SWIG Internals Manual

(Note : This is a work in progress.)

Table of Contents

• 1. Introduction
  • 1.1 Directory Guide
  • 1.2 Overall Program Flow
• 2. DOH
  • 2.1 Motivation and Background
  • 2.2 Basic Types
  • 2.3 Creating, Copying and Destroying Objects
  • 2.4 A Word About Mutability and Copying
  • 2.5 Strings
  • 2.6 Lists
  • 2.7 Hash Tables
  • 2.8 Files
  • 2.9 Void Objects
  • 2.10 Utility Functions
• 3. Types and Typemaps
• 4. Parsing
• 5. C/C++ Wrapper Support Functions
• 6. Symbol Naming Guidelines for Generated C/C++ Code
• 7. Debugging SWIG
  • 7.1 Debugging DOH Types The Hard Way
  • 7.2 Debugging DOH memory allocation problems

The audience is assumed to be SWIG developers (who should also read the SWIG Engineering Manual before starting to code).

1.1 Directory Guide

• Modules/swigmain.cxx:main() is the program entry point. It parses the language-specifying command-line option (for example, -java), creating a new language-specific wrapping object (each language is a C++ class derived from base class Language). This object and the command-line is passed to SWIG_main(), whose return value is the program exit value.
• Modules/main.cxx:SWIG_main() is the "real" main. It initializes the preprocessor and typemap machinery, defines some preprocessor symbols, locates the SWIG library, processes common command-line options, and then calls the language-specific command-line parser. From here there are three paths: "help", "checkout" and everything else.
  • In "help" mode, clean up open files and exit.
  • In "checkout" mode, copy specified files from the SWIG library to the current directory. Errors cause error messages but no non-local exits.
  • Otherwise, do wrapping: determine output file name(s), define some preprocessor symbols and run the preprocessor, initialize the interface-definition parser, set up the typemap for handling new return strings, and finally do the language-specific parse (by calling the language object's parse() method), which creates output files by side-effect.
  Afterwards, remove temporary files, and clean up. If the command-line included -freeze, go into an infinite loop; otherwise return the error count.
• The language-specific parse() (and all other language-specific code) lives in Modules/foo.{h,cxx} for language Foo. Typically, FOO::parse() calls FOO::headers() and then the global function yyparse(), which uses the callbacks registered by SWIG_main() above.

DOH takes a different approach to tackling the complexity problem. First, rather than going overboard with dozens of types and class definitions, DOH only defines a handful of simple yet very useful objects that are easy to remember. Second, DOH uses dynamic typing---one of the features that make scripting languages so useful and which make it possible to accomplish things with much less code. Finally, DOH utilizes a few coding tricks that allow it to perform a limited form of function overloading for certain C datatypes (more on that a little later).

The key point to using DOH is that instead of thinking about code in terms of highly specialized C data structures, just about everything ends up being represented in terms of a just a few datatypes. For example, structures are replaced by DOH hash tables whereas arrays are replaced by DOH lists. At first, this is probably a little strange to most C/C++ programmers, but in the long run in makes the system extremely flexible and highly extensible. Also, in terms of coding, many of the newly DOH-based subsystems are less than half the size (in lines of code) of the earlier C++ implementation.

2.2 Basic Types

• String. A string of characters with automatic memory management and high-level operations such as string replacement. In addition, strings support file I/O operations that make it possible to use them just about anyplace a file can be used.

• List. A list of arbitrary DOH objects (of possibly mixed types).

• Hash. A hash table that maps a set of string keys to a set of arbitrary DOH objects. The DOH version of an associative array for all of you Perl fans.

• File. A DOH wrapper around the C FILE * structure. This is provided since other objects sometimes want to behave like files (strings for instance).

• Void. A DOH wrapper around an arbitrary C pointer. This can be used if you want to place arbitrary C data structures in DOH lists and hash tables.

• DOHString *. A String object.
• DOHList *. A list object.
• DOHHash *. A hash object.
• DOHFile *. A file object.
• DOHVoid *. A void object.
• DOHString_or_char *. A DOH String object or a raw C "char *".

• NewString(DOHString_or_char *value)
  Create a new string object with contents initially set to value. value can be either a C string or a DOH string object.

• NewStringf(char *fmt, ...)
  Create a new string object with contents initially set to a formatted string. Think of this as being sprintf() combined with an object constructor.

• NewList()
  Create a new list object that is initially empty.

• NewHash()
  Create a new hash object that is initially empty.

• NewFile(DOHString_or_char *filename, char *mode)
  Open a file and return a file object. This is a wrapper around the C fopen() library call.

• NewFileFromFile(FILE *f)
  Create a new file object given an already opened FILE * object.

• NewVoid(void *obj, void (*del)(void *))
  Create a new DOH object that is a wrapper around an arbitrary C pointer. del is an optional destructor function that will be called when the object is destroyed.

DOH *a, *b, *c, *d;
a = NewString("Hello World");
b = NewList();
c = Copy(a);         /* Copy the string a */
d = Copy(b);         /* Copy the list b */

Objects can be deleted using the Delete() function. For example:

DOH *a = NewString("Hello World");
...
Delete(a);              /* Destroy a */

DOH *a, *b;
a = NewString("Hello World");
b = NewList();
Append(b,a);         /* Increases refcnt of a to 2 */
Delete(a);           /* Decreases refcnt of a to 1 */
Delete(b);           /* Destroys b, and destroys a */

DOH *a = NewString("Hello");
DohIncref(a);

Example usage of lists:

/* Create and populate */
List *list = NewList();
Append(list, NewString("listval1"));
Append(list, NewString("listval2"));
Append(list, NewString("listval3"));
Append(list, NewString("listval4"));
Append(list, NewString("listval5"));

/* Size */
Printf(stdout, "list len: %d\n", Len(list));

/* Delete */
Delitem(list, 3);

/* Replace */
Setitem(list, 0, NewString("newlistval1"));

/* Get */
String *item = Getitem(list,1);
if (item)
  Printf(stdout, "get: %s\n", item);
else
  Printf(stdout, "get: [non-existent]\n");

/* Iterate through the container */
int len = Len(list);
for (int i=0; i<len; i++) {
  String *item = Getitem(list,i);
  Printf(stdout, "list item: %s\n", item);
}

Resulting output:

hash len: 5
get: hashval2
hash item: hashval5 [h5]
hash item: hashval1 [h1]
hash item: hashval2 [h2]
hash item: hashval3 [h3]

Example usage of hash tables:

/* Create and populate */
Hash *hash = NewHash();
Setattr(hash, "h1", NewString("hashval1"));
Setattr(hash, "h2", NewString("hashval2"));
Setattr(hash, "h3", NewString("hashval3"));
Setattr(hash, "h4", NewString("hashval4"));
Setattr(hash, "h5", NewString("hashval5"));

/* Size */
Printf(stdout, "hash len: %d\n", Len(hash));

/* Delete */
Delattr(hash, "h4");

/* Get */
String *item = Getattr(hash, "h2");
if (item)
  Printf(stdout, "get: %s\n", item);
else
  Printf(stdout, "get: [non-existent]\n");

/* Iterate through the container */
Iterator it;
for (it = First(hash); it.key; it= Next(it))
  Printf(stdout, "hash item: %s [%s]\n", (it.item), (it.key));

Resulting output:

list len: 5
get: listval2
list item: newlistval1
list item: listval2
list item: listval3
list item: listval5

The representation and manipulation of types is currently in the process of being reorganized and (hopefully) simplified. The following list describes the current set of functions that are used to manipulate datatypes.

• SwigType_str(SwigType *t, char *name).
  This function produces the exact string representation of the datatype t. name is an optional parameter that specifies a declaration name. This is used when dealing with more complicated datatypes such as arrays and pointers to functions where the output might look something like "int (*name)(int, double)".

• SwigType_lstr(SwigType *t, char *name).
  This function produces a string representation of a datatype that can be safely be assigned a value (i.e., can be used as the "lvalue" of an expression). To do this, qualifiers such as "const", arrays, and references are stripped away or converted into pointers. For example:

  Original Datatype              lstr()
  ------------------             --------
  const char *a                  char *a
  double a[20]                   double *a
  double a[20][30]               double *a
  double &a                      double *a
  

  The intent of the lstr() function is to produce local variables inside wrapper functions--all of which must be reassignable types since they are the targets of conversions from a scripting representation.

• SwigType_rcaststr(SwigType *t, char *name).
  This function produces a string that casts a type produced by the lstr() function to the type produced by the str() function. You might view it as the inverse of lstr(). This function only produces output when it needs to (when str() and lstr() produce different results). Furthermore, an optional name can be supplied when the cast is to be applied to a specific name. Examples:

  Original Datatype             rcaststr()
  ------------------            ---------
  char *a                       
  const char *a                 (const char *) name
  double a[20]                  (double *) name
  double a[20][30]              (double (*)[30]) name
  double &a                     (double &) *name
  

• SwigType_lcaststr(SwigType *t, char *name).
  This function produces a string that casts a type produced by the str() function to the type produced by the lstr() function. This function only produces output when it needs to (when str() and lstr() produce different results). Furthermore, an optional name can be supplied when the cast is to be applied to a specific name.

  Original Datatype             lcaststr()
  ------------------            ---------
  char *a                       
  const char *a                 (char *) name
  double a[20]                  (double *) name
  double a[20][30]              (double *) name
  double &a                     (double *) &name
  

• SwigType_manglestr(SwigType *t).
  Produces a type-string that is used to identify this datatype in the target scripting language. Usually this string looks something like "_p_p_double" although the target language may redefine the output for its own purposes. Normally this function strips all qualifiers, references, and arrays---producing a mangled version of the type produced by the lstr() function.

int foo(int a, double b[20][30], const char *c, double &d);

wrapper_foo() {
   lstr("int","result")
   lstr("int","arg0")
   lstr("double [20][30]", "arg1")
   lstr("const char *", "arg2")
   lstr("double &", "arg3")
   ...
   get arguments
   ...
   result = (lcaststr("int"))  foo(rcaststr("int","arg0"),
                               rcaststr("double [20][30]","arg1"),
                               rcaststr("const char *", "arg2"),
                               rcaststr("double &", "arg3"))
   ...
}

wrapper_foo() {
   int      result;
   int      arg0;
   double  *arg1;
   char    *arg2;
   double  *arg3;
   ...
   get arguments
   ...
   result = (int) foo(arg0,
                      (double (*)[30]) arg1,
                      (const char *) arg2,
                      (double &) *arg3);
   ...
}

• For convenience, the string generation functions return a "char *" that points to statically allocated memory living inside the type library. Therefore, it is never necessary (and it's an error) to free the pointer returned by the functions. Also, if you need to save the result, you should make a copy of it. However, with that said, it is probably worth nothing that these functions do cache the last 8 results. Therefore, it's fairly safe to make a handful of repeated calls without making any copies.

When SWIG generates wrappers, it tries to provide a mostly seamless integration with the original code. However, there are a number of problematic features of C/C++ programs that complicate this interface.

• Passing and returning structures by value. When used, SWIG converts all pass-by-value functions into wrappers that pass by reference. For example:

  double dot_product(Vector a, Vector b);
  

  gets turned into a wrapper like this:

  double wrap_dot_product(Vector *a, Vector *b) {
       return dot_product(*a,*b);
  }
  

  Functions that return by value require a memory allocation to store the result. For example:

  Vector cross_product(Vector *a, Vector *b);
  

  become

  Vector *wrap_cross_product(Vector *a, Vector *b) {
     Vector *result = (Vector *) malloc(sizeof(Vector));
     *result = cross_product(a,b);
     return result;
  }
  

  Note: If C++ is being wrapped, the default copy constructor is used instead of malloc() to create a copy of the return result.

• C++ references. C++ references are handled exactly the same as pass/return by value except that a memory allocation is not made for functions that return a reference.

• Qualifiers such as "const" and "volatile". SWIG strips all qualifiers from the interface presented to the target language. Besides, what in the heck is "const" in Perl anyways?

• Instance Methods. Method invocations are handled as a function call in which a pointer to the object (the "this" pointer) appears as the first argument. For example, in the following class:

  class Foo {
  public:
      double bar(double);
  };
  

  The "bar" method is wrapped by a function like this:

  double Foo_bar(Foo *self, double arg0) {
     return self->bar(arg0);
  }
  

• Structure/class data members. Data members are handled by creating a pair of wrapper functions that set and get the value respectively. For example:

  struct Foo {
      int x;
  };
  

  gets wrapped as follows:

  int Foo_x_get(Foo *self) {
      return self->x;
  }
  int Foo_x_set(Foo *self, int value) {
      return (self->x = value);
  }
  

• Constructors. Constructors for C/C++ data structures are wrapped by a function like this:

  Foo *new_Foo() {
      return new Foo;
  }
  

  Note: For C, new objects are created using the calloc() function.

• Destructors. Destructors for C/C++ data structures are wrapper like this:

  void delete_Foo(Foo *self) {
      delete self;
  }
  

  Note: For C, objects are destroyed using free().

• char *Swig_clocal(DataType *t, char *name, char *value)
  This function creates a string containing the declaration of a local variable with type t, name name, and default value value. This local variable is stripped of all qualifiers and will be a pointer if the type is a reference or user defined type.

• DataType *Swig_clocal_type(DataType *t)
  Returns a type object corresponding to the type string produced by the Swig_clocal() function.

• char *Swig_clocal_deref(DataType *t, char *name)
  This function is the inverse of the clocal() function. Given a type and a name, it produces a string containing the code needed to cast/convert the type produced by Swig_clocal() back into its original type.

• char *Swig_clocal_assign(DataType *t, char *name)
  Given a type and name, this produces a string containing the code (and an optional cast) needed to make an assignment from the real datatype to the local datatype produced by Swig_clocal(). Kind of the opposite of deref().

• int Swig_cargs(Wrapper *w, ParmList *l)
  Given a wrapper function object and a list of parameters, this function declares a set of local variables for holding all of the parameter values (using Swig_clocal()). Returns the number of parameters. In addition, this function sets the local name of each parameter which can be retrieved using the Parm_Getlname() function.

• void Swig_cresult(Wrapper *w, DataType *t, char *resultname, char *decl)
  Generates the code needed to set the result of a wrapper function and performs all of the needed memory allocations for ANSI C (if necessary). t is the type of the result, resultname is the name of the result variable, and decl is a string that contains the C code which produces the result.

• void Swig_cppresult(Wrapper *w, DataType *t, char *resultname, char *decl)
  Generates the code needed to set the result of a wrapper function and performs all of the needed memory allocations for C++ (if necessary). t is the type of the result, resultname is the name of the result variable, and decl is a string that contains the C code which produces the result.

• Wrapper *Swig_cfunction_wrapper(char *fname, DataType *rtype, ParmList *parms, char *code)
  Create a wrapper around a normal function declaration. fname is the name of the wrapper, rtype is the return type, parms are the function parameters, and code is a string containing the code in the function body.

• Wrapper *Swig_cmethod_wrapper(char *classname, char *methodname, DataType *rtype, DataType *parms, char *code)
  

• char *Swig_cfunction_call(char *name, ParmList *parms) This function produces a string containing the code needed to call a C function. The string that is produced contains all of the transformations needed to convert pass-by-value into pass-by-reference as well as handle C++ references. Produces a string like "name(arg0, arg1, ..., argn)".

double dot_product(Vector a, Vector b);

ParmList *l = ... parameter list of the function ...
DataType *t = ... return type of the function ...
char     *name = ... name of the function ...
Wrapper *w = NewWrapper();
Printf(w->def,"void wrap_%s() {\n", name);

/* Declare all of the local variables */
Swig_cargs(w, l);

/* Convert all of the arguments */
...

/* Make the function call and declare the result variable */
Swig_cresult(w,t,"result",Swig_cfunction(name,l));

/* Convert the result into whatever */
...

Printf(w->code,"}\n");
Wrapper_print(w,out);

void wrap_dot_product() {
    Vector *arg0;
    Vector *arg1;
    double  result;

    ...
    result = dot_product(*arg0, *arg1);
    ...
}

Notes:

• The intent of these functions is to provide consistent handling of function parameters and return values so that language module writers don't have to worry about it too much.

• These functions may be superseded by features in the new typemap system which provide hooks for specifying local variable declarations and argument conversions.

17.4.3.1.2 Global names [lib.global.names]

1 Certain sets of names and function signatures are always reserved to the implementation:

    * Each name that contains a double underscore (__) or begins with an underscore followed 
      by an upper case letter (2.11) is reserved to the implementation for any use.
    * Each name that begins with an underscore is reserved to the implementation for use as 
      a name in the global namespace.165)

    165) Such names are also reserved in namespace ::std (17.4.3.1). [back to text] 

For C++ code that doesn't attempt to mangle a symbol being wrapped (for example SWIG convenience functions):

• Put symbols in the Swig namespace, for example class Swig::Director. Qualify using the Swig namespace whenever the symbol is referenced, even within the Swig namespace, for example new Swig::Director() not new Director().
• Use swig_ as a prefix for all member variables and member functions that are involved in an inheritance chain with wrapped classes, for example Swig::Director::swig_get_up() and bool Swig::Director::swig_up.
• Alternatively class names can be prefixed with Swig in the global namespace for example template<class T> class SwigValueWrapper.

For code compiled as C or C++ that doesn't attempt to mangle a symbol being wrapped (for example SWIG convenience functions):

• Use SWIG_ as a prefix for structures for example SWIG_JavaExceptions_t.
• Use SWIG_ as a prefix for global functions for example SWIG_TypeRegister.
• Use SWIG_ as a prefix for macros for example #define SWIG_PY_INT 1

• Use SWIGxxx or Swigxxx as a prefix where xxx is chosen which would make SWIGxxx/Swigxxx a unique symbol in the global namespace, for example class SwigDirectorFoo when wrapping class Foo. Don't use a trailing underscore for the prefix as this may generate a double underscore when wrapping a symbol which starts with a single underscore.

The DOH types used in the SWIG source code are all typedefined to void. Consequently, it is impossible for debuggers to automatically extract any information about DOH objects. The easiest approach to debugging and viewing the contents of DOH objects is to make a call into one of the family of SWIG print functions from the debugger. The "Debugging Functions" section in SWIG Parse Tree Handling lists them. It is sometimes easier to debug by placing a few calls to these functions in code of interest and recompile, especially if your debugger cannot easily make calls into functions within a debugged binary.

The SWIG distribution comes with some additional support for the gdb debugger in the Tools/swig.gdb file. Follow the instructions in this file for 'installing'. This support file provides an easy way to call into some of the family of SWIG print functions via additional user-defined gdb commands. Some usage of the swigprint and locswigprint user-defined commands are demonstrated below.

More often than not, a parse tree node needs to be examined. The session below displays the node n in one of the Java language module wrapper functions. The swigprint method is used to show the symbol name (symname - a DOH String type) and the node (n - a DOH Hash type).

Breakpoint 1, JAVA::functionWrapper (this=0x97ea5f0, n=0xb7d2afc8) at Modules/java.cxx:799
799	    String *symname = Getattr(n, "sym:name");
(gdb) next
800	    SwigType *t = Getattr(n, "type");
(gdb) swigprint symname
Shape_x_set
(gdb) swigprint n
Hash(0xb7d2afc8) {
  'membervariableHandler:view' : variableHandler, 
  'feature:except' : 0, 
  'name' : x, 
  'ismember' : 1, 
  'sym:symtab' : Hash(0xb7d2aca8) {......}, 
  'nodeType' : cdecl, 
  'nextSibling' : Hash(0xb7d2af98) {.............}, 
  'kind' : variable, 
  'variableHandler:feature:immutable' : <Object 'VoidObj' at 0xb7cfa008>, 
  'sym:name' : Shape_x_set, 
  'view' : membervariableHandler, 
  'membervariableHandler:sym:name' : x, 
  'membervariableHandler:type' : double, 
  'membervariableHandler:parms' : <Object 'VoidObj' at 0xb7cfa008>, 
  'parentNode' : Hash(0xb7d2abc8) {..............................}, 
  'feature:java:enum' : typesafe, 
  'access' : public, 
  'parms' : Hash(0xb7cb9408) {......}, 
  'wrap:action' : if (arg1) (arg1)->x = arg2;, 
  'type' : void, 
  'memberset' : 1, 
  'sym:overname' : __SWIG_0, 
  'membervariableHandler:name' : x, 
}

Note that all the attributes in the Hash are shown, including the 'sym:name' attribute which was assigned to the symname variable.

Hash types can be shown either expanded or collapsed. When a Hash is shown expanded, all the attributes are displayed along with their values, otherwise a '.' replaces each attribute when collapsed. Therefore a count of the dots provides the number of attributes within an unexpanded Hash. Below shows the 'parms' Hash being displayed with the default Hash expansion of 1, then with 2 provided as the second argument to swigprint to expand to two Hash levels in order to view the contents of the collapsed 'nextSibling' Hash.

(gdb) swigprint 0xb7cb9408
Hash(0xb7cb9408) {
  'name' : self, 
  'type' : p.Shape, 
  'self' : 1, 
  'nextSibling' : Hash(0xb7cb9498) {...}, 
  'hidden' : 1, 
  'nodeType' : parm, 
}
(gdb) swigprint 0xb7cb9408 2
Hash(0xb7cb9408) {
  'name' : self, 
  'type' : p.Shape, 
  'self' : 1, 
  'nextSibling' : Hash(0xb7cb9498) {
    'name' : x, 
    'type' : double, 
    'nodeType' : parm, 
  }, 
  'hidden' : 1, 
  'nodeType' : parm, 
}

The same Hash can also be displayed with file and line location information via the locswigprint command.

(gdb) locswigprint 0xb7cb9408
example.h:11: [Hash(0xb7cb9408) {
Hash(0xb7cb9408) {
  'name' : self, 
  'type' : p.Shape, 
  'self' : 1, 
  'nextSibling' : Hash(0xb7cb9498) {...}, 
  'hidden' : 1, 
  'nodeType' : parm, 
}]

Tip: Commands in gdb can be shortened with whatever makes them unique and can be command completed with the tab key. Thus swigprint can usually be shortened to sw and locswigprint to loc. The help for each command can also be obtained within the debugging session, for example, 'help swigprint'.

The sub-section below gives pointers for debugging DOH objects using casts and provides an insight into why it can be hard to debug SWIG without the family of print functions.

7.1 Debugging DOH Types The Hard Way

Below are a list of common SWIG types. With each is the cast that can be used in the debugger to extract the underlying type information and the underlying char * string.

• String *s;
(struct String *)((DohBase *)s)->data
The underlying char * string can be displayed with
(*(struct String *)(((DohBase *)s)->data)).str
• SwigType *t;
(struct String *)((DohBase *)t)->data
The underlying char * string can be displayed with
(*(struct String *)(((DohBase *)t)->data)).str
• const_String_or_char_ptr sc;
Either
(*(struct String *)(((DohBase *)sc)->data)).str
or
(char *)sc
will work depending on whether the underlying type is really a String * or char *.

The DOH objects are reference counted and use pools for memory allocation. The implementation is in memory.c. When there are memory corruption problems, various memory allocator tools are normally used to diagnose problems. These can be used on SWIG and can be very useful. However, they won't necessarily find use of stale DOH objects, that is, DOH objects that are used after they have been deleted. This is because the DOH memory allocator grabs a chunk of memory from the C memory allocator and manages the usage internally. Stale DOH object usage can be checked for by defining DOH_DEBUG_MEMORY_POOLS in memory.c. If an attempt to use an object is made after the reference count is zero, an assertion is triggered instead of quietly re-using the stale object...

swig: DOH/memory.c:91: DohCheck: Assertion `!DOH_object_already_deleted' failed.

This can be memory intensive as previously used memory in the pool is not re-used so is only recommended for diagnosing memory corruption problems.