Swig Debug

Debugging the SWIG compiler

When to Use

• Investigating a SWIG crash or assertion (segfault, abort, internal error)
• Understanding why a parse tree node has unexpected attributes
• Tracing template instantiation, typemap selection, or symbol table resolution
• Diagnosing DOH memory corruption / use after free problems

Why DOH needs special handling

All DOH types (String *, SwigType *, Hash *, List *, Node *, etc.) are typedefined to void, so debuggers cannot inspect them automatically. Use SWIG's print helpers — never raw print on a DOH pointer.

See Doc/Devel/internals.html section 7 ("Debugging SWIG") for the authoritative reference.

Setup

1. Build SWIG with debug symbols and no optimisation:

   ./configure CFLAGS='-g -O0' CXXFLAGS='-g -O0'
   make
   

2. Load Tools/swig.gdb into your gdb session. Either add to ~/.gdbinit:

   source /path/to/swig/Tools/swig.gdb
   

   or source it interactively each session. This defines swigprint and locswigprint.
3. Confirm helpers are loaded: (gdb) show user lists them; (gdb) help swigprint shows usage.

Running SWIG under gdb

The test suite invokes ./preinst-swig (which sets SWIG_LIB=./Lib). To debug the actual binary, run the underlying ./swig directly with the same args, or set the env var manually:

gdb --args ./swig -python -c++ -I./Lib -outdir /tmp Examples/test-suite/foo.i
(gdb) set environment SWIG_LIB=./Lib
(gdb) run

To capture the exact command a test uses, run the test once with verbose output and copy the swig invocation:

cd Examples/test-suite/python
make foo.cpptest    # without -s, so commands are printed

Running the test suite under tools

The shared Examples/test-suite/common.mk exposes two hook variables that wrap stages of the test pipeline:

• SWIGTOOL — wraps the SWIG invocation itself
• RUNTOOL — wraps the runtime test execution (the language interpreter)

The make target suffix must match the test source type — getting it wrong silently runs nothing or the wrong recipe. Use:

• .cpptest for C++ tests (the .i uses C++ — most tests in Examples/test-suite/)
• .ctest for plain C tests
• .multicpptest for multi module C++ tests (testcase split across multiple .i files)

There is no auto detection; check the testcase to see which it is. Examples below pick the right suffix for ret_by_value (a C test):

# gdb on a single SWIG invocation
make ret_by_value.ctest SWIGTOOL="gdb --args"

# valgrind the runtime test
make ret_by_value.ctest RUNTOOL="valgrind --leak-check=full"

# valgrind SWIG itself (needs DOH_DEBUG_MEMORY_POOLS rebuild — see Memory section)
make ret_by_value.ctest SWIGTOOL="valgrind --tool=memcheck --trace-children=yes"

# gdb on the runtime side
make ret_by_value.ctest RUNTOOL="gdb --args"

For a C++ testcase the same calls would use .cpptest (e.g. make bools.cpptest SWIGTOOL="gdb --args"); for a multi module testcase, .multicpptest (e.g. make imports.multicpptest). make <testcase>.clean (same suffix rules) removes generated files.

Adding flags via SWIG_FEATURES

SWIG_FEATURES is an environment variable read by SWIG and merged with command line options. Useful to inject -debug-* flags into a test or the whole suite without editing makefiles:

make ret_by_value.ctest SWIG_FEATURES="-debug-tmsearch"
env SWIG_FEATURES=-O make check-python-test-suite

Inspecting parse tree nodes

The most common debugging task. Set a breakpoint in the language module's wrapper entry point and swigprint the node:

(gdb) break JAVA::functionWrapper
(gdb) run
Breakpoint 1, JAVA::functionWrapper (this=0x..., n=0xb7d2afc8) at Modules/java.cxx:799
(gdb) swigprint n
Hash(0xb7d2afc8) {
  'name' : x,
  'sym:name' : Shape_x_set,
  'nodeType' : cdecl,
  'type' : void,
  ...
}

swigprint accepts:

• a DOH variable: swigprint symname
• a raw address: swigprint 0xb7cb9408
• an optional second arg controlling Hash nesting depth (default 1): swigprint n 2

Collapsed Hashes appear as Hash(0x...) {......}; the dot count equals the number of attributes.

locswigprint is identical but prefixes the output with the source file/line where the node originated:

(gdb) locswigprint 0xb7cb9408
example.h:11: [Hash(0xb7cb9408) { ... }]

Tip: gdb tab completes user defined commands, so sw / loc are sufficient prefixes.

Common breakpoints

Calling print functions from instrumented code

If gdb cannot call into the binary (optimised build, release distro, awkward environment), insert calls to the print family directly in the SWIG source and recompile. The functions are listed under "Debugging Functions" in Doc/Devel/tree.html. Swig_print_node(n), Swig_print_tree(n), and Printf(stdout, "%s\n", obj) are the most useful.

Try -debug-* first

The built in command line flags often answer the question without needing gdb at all. Full list (from Doc/Manual/Extending.html §40.11):

./swig -help prints the canonical list.

The four processing stages

-debug-top <n> and -debug-module <n> accept stages 1–4. Stages roughly correspond to: 1 = post parse raw tree, 2 = after type passes, 3 = after target language independent transforms, 4 = after the target language module has finished (this is the final tree). -debug-top 4 or the equivalent -xmlout is the right choice to see what the language module actually saw.

Conflicts and overload diagnostics are reported against sym:name values, which is why sym:name shows up so prominently in these dumps.

Recommended starting incantations:

./swig -c++ -python -debug-tags example.i                    # structural skeleton
./swig -c++ -python -debug-module 1 -debug-quiet example.i   # user module tree, post parse
./swig -c++ -python -debug-top 4 -debug-quiet example.i      # final tree, all stages applied

Preprocessor dump

swig -E interface.i writes the post preprocessor text that SWIG hands to its C++ parser (analogous to gcc -E). Useful for seeing how %include, macros, and %define expanded before parsing — often the first thing to check when a declaration is "missing" from the parse tree.

XML dump

The parse tree can also be serialised as XML. Three modes (from Extending.html §40.4.4):

swig -python -c++ -xmlout out.xml  example.i   # final tree (~ -debug-top 4), with target language processing
swig -xml      -o out.xml          example.i   # tree without any target language (~ -debug-top 3)
swig -xml -xmllite -o out.xml      example.i   # same, but skips typescope / typetab nodes (smaller)

XML output is bigger than -debug-top but easier to grep, diff, and postprocess. Format is unstable across SWIG versions.

%feature attributes

%feature directives attach as attributes in the feature: namespace on matching nodes. To see what features are stuck where, dump the final tree and grep for feature: — -debug-top 4 -debug-quiet is the usual incantation.

errors test suite

A dedicated suite under Examples/test-suite/errors/ tests SWIG's warning and error messages themselves. Run with make check-errors-test-suite. Mechanism is documented in Examples/test-suite/errors/Makefile.in — useful if you're changing diagnostic output or adding a new warning.

DOH inspection without the helpers

If swigprint is unavailable (e.g. core dump on a stripped binary), DOH objects can be cast manually. Compile with optimisations off first or these casts may fail:

• String *s → (*(struct String *)(((DohBase *)s)->data)).str
• SwigType *t → (*(struct String *)(((DohBase *)t)->data)).str
• const_String_or_char_ptr sc → either the String * cast above or (char *)sc, depending on which it actually is

These give you the underlying C string. Anything more structured (Hash contents, List items) is far easier with the helpers.

Memory corruption

DOH uses reference counting and an internal pool allocator (Source/DOH/memory.c), so stale pointer bugs do not show up under valgrind by default — the pool keeps freed memory mapped and reused.

To catch use after free of DOH objects:

1. Define DOH_DEBUG_MEMORY_POOLS in Source/DOH/memory.c.
2. Rebuild SWIG.
3. Rerun under valgrind. Stale DOH usage now triggers Fatal internal error: Attempt to delete a non-DOH object. instead of silently succeeding.

Memory intensive — only enable while diagnosing.

Notes

• Always build -g -O0 before debugging; optimised inlining destroys most DOH casts.
• make after editing source — the test suite uses ./preinst-swig, which calls the freshly built ./swig, so no install is needed.
• Authoritative references:
  • Doc/Devel/internals.html §7 — debugging SWIG, DOH casts, memory pools
  • Doc/Devel/tree.html — parse tree node attributes and the "Debugging Functions" list
  • Doc/Manual/Extending.html §40.4.3 (parse trees), §40.10 (test suite tooling: SWIGTOOL/RUNTOOL/SWIG_FEATURES/valgrind/gdb), §40.11 (-debug-* options), §40.12 (per node attribute reference)
  • Tools/swig.gdb — helper command definitions