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 compiled for
debugging using cmake -DCMAKE_BUILD_TYPE=Debug, after which
all functions and predicates listed below 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 likelihood 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(A)
- 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.
These predicates require the system to be compiled for debugging
using
cmake -DCMAKE_BUILD_TYPE=Debug.
- int PL_prolog_debug(const
char *topic)
- int PL_prolog_nodebug(const
char *topic)
- (De)activate debug topics. The topics argument is a
comma-separated string of topics to enable or disable. Matching is
case-insensitive. See also prolog_debug/1
and prolog_nodebug/1.
These functions require the system to be compiled for debugging using
cmake -DCMAKE_BUILD_TYPE=Debug.
0 
