Calling a C function with a varargs argument dynamically

Question

I am programming in C against a third party library (in HP/Mercury Loadrunner) that allows a varargs-style variable size argument list for one of it's functions. I want to call this function but I do not know up front how many arguments I will have.

There is a function made by one of my predecessors that serves somewhat but the problem here is that this function assumes the worst case scenario (over 3000 arguments) and hand-codes for that.

To illuminate, here's the (beginning of) the code. The function we call is web_submit_data(). It will HTTP post a set of form data. This implementation came about when dealing with dynamically generated forms with an arbitrary number of fields. (Cleaned up a fair bit from the original, which hand coded indexes by hand as well..)

---

web_submit_data_buffer_gazillion_items( const char *bufferName, const char *bufferValue)
{
    const int size = 129;
    int i = 0;
    int j = 11;

    web_submit_data(&bufferName[i++ * size], //"some form"
    &bufferName[i++ * size], //"Action=https://blah.blah/form");
    &bufferName[i++ * size], //"Method=POST");
    &bufferName[i++ * size], //"TargetFrame=");
    &bufferName[i++ * size], //"RecContentType=text/html");
    &bufferName[i++ * size], //"Referer=https://blah.blah/index.html");
    &bufferName[i++ * size], //"Snapshot=t1.inf");
    &bufferName[i++ * size], //"Mode=HTML");
    ITEMDATA,  // missing in action: indexes 8 through 10
    &bufferName[j * size],&bufferValue[j++ * size], ENDITEM, 
    &bufferName[j * size],&bufferValue[j++ * size], ENDITEM, 
    &bufferName[j * size],&bufferValue[j++ * size], ENDITEM, 
..
(repeat the last 3 lines ad nauseum)
..
    &bufferName[j * size],&bufferValue[j++ * size], ENDITEM,
    &bufferName[j * size]);  
}

Now I have found an external library that might work (http://www.dyncall.org) but I would much rather not a) be completely processor dependant and b) attempt to teach Loadrunner about linking in external sources..

Edit: The original function used hardcoded indexes instead of using a variable. Can still revert to that if it turns out to be too unpredictable. However, as I am unlikely to run this with a different compiler or hardware / OS I doubt that really is worth it.

Also: I don't have control over the implementation of web_submit_data(). So just pushing the problem down one level isn't going to cut it..

Another thing to note: The spec for web_submit_data() uses a constant called LAST to mark the end of the argument list. The original implementation doesn't use it. Presumably the callsite does ..

Answer 1

There is no portable way to build up an argument list for a variable argument function in C at run time. There are a few implementation-dependent tricks out there, the dyncall library you found looks like a good one and probably more portable than most.

Answer 2

Note: the code is already compiler-dependent (though perhaps not processor-dependent), because the invocation of web_submit_data assumes there that the argument subexpressions in a procedure call are evaluated from left-to-right order, but the C language leaves the order of argument evaluation unspecified.

See for reference: http://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_value

So perhaps the non-portable solution is not going to make things significantly worse for you.

Answer 3

Can you restructure your code so that this isn't necessary? Perhaps you could take the incoming buffer and make it more deterministic:

struct form_field
{
  char[FIELD_NAME_MAX] name;
  char[FIELD_VALUE_MAX] val;
};

web_submit_data_buffer_gazillion_items( const char *bufferName, const char *bufferValue)
{
    /*
      loop over bufferName somehow, either with a known size or terminating record,
      and build an array of form_field records
    */
    //loop
    {
      // build array of records
    }


    web_submit_data(record_array, array_len);

}

Sorry this couldn't be more fleshed out - my wife called me in for breakfast. :-)

Answer 4

Write it once with the preprocessor and never look back.

#define WEB_SUBMIT_BUFFER(name, val)         \
    do {                                     \
        const int size = 129;                \
        int i = 0;                           \
        int j = 11;                          \
        web_submit_data(&(name)[i++ * size], \
                        &(name)[i++ * size], \
        /* etc ad nauseum */                 \
    } while (0)

Or if the number of arguments is fixed for each specific call, write a script to generate preprocessor defines to hide how heinous that call is.

#define WEB_SUBMIT_BUFFER_32(name, val)      \
    do {                                     \
        const int size = 129;                \
        int i = 0;                           \
        int j = 11;                          \
        web_submit_data(&(name)[i++ * size], \
                        &(name)[i++ * size], \
        /* 32 times */                       \
    } while (0)
#define WEB_SUBMIT_BUFFER_33(name, val) ...
#define WEB_SUBMIT_BUFFER_34(name, val) /* etc */

Answer 5

Note that the code sample you posted has undefined behavior - the commas that separate function parameters are not sequence points (those commas are not the comma operator), so modifying i and or j multiple times in the function call argument list results in undefined behavior.

This is not to mention that the evaluation order of function call arguments is not specified by the standard - so even if you did the modification of i and j using functions to evaluate the arguments (function calls themselves are sequence points), you would be pretty much passing the pointers in an indeterminate order.

Also, I don't see how web_submit_data() knows how many arguments it's been passed - I don't see a count or a definitive sentinel argument at the end. But I guess your example may be just that - an example that might not have complete, accurate details. On the other hand, it's web_submit_data()'s problem anyway, right?

Answer 6

Since it's generally not a problem to pass more arguments to a function taking variable arguments than the function expects (see footnote #1), you can do something like the following:

// you didn't give a clear specification of what you want/need, so this 
// example may not be quite what you want as I've had to guess at
// some of the specifications. Hopefully the comments will make clear
// what I may have assumed.
//
// NOTE:  while I have compiled this example, I have not tested it,
//        so there is a distinct possiblity of bugs (particularly
//        off-by-one errors). Check me on this stuff, please.

// I made these up so I could compile the example
#define ITEMDATA ((char const*) NULL)
#define ENDITEM  ((char const*) 0xffffffff)

void web_submit_data_wrapper( const char*bufferName, 
                              const char* bufferValue, 
                              size_t headerCount,       // number of header pointers to pass (8 in your example)
                              size_t itemStartIndex,    // index where items start in the buffers (11 in your example)
                              size_t itemCount,         // number of items to pass (unspecified in your example)
                              size_t dataSize )         // size of each header or item (129 in your example)
{
    // kMaxVarArgs would be 3000 or a gazillion in your case

    // size_t const kMaxVarArgs = 20;  // I'd prefer to use this in C++
    #define kMaxVarArgs (20)

    typedef char const* char_ptr_t;
    typedef char_ptr_t char_ptr_array_t[kMaxVarArgs];

    char_ptr_array_t varargs = {0};

    size_t idx = 0;

    // build up the array of pararmeters we'll pass to the variable arg list

    // first the headers
    while (headerCount--) {
        varargs[idx++] = &bufferName[idx * dataSize];
    }

    // mark the end of the header data
    varargs[idx++] = ITEMDATA;

    // now the "items"
    while (itemCount--) {
        varargs[idx++] = &bufferName[itemStartIndex * dataSize];
        varargs[idx++] = &bufferValue[itemStartIndex * dataSize];
        varargs[idx++] = ENDITEM;

        ++itemStartIndex;
    }

    // the thing after the last item 
    // (I'm not sure what this is from your example)
    varargs[idx] = &bufferName[itemStartIndex * dataSize];

    // now call the target function - the fact that we're passing more arguments
    //  than necessary should not matter due to the way VA_ARGS are handled 
    //  but see the Footnote in the SO answer for a disclaimer

    web_submit_data( 
        varargs[0],
        varargs[1],
        varargs[2],

        //... ad nasuem until

        varargs[kMaxVarArgs-1]
        );

}

---

Footnote #1: If you think about how the macros in stdargs.h act this becomes clear. However, I do not claim that this technique would be standards compliant. In fact, in recent history the stackoverflow answers I've posted where I;ve made this disclaimer have in fact been found to be non-standards compliant (usually by the ever vigilant litb). So use this technique at your own risk, and verify, verify, verify).

Answer 7

There are two way to pass a variable number of arguments: to a function that accepts "..." or to a function that accepts va_list.

You can not dynamically define the number of arguments for the "..." interface, but you should be able to do so for the va_list one. Google for va_start, va_end, and va_list.

Answer 8

In CamelBones I use libffi to call objc_msgSend(), which is a varargs function. Works a treat.

Answer 9

Variable length arguments are basically just a pointer to a bunch of packed data that is passed to the required function. It is the responsibility of the called function to interpret this packed data.

The architecture safe way to do this is to use the va_list macros (that n-alexander mentioned), otherwise you may run into issues with how various data types are padded in memory.

The proper way to design varargs functions is to actually have two versions, one that accepts the '...', which in turn extracts the va_list and passes it to a function that takes a va_list. This way you can dynamically construct the arguments if you need to and can instead call the va_list version of the function.

Most standard IO functions have varargs versions: vprintf for printf, vsprintf for sprintf... you get the idea. See if your library implements a function named "vweb_submit_data" or something to that effect. If they don't, email them and tell them to fix their library.

3000 lines of the same thing (even if it is preprocessor induced) makes me cringe