocessor. If---for example---your foreign package uses a function warning(), which happens to exist in SWI-Prolog as well, the following macro should fix the problem:

#define warning warning_

Note that shared libraries do not have this problem as the shared library loader will only look for symbols in the main executable for symbols that are not defined in the library itself.

12.8.6 Compatibility of the Foreign Interface

The term reference mechanism was first used by Quintus Prolog version 3. SICStus Prolog version 3 is strongly based on the Quintus interface. The described SWI-Prolog interface is similar to using the Quintus or SICStus interfaces, defining all foreign-predicate arguments of type +term. SWI-Prolog explicitly uses type functor_t, while Quintus and SICStus use <name> and <arity>. As the names of the functions differ from Prolog to Prolog, a simple macro layer dealing with the names can also deal with this detail. For example:

#define QP_put_functor(t, n, a) \
        PL_put_functor(t, PL_new_functor(n, a))

The PL_unify_*() functions are lacking from the Quintus and SICStus interface. They can easily be emulated, or the put/unify approach should be used to write compatible code.

The PL_open_foreign_frame()/PL_close_foreign_frame() combination is lacking from both other Prologs. SICStus has PL_new_term_refs(0), followed by PL_reset_term_refs(), that allows for discarding term references.

The Prolog interface for the graphical user interface package XPCE shares about 90% of the code using a simple macro layer to deal with different naming and calling conventions of the interfaces.