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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Variable Length Arguments</title>
</head>
<body bgcolor="#ffffff">
<a name="n1"></a><H1>10 Variable Length Arguments</H1>
<!-- INDEX -->
<ul>
<li><a href="#n2">Introduction</a>
<li><a href="#n3">The Problem</a>
<li><a href="#n4">Default varargs support</a>
<li><a href="#n5">Argument replacement using %varargs</a>
<li><a href="#n6">Varargs and typemaps</a>
<li><a href="#n7">Varargs wrapping with libffi</a>
<li><a href="#n8">Wrapping of va_list</a>
<li><a href="#n9">C++ Issues</a>
<li><a href="#n10">Discussion</a>
</ul>
<!-- INDEX -->
<b>(a.k.a, "The horror. The horror.")</b>
<p>
This chapter describes the problem of wrapping functions that take a
variable number of arguments. For instance, generating wrappers for
the C <tt>printf()</tt> family of functions.
<p>
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.
<a name="n2"></a><H2>10.1 Introduction</H2>
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:
<blockquote>
<pre>
int printf(const char *fmt, ...)
int fprintf(FILE *, const char *fmt, ...);
int sprintf(char *s, const char *fmt, ...);
</pre>
</blockquote>
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:
<blockquote>
<pre>
int traceprintf(const char *fmt, ...);
</pre>
</blockquote>
In this case, you may want to have some kind of access from the target language.
<p>
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 <tt>printf()</tt> style output
functions as shown. Closely related to this would be
<tt>scanf()</tt> 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:
<blockquote>
<pre>
int execlp(const char *path, const char *arg1, ...);
...
/* Example */
execlp("ls","ls","-l",NULL);
</pre>
</blockquote>
In addition, varargs is sometimes used to fake default arguments in older
C libraries. For instance, the low level <tt>open()</tt> system call
is often declared as a varargs function so that it will accept two
or three arguments:
<blockquote>
<pre>
int open(const char *path, int oflag, ...);
...
/* Examples */
f = open("foo", O_RDONLY);
g = open("bar", O_WRONLY | O_CREAT, 0644);
</pre>
</blockquote>
Finally, to implement a varargs function, recall that you have to use
the C library functions defined in <tt>&lt;stdarg.h&gt;</tt>. For
example:
<blockquote>
<pre>
List make_list(const char *s, ...) {
va_list ap;
List *x = new List();
...
va_start(ap, s);
while (s) {
x.append(s);
s = va_arg(ap, const char *);
}
va_end(ap);
return x;
}
</pre>
</blockquote>
<a name="n3"></a><H2>10.2 The Problem</H2>
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.
<p>
Although it is possible to write functions that accept the special
type <tt>va_list</tt>, this is something entirely different. You
can't take a <tt>va_list</tt> structure and pass it in place of the
variable length arguments to another varargs function. It just
doesn't work.
<p>
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:
<blockquote>
<pre>
printf("Hello %s. Your number is %d\n", name, num);
</pre>
</blockquote>
When the compiler looks at this, it knows that you are calling
<tt>printf()</tt> with exactly three arguments. Furthermore, it knows
that the number of arguments as well are their types and sizes is
<em>never</em> 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 <tt>printf()</tt>.
<p>
In contrast, suppose you attempted to make some kind of wrapper around
<tt>printf()</tt> using code like this:
<blockquote>
<pre>
int wrap_printf(const char *fmt, ...) {
va_list ap;
va_start(ap,fmt);
...
printf(fmt,ap);
...
va_end(ap);
};
</blockquote>
</pre>
Athough this code might compile, it won't do what you expect. This is
because the call to <tt>printf()</tt> 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
<tt>printf()</tt>. Needless to say, it won't work.
<p>
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.
<p>
In theory, it <em>is</em> 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.
<p>
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.
<a name="n4"></a><H2>10.3 Default varargs support</H2>
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,
<blockquote>
<pre>
void traceprintf(const char *fmt, ...);
</pre>
</blockquote>
it would be wrapped as if it had been declared as follows:
<blockquote>
<pre>
void traceprintf(const char *fmt);
</pre>
</blockquote>
When the function is called inside the wrappers, it is called as follows:
<blockquote>
<pre>
traceprintf(arg1, NULL);
</pre>
</blockquote>
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):
<blockquote>
<pre>
>>> traceprintf("Hello World")
>>> traceprintf("Hello %s. Your number is %d\n" % (name, num))
</pre>
</blockquote>
Notice how string formatting is being done in Python instead of C.
<a name="n5"></a><H2>10.4 Argument replacement using %varargs</H2>
Instead of dropping the variable length arguments, an alternative approach is to replace
<tt>(...)</tt> with a set of suitable arguments. SWIG provides a special <tt>%varargs</tt> directive
that can be used to do this. For example,
<blockquote>
<pre>
%varargs(int mode = 0) open;
...
int open(const char *path, int oflags, ...);
</pre>
</blockquote>
is equivalent to this:
<blockquote>
<pre>
int open(const char *path, int oflags, int mode = 0);
</pre>
</blockquote>
In this case, <tt>%varargs</tt> 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, <tt>%varargs</tt> can also
accept a numerical argument count as follows:
<blockquote>
<pre>
%varargs(10,char *arg = NULL) execlp;
...
int execlp(const char *path, const char *arg1, ...);
</pre>
</blockquote>
This would wrap <tt>execlp()</tt> as a function that accepted up to 10 optional arguments.
Depending on the application, this may be more than enough for practical purposes.
<p>
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 <tt>%varargs</tt> directive.
<p>
Argument replacement is not as useful when working with functions that accept
mixed argument types such as <tt>printf()</tt>. Providing general purpose
wrappers to such functions presents special problems (covered shortly).
<a name="n6"></a><H2>10.5 Varargs and typemaps</H2>
Variable length arguments may be used in typemap specifications. For example:
<blockquote>
<pre>
%typemap(in) (...) {
// Get variable length arguments (somehow)
...
}
%typemap(in) (const char *fmt, ...) {
// Multi-argument typemap
}
</pre>
</blockquote>
However, this immediately raises the question of what "type" is actually used
to represent <tt>(...)</tt>. For lack of a better alternative, the type of
<tt>(...)</tt> is set to <tt>void *</tt>. Since there is no
way to dynamically pass arguments to a varargs function (as previously described),
the <tt>void *</tt> 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 <tt>void *</tt> value as an argument to
the function. Therefore, you could use the pointer to hold a valid argument value if you wanted.
<p>
To illustrate, here is a safer version of wrapping <tt>printf()</tt> in Python:
<blockquote>
<pre>
%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, ...);
</pre>
</blockquote>
In this example, the format string is implicitly set to <tt>"%s"</tt>.
This prevents a program from passing a bogus format string to the
extension. Then, the passed input object is decoded and placed in the
<tt>void *</tt> argument defined for the <tt>(...)</tt> argument. When the
actual function call is made, the underlying wrapper code will look roughly
like this:
<blockquote>
<pre>
wrap_printf() {
char *arg1;
void *arg2;
int result;
arg1 = "%s";
arg2 = (void *) PyString_AsString(arg2obj);
...
result = printf(arg1,arg2);
...
}
</pre>
</blockquote>
Notice how both arguments are passed to the function and it does what you
would expect.
<p>
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.
<p>
If a typemap is defined for any form of <tt>(...)</tt>, 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
<tt>execlp()</tt> function shown earlier. To do this using a typemap
instead of using <tt>%varargs</tt>, you might first write a typemap
like this:
<blockquote>
<pre>
%typemap(in) (...)(char *args[10]) {
int i;
int argc;
for (i = 0; i &lt; 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 &lt; 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;
}
</pre>
</blockquote>
In this typemap, the special variable <tt>varargs</tt> 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 <tt>args</tt>. Then, the array is
assigned to <tt>$1</tt> (recall that this is the <tt>void *</tt>
variable corresponding to <tt>(...)</tt>). 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 <tt>%feature</tt> directive like
this:
<blockquote>
<pre>
%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, ...);
</pre>
</blockquote>
<p>
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.
<a name="n7"></a><H2>10.6 Varargs wrapping with libffi</H2>
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.
<P>
One way to do this is to use a special purpose library such as libffi
(<a
href="http://sources.redhat.com/libffi/">http://sources.redhat.com/libffi</a>).
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.
<p>
To illustrate the use of libffi, suppose that you <em>really</em> wanted to create a
wrapper for <tt>execlp()</tt> that accepted <em>any</em> number of
arguments. To do this, you might make a few adjustments to the previous
example. For example:
<blockquote>
<pre>
/* 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 &lt; 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 &lt;= 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, ...);
</pre>
</blockquote>
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.
<p>
As a more extreme example of libffi, here is some code that attempts to wrap <tt>printf()</tt>,
<blockquote>
<pre>
/* 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 &lt; 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 &lt; 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, ...);
</pre>
</blockquote>
Much to your amazement, it even seems to work if you try it:
<blockquote>
<pre>
>>> import example
>>> example.printf("Grade: %s %d/60 = %0.2f%%\n", "Dave", 47, 47.0*100/60)
Grade: Dave 47/60 = 78.33%
>>>
</pre>
</blockquote>
Of course, there are still some limitations to consider:
<blockquote>
<pre>
>>> example.printf("la de da de da %s", 42)
Segmentation fault (core dumped)
</pre>
</blockquote>
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 <tt>varargs</tt> 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.
<a name="n8"></a><H2>10.7 Wrapping of va_list</H2>
Closely related to variable length argument wrapping, you may encounter functions that accept a parameter
of type <tt>va_list</tt>. For example:
<blockquote>
<pre>
int vfprintf(FILE *f, const char *fmt, va_list ap);
</pre>
</blockquote>
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 <tt>va_list</tt> structure are closely tied to the underlying
call-stack. It's not clear that exporting a <tt>va_list</tt> would
have any use or that it would work at all.
<a name="n9"></a><H2>10.8 C++ Issues</H2>
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 <tt>%varargs</tt> directive. This is portable
and it fully supports classes much like the <tt>%rename</tt> directive. For example:
<blockquote>
<pre>
%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
};
</pre>
</blockquote>
<tt>%varargs</tt> also works with constructors, operators, and any
other C++ programming construct that accepts variable arguments.
<p>
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 <tt>this</tt> 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).
<p>
If you do decide to change the underlying action code, be aware that SWIG
always places the <tt>this</tt> pointer in <tt>arg1</tt>. Other arguments
are placed in <tt>arg2</tt>, <tt>arg3</tt>, and so forth. For example:
<blockquote>
<pre>
%feature("action") Foo::bar {
...
result = arg1->bar(arg2, arg3, etc.);
...
}
</pre>
</blockquote>
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.
<a name="n10"></a><H2>10.9 Discussion</H2>
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 <tt>%varargs</tt> directive is
probably the easiest way to tackle the problem. However, using typemaps, it is possible to do some very advanced
kinds of wrapping.
<p>
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:
<blockquote>
<pre>
%typemap(const char *fmt, ...) {
...
}
%feature("action") traceprintf {
...
}
/* Include some header file with traceprintf in it */
%include "someheader.h"
</pre>
</blockquote>
Second, careful scrutiny will reveal that the typemaps involving <tt>(...)</tt> 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:
<blockquote>
<pre>
#ifdef USE_LIBFFI
%feature("action") printf {
...
}
#endif
#ifdef USE_OTHERFFI
%feature("action") printf {
...
}
#endif
</pre>
</blockquote>
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.
<p>
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.
<p><hr>
<address>SWIG 1.3 - Last Modified : March 24, 2002</address>
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