Variable Length Arguments

# 13 Variable Length Arguments

- Introduction
- The Problem
- Default varargs support
- Argument replacement using %varargs
- Varargs and typemaps
- Varargs wrapping with libffi
- Wrapping of va_list
- C++ Issues
- Discussion

(a.k.a, "The horror. The horror.")

This chapter describes the problem of wrapping functions that take a variable number of arguments. For instance, generating wrappers for the C `printf()` family of functions.

This topic is sufficiently advanced to merit its own chapter. In fact, support for varargs is an often requested feature that was first added in SWIG-1.3.12. Most other wrapper generation tools have wisely chosen to avoid this issue.

## 13.1 Introduction

Some C and C++ programs may include functions that accept a variable number of arguments. For example, most programmers are familiar with functions from the C library such as the following:

    int printf(const char *fmt, ...)
    int fprintf(FILE *, const char *fmt, ...);
    int sprintf(char *s, const char *fmt, ...);

Although there is probably little practical purpose in wrapping these specific C library functions in a scripting language (what would be the point?), a library may include its own set of special functions based on a similar API. For example:

    int  traceprintf(const char *fmt, ...);

In this case, you may want to have some kind of access from the target language.

Before describing the SWIG implementation, it is important to discuss the common uses of varargs that you are likely to encounter in real programs. Obviously, there are the `printf()` style output functions as shown. Closely related to this would be `scanf()` style input functions that accept a format string and a list of pointers into which return values are placed. However, variable length arguments are also sometimes used to write functions that accept a NULL-terminated list of pointers. A good example of this would be a function like this:

    int execlp(const char *path, const char *arg1, ...);
    ...

    /* Example */
    execlp("ls","ls","-l",NULL);

In addition, varargs is sometimes used to fake default arguments in older C libraries. For instance, the low level `open()` system call is often declared as a varargs function so that it will accept two or three arguments:

    int open(const char *path, int oflag, ...);
    ...

    /* Examples */
    f = open("foo", O_RDONLY);
    g = open("bar", O_WRONLY | O_CREAT, 0644);

Finally, to implement a varargs function, recall that you have to use the C library functions defined in `<stdarg.h>`. For example:

    List make_list(const char *s, ...) {
        va_list ap;
        List    x;
        ...
        va_start(ap, s);
        while (s) {
           x.append(s);
           s = va_arg(ap, const char *);
        }
        va_end(ap);
        return x;
    }

## 13.2 The Problem

Generating wrappers for a variable length argument function presents a number of special challenges. Although C provides support for implementing functions that receive variable length arguments, there are no functions that can go in the other direction. Specifically, you can't write a function that dynamically creates a list of arguments and which invokes a varargs function on your behalf.

Although it is possible to write functions that accept the special type `va_list`, this is something entirely different. You can't take a `va_list` structure and pass it in place of the variable length arguments to another varargs function. It just doesn't work.

The reason this doesn't work has to do with the way that function calls get compiled. For example, suppose that your program has a function call like this:

    printf("Hello %s. Your number is %d\n", name, num);

When the compiler looks at this, it knows that you are calling `printf()` with exactly three arguments. Furthermore, it knows that the number of arguments as well are their types and sizes is never going to change during program execution. Therefore, this gets turned to machine code that sets up a three-argument stack frame followed by a call to `printf()`.

In contrast, suppose you attempted to make some kind of wrapper around `printf()` using code like this:

    int wrap_printf(const char *fmt, ...) {
       va_list ap;
       va_start(ap,fmt);
       ...
       printf(fmt,ap);
       ...
       va_end(ap);
    };

Although this code might compile, it won't do what you expect. This is because the call to `printf()` is compiled as a procedure call involving only two arguments. However, clearly a two-argument configuration of the call stack is completely wrong if your intent is to pass an arbitrary number of arguments to the real `printf()`. Needless to say, it won't work.

Unfortunately, the situation just described is exactly the problem faced by wrapper generation tools. In general, the number of passed arguments will not be known until run-time. To make matters even worse, you won't know the types and sizes of arguments until run-time as well. Needless to say, there is no obvious way to make the C compiler generate code for a function call involving an unknown number of arguments of unknown types.

In theory, it is possible to write a wrapper that does the right thing. However, this involves knowing the underlying ABI for the target platform and language as well as writing special purpose code that manually constructed the call stack before making a procedure call. Unfortunately, both of these tasks require the use of inline assembly code. Clearly, that's the kind of solution you would much rather avoid.

With this nastiness in mind, SWIG provides a number of solutions to the varargs wrapping problem. Most of these solutions are compromises that provide limited varargs support without having to resort to assembly language. However, SWIG can also support real varargs wrapping (with stack-frame manipulation) if you are willing to get hands dirty. Keep reading.

## 13.3 Default varargs support

When variable length arguments appear in an interface, the default behavior is to drop the variable argument list entirely, replacing them with a single NULL pointer. For example, if you had this function,

    void traceprintf(const char *fmt, ...);

it would be wrapped as if it had been declared as follows:

    void traceprintf(const char *fmt);

When the function is called inside the wrappers, it is called as follows:

    traceprintf(arg1, NULL);

Arguably, this approach seems to defeat the whole point of variable length arguments. However, this actually provides enough support for many simple kinds of varargs functions to still be useful. For instance, you could make function calls like this (in Python):

    >>> traceprintf("Hello World")
    >>> traceprintf("Hello %s. Your number is %d\n" % (name, num))

Notice how string formatting is being done in Python instead of C.

## 13.4 Argument replacement using %varargs

Instead of dropping the variable length arguments, an alternative approach is to replace `(...)` with a set of suitable arguments. SWIG provides a special `%varargs` directive that can be used to do this. For example,

    %varargs(int mode = 0) open;
    ...
    int open(const char *path, int oflags, ...);

is equivalent to this:

    int open(const char *path, int oflags, int mode = 0);

In this case, `%varargs` is simply providing more specific information about the extra arguments that might be passed to a function. If the parameters to a varargs function are of uniform type, `%varargs` can also accept a numerical argument count as follows:

    %varargs(10,char *arg = NULL) execlp;
    ...
    int execlp(const char *path, const char *arg1, ...);

This would wrap `execlp()` as a function that accepted up to 10 optional arguments. Depending on the application, this may be more than enough for practical purposes.

Argument replacement is most appropriate in cases where the types of the extra arguments is uniform and the maximum number of arguments is known. When replicated argument replacement is used, at least one extra argument is added to the end of the arguments when making the function call. This argument serves as a sentinel to make sure the list is properly terminated. It has the same value as that supplied to the `%varargs` directive.

Argument replacement is not as useful when working with functions that accept mixed argument types such as `printf()`. Providing general purpose wrappers to such functions presents special problems (covered shortly).

## 13.5 Varargs and typemaps

Variable length arguments may be used in typemap specifications. For example:

    %typemap(in) (...) {
        // Get variable length arguments (somehow)
        ...
    }

    %typemap(in) (const char *fmt, ...) {
        // Multi-argument typemap
    }

However, this immediately raises the question of what "type" is actually used to represent `(...)`. For lack of a better alternative, the type of `(...)` is set to `void *`. Since there is no way to dynamically pass arguments to a varargs function (as previously described), the `void *` argument value is intended to serve as a place holder for storing some kind of information about the extra arguments (if any). In addition, the default behavior of SWIG is to pass the `void *` value as an argument to the function. Therefore, you could use the pointer to hold a valid argument value if you wanted.

To illustrate, here is a safer version of wrapping `printf()` in Python:

    %typemap(in) (const char *fmt, ...) {
        $1 = "%s";                                /* Fix format string to %s */
        $2 = (void *) PyString_AsString($input);  /* Get string argument */
    };
    ...
    int printf(const char *fmt, ...);

In this example, the format string is implicitly set to `"%s"`. This prevents a program from passing a bogus format string to the extension. Then, the passed input object is decoded and placed in the `void *` argument defined for the `(...)` argument. When the actual function call is made, the underlying wrapper code will look roughly like this:

    wrap_printf() {
       char *arg1;
       void *arg2;
       int   result;

       arg1 = "%s";
       arg2 = (void *) PyString_AsString(arg2obj);
       ...
       result = printf(arg1,arg2);
       ...
    }

Notice how both arguments are passed to the function and it does what you would expect.

The next example illustrates a more advanced kind of varargs typemap. Disclaimer: this requires special support in the target language module and is not guaranteed to work with all SWIG modules at this time. It also starts to illustrate some of the more fundamental problems with supporting varargs in more generality.

If a typemap is defined for any form of `(...)`, many SWIG modules will generate wrappers that accept a variable number of arguments as input and will make these arguments available in some form. The precise details of this depends on the language module being used (consult the appropriate chapter for more details). However, suppose that you wanted to create a Python wrapper for the `execlp()` function shown earlier. To do this using a typemap instead of using `%varargs`, you might first write a typemap like this:

    %typemap(in) (...)(char *args[10]) {
        int i;
        int argc;
        for (i = 0; i < 10; i++) args[i] = 0;
        argc = PyTuple_Size(varargs);
        if (argc > 10) {
           PyErr_SetString(PyExc_ValueError,"Too many arguments");
           return NULL;
        }
        for (i = 0; i < argc; i++) {
           PyObject *o = PyTuple_GetItem(varargs,i);
           if (!PyString_Check(o)) {
               PyErr_SetString(PyExc_ValueError,"Expected a string");
               return NULL;
           }
           args[i] = PyString_AsString(o);
        }
        $1 = (void *) args;
    }

In this typemap, the special variable `varargs` is a tuple holding all of the extra arguments passed (this is specific to the Python module). The typemap then pulls this apart and sticks the values into the array of strings `args`. Then, the array is assigned to `$1` (recall that this is the `void *` variable corresponding to `(...)`). However, this assignment is only half of the picture----clearly this alone is not enough to make the function work. To patch everything up, you have to rewrite the underlying action code using the `%feature` directive like this:

    %feature("action") execlp {
       char *args = (char **) arg3;
       result = execlp(arg1, arg2, args[0], args[1], args[2], args[3], args[4],
                       args[5],args[6],args[7],args[8],args[9], NULL);
    }

    int execlp(const char *path, const char *arg, ...);

This patches everything up and creates a function that more or less works. However, don't try explaining this to your coworkers unless you know for certain that they've had several cups of coffee. If you really want to elevate your guru status and increase your job security, continue to the next section.

## 13.6 Varargs wrapping with libffi

All of the previous examples have relied on features of SWIG that are portable and which don't rely upon any low-level machine-level details. In many ways, they have all dodged the real issue of variable length arguments by recasting a varargs function into some weaker variation with a fixed number of arguments of known types. In many cases, this works perfectly fine. However, if you want more generality than this, you need to bring out some bigger guns.

One way to do this is to use a special purpose library such as libffi (http://sources.redhat.com/libffi). libffi is a library that allows you to dynamically construct call-stacks and invoke procedures in a relatively platform independent manner. Details about the library can be found in the libffi distribution and are not repeated here.

To illustrate the use of libffi, suppose that you really wanted to create a wrapper for `execlp()` that accepted any number of arguments. To do this, you might make a few adjustments to the previous example. For example:

    /* Take an arbitrary number of extra arguments and place into an array
       of strings */

    %typemap(in) (...) {
       char **argv;
       int    argc;
       int    i;

       argc = PyTuple_Size(varargs);
       argv = (char **) malloc(sizeof(char *)*(argc+1));
       for (i = 0; i < argc; i++) {
          PyObject *o = PyTuple_GetItem(varargs,i);
          if (!PyString_Check(o)) {
              PyErr_SetString(PyExc_ValueError,"Expected a string");
    	  free(argv);
              return NULL;
          }
          argv[i] = PyString_AsString(o);
       }
       argv[i] = NULL;
       $1 = (void *) argv;
    }

    /* Rewrite the function call, using libffi */

    %feature("action") execlp {
      int       i, vc;
      ffi_cif   cif;
      ffi_type  **types;
      void      **values;
      char      **args;

      vc = PyTuple_Size(varargs);
      types  = (ffi_type **) malloc((vc+3)*sizeof(ffi_type *));
      values = (void **) malloc((vc+3)*sizeof(void *));
      args   = (char **) arg3;

      /* Set up path parameter */
      types[0] = &ffi_type_pointer;
      values[0] = &arg1;

      /* Set up first argument */
      types[1] = &ffi_type_pointer;
      values[1] = &arg2;

      /* Set up rest of parameters */
      for (i = 0; i <= vc; i++) {
        types[2+i] = &ffi_type_pointer;
        values[2+i] = &args[i];
      }
      if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, vc+3,
                       &ffi_type_uint, types) == FFI_OK) {
        ffi_call(&cif, (void (*)()) execlp, &result, values);
      } else {
        PyErr_SetString(PyExc_RuntimeError, "Whoa!!!!!");
        free(types);
        free(values);
        free(arg3);
        return NULL;
      }
      free(types);
      free(values);
      free(arg3);
    }

    /* Declare the function. Whew! */
    int execlp(const char *path, const char *arg1, ...);

Looking at this example, you may start to wonder if SWIG is making life any easier. Given the amount of code involved, you might also wonder why you didn't just write a hand-crafted wrapper! Either that or you're wondering "why in the hell am I trying to wrap this varargs function in the first place?!?" Obviously, those are questions you'll have to answer for yourself.

As a more extreme example of libffi, here is some code that attempts to wrap `printf()`,

    /* A wrapper for printf() using libffi */

    %{
    /* Structure for holding passed arguments after conversion */
      typedef struct {
        int type;
        union {
          int    ivalue;
          double dvalue;
          void   *pvalue;
        } val;
      } vtype;
      enum { VT_INT, VT_DOUBLE, VT_POINTER };
    %}

    %typemap(in) (const char *fmt, ...) {
      vtype *argv;
      int    argc;
      int    i;

      /* Format string */
      $1 = PyString_AsString($input);

      /* Variable length arguments */
      argc = PyTuple_Size(varargs);
      argv = (vtype *) malloc(argc*sizeof(vtype));
      for (i = 0; i < argc; i++) {
        PyObject *o = PyTuple_GetItem(varargs,i);
        if (PyInt_Check(o)) {
          argv[i].type = VT_INT;
          argv[i].val.ivalue = PyInt_AsLong(o);
        } else if (PyFloat_Check(o)) {
          argv[i].type = VT_DOUBLE;
          argv[i].val.dvalue = PyFloat_AsDouble(o);
        } else if (PyString_Check(o)) {
          argv[i].type = VT_POINTER;
          argv[i].val.pvalue = (void *) PyString_AsString(o);
        } else {
          PyErr_SetString(PyExc_ValueError,"Unsupported argument type");
          free(argv);
          return NULL;
        }
      }
      $2 = (void *) argv;
    }

    /* Rewrite the function call using libffi */
    %feature("action") printf {
      int       i, vc;
      ffi_cif   cif;
      ffi_type  **types;
      void      **values;
      vtype     *args;

      vc = PyTuple_Size(varargs);
      types  = (ffi_type **) malloc((vc+1)*sizeof(ffi_type *));
      values = (void **) malloc((vc+1)*sizeof(void *));
      args   = (vtype *) arg2;

      /* Set up fmt parameter */
      types[0] = &ffi_type_pointer;
      values[0] = &arg1;

      /* Set up rest of parameters */
      for (i = 0; i < vc; i++) {
        switch(args[i].type) {
        case VT_INT:
          types[1+i] = &ffi_type_uint;
          values[1+i] = &args[i].val.ivalue;
          break;
        case VT_DOUBLE:
          types[1+i] = &ffi_type_double;
          values[1+i] = &args[i].val.dvalue;
          break;
        case VT_POINTER:
          types[1+i] = &ffi_type_pointer;
          values[1+i] = &args[i].val.pvalue;
          break;
        default:
          abort();    /* Whoa! We're seriously hosed */
          break;
        }
      }
      if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, vc+1,
                       &ffi_type_uint, types) == FFI_OK) {
        ffi_call(&cif, (void (*)()) printf, &result, values);
      } else {
        PyErr_SetString(PyExc_RuntimeError, "Whoa!!!!!");
        free(types);
        free(values);
        free(args);
        return NULL;
      }
      free(types);
      free(values);
      free(args);
    }

    /* The function */
    int printf(const char *fmt, ...);

Much to your amazement, it even seems to work if you try it:

    >>> import example
    >>> example.printf("Grade: %s   %d/60 = %0.2f%%\n", "Dave", 47, 47.0*100/60)
    Grade: Dave   47/60 = 78.33%
    >>>

Of course, there are still some limitations to consider:

    >>> example.printf("la de da de da %s", 42)
    Segmentation fault (core dumped)

And, on this note, we leave further exploration of libffi to the reader as an exercise. Although Python has been used as an example, most of the techniques in this section can be extrapolated to other language modules with a bit of work. The only details you need to know is how the extra arguments are accessed in each target language. For example, in the Python module, we used the special `varargs` variable to get these arguments. Modules such as Tcl8 and Perl5 simply provide an argument number for the first extra argument. This can be used to index into an array of passed arguments to get values. Please consult the chapter on each language module for more details.

## 13.7 Wrapping of va_list

Closely related to variable length argument wrapping, you may encounter functions that accept a parameter of type `va_list`. For example:

    int vfprintf(FILE *f, const char *fmt, va_list ap);

As far as we know, there is no obvious way to wrap these functions with SWIG. This is because there is no documented way to assemble the proper va_list structure (there are no C library functions to do it and the contents of va_list are opaque). Not only that, the contents of a `va_list` structure are closely tied to the underlying call-stack. It's not clear that exporting a `va_list` would have any use or that it would work at all.

## 13.8 C++ Issues

Wrapping of C++ member functions that accept a variable number of arguments presents a number of challenges. By far, the easiest way to handle this is to use the `%varargs` directive. This is portable and it fully supports classes much like the `%rename` directive. For example:

    %varargs (10, char * = NULL) Foo::bar;

    class Foo {
    public:
         virtual void bar(char *arg, ...);   // gets varargs above
    };

    class Spam: public Foo {
    public:
         virtual void bar(char *arg, ...);   // gets varargs above
    };

`%varargs` also works with constructors, operators, and any other C++ programming construct that accepts variable arguments.

Doing anything more advanced than this is likely to involve a serious world of pain. In order to use a library like libffi, you will need to know the underlying calling conventions and details of the C++ ABI. For instance, the details of how `this` is passed to member functions as well as any hidden arguments that might be used to pass additional information. These details are implementation specific and may differ between compilers and even different versions of the same compiler. Also, be aware that invoking a member function is further complicated if it is a virtual method. In this case, invocation might require a table lookup to obtain the proper function address (although you might be able to obtain an address by casting a bound pointer to a pointer to function as described in the C++ ARM section 18.3.4).

If you do decide to change the underlying action code, be aware that SWIG always places the `this` pointer in `arg1`. Other arguments are placed in `arg2`, `arg3`, and so forth. For example:

    %feature("action") Foo::bar {
       ...
       result = arg1->bar(arg2, arg3, etc.);
       ...
    }

Given the potential to shoot yourself in the foot, it is probably easier to reconsider your design or to provide an alternative interface using a helper function than it is to create a fully general wrapper to a varargs C++ member function.

## 13.9 Discussion

This chapter has provided a number of techniques that can be used to address the problem of variable length argument wrapping. If you care about portability and ease of use, the `%varargs` directive is probably the easiest way to tackle the problem. However, using typemaps, it is possible to do some very advanced kinds of wrapping.

One point of discussion concerns the structure of the libffi examples in the previous section. Looking at that code, it is not at all clear that this is the easiest way to solve the problem. However, there are a number of subtle aspects of the solution to consider--mostly concerning the way in which the problem has been decomposed. First, the example is structured in a way that tries to maintain separation between wrapper-specific information and the declaration of the function itself. The idea here is that you might structure your interface like this:

    %typemap(const char *fmt, ...) {
       ...
    }
    %feature("action") traceprintf {
       ...
    }

    /* Include some header file with traceprintf in it */
    %include "someheader.h"

Second, careful scrutiny will reveal that the typemaps involving `(...)` have nothing whatsoever to do with the libffi library. In fact, they are generic with respect to the way in which the function is actually called. This decoupling means that it will be much easier to consider other library alternatives for making the function call. For instance, if libffi wasn't supported on a certain platform, you might be able to use something else instead. You could use conditional compilation to control this:

    #ifdef USE_LIBFFI
    %feature("action") printf {
       ...
    }
    #endif
    #ifdef USE_OTHERFFI
    %feature("action") printf {
    ...
    }
    #endif

Finally, even though you might be inclined to just write a hand-written wrapper for varargs functions, the techniques used in the previous section have the advantage of being compatible with all other features of SWIG such as exception handling.

As a final word, some C programmers seem to have the assumption that the wrapping of variable length argument functions is an easily solved problem. However, this section has hopefully dispelled some of these myths. All things being equal, you are better off avoiding variable length arguments if you can. If you can't avoid them, please consider some of the simple solutions first. If you can't live with a simple solution, proceed with caution. At the very least, make sure you carefully read the section "A7.3.2 Function Calls" in Kernighan and Ritchie and make sure you fully understand the parameter passing conventions used for varargs. Also, be aware of the platform dependencies and reliability issues that this will introduce. Good luck.