How to pass function pointers with call stack?

Question

I'm not sure I have the best title for this question, feel free to improve.

If I have

typedef void (*VoidFunction)(void);

And then a family of functions that fit this type, I could write a sort of "transaction" wrapper function that looked something like:

void doTransaction(VoidFunction function)
{
    doSomePreambleWork();
    function();
    doSomePostambleWork();
}

If I had a family of functions that took single int arguments, I could wash-rinse-repeat:

typedef void (*VoidOneIntFunction)(int a);
void doTransactionOneInt(VoidFunctionOneInt function, int a)
{
    doSomePreambleWork();
    function(a);
    doSomePostambleWork();
}

Leaving the issue of return types (iow, assuming a void return type), is it possible to genericize this pattern, so that I only have to do write one wrapper function, something like:

// ????? I'm not sure how i'd type the passed function
void doTransactionGeneric(void * function, ...)
{
    doSomePreambleWork();
    function(); // ????? and i don't know how i'd go about calling it...
    doSomePostambleWork();
}

Take a look at the <stdarg.h> header. The solution is to use a variable argument list. — Will
– Will, Commented Apr 24, 2013 at 17:28
I was hoping for something more "meta" than these answers. Maybe it's just not possible with C. I thought I could mess with the stack somehow, to invoke the first argument of the call stack (known to be a function) with the remainder of the call stack magically somehow. :( — Travis Griggs
– Travis Griggs, Commented Apr 25, 2013 at 6:40

Remy Lebeau · Accepted Answer · 2013-04-24 17:36:37Z

2

If you want to use the ... syntax, known as variadic parameters, then you use va_list to access the parameter list, eg:

#include <stdarg.h>

typedef void (*VoidArgsFunction)(va_list args);

void doTransactionGeneric(VoidArgsFunction function, ...)
{
    doSomePreambleWork();

    va_list args;
    va_start(args, function);
    function(args);
    va_end(args);

    doSomePostambleWork();
}

Inside of the called function, it can use va_arg() to access the individual parameter values from the va_list as needed, eg:

#include <stdarg.h>

void someFunction(va_list args)
{
    int param1 = va_arg(args, int);
    char *param2 = va_arg(args, char*);
    ...
}

doTransactionGeneric(&somefunction, 12345, "Hello World");

answered Apr 24, 2013 at 17:36

Remy Lebeau

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Comments

user529758 · Accepted Answer · 2013-04-24 17:27:47Z

0

I'm not sure how i'd type the passed function

Just like you would declare a function pointer anywhere else:

void doTransactionGeneric(void (*function)(void), ...)

and I don't know how I'd go about calling it...

Just like you already did:

function();

answered Apr 24, 2013 at 17:27

user529758

1 Comment

Remy Lebeau Over a year ago

If you declare function like that, you would not be able to pass variadic parameters to doTransactionGeneric() itself and pass them along to the specified function.

Valeri Atamaniouk · Accepted Answer · 2013-04-24 17:43:40Z

Here is a suggestion.

Assuming you have several functions (like in your question):

typedef void (*VoidFunction)(void);
typedef void (*VoidFunctionOneInt)(int);
// we can add more

Let's define type enumeration (we don't have to use the enumeration, but this is more illustrative):

typedef enum
{
   e_fn_void,     // void return, no args
   e_fn_void_int, // void return, one int
                  // we can add more...
} void_function_kind_t;

Then, lets say, we have some generic processor. We need to supply type of the function, function pointer, and arguments:

void some_generic_function(void_function_kind_t kind, void *fn, ...)
{
   va_list args;
   va_start(args, fn);

   // here - make a call
   switch (kind)
   {
   case e_fn_void:
        // no arguments
        ((VoidFunction)fn)();
        break;
   case e_fn_void_int:
        {
           int arg0 = va_arg(args,int);
           ((VoidFunctionOneInt)fn)(arg0);
        }
        break;
   default: // error - unsupported function type
        break;
   }

   va_end(args);
}

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