void pointers and ffcall library

Question

I'm using the ffcall (specifically the avcall package of ffcall) library to dynamically push parameters to variadic functions. i.e. we have

int blah (char *a, int b, double c, ...);

and we want to call this function with values taken from the user. To do this, we create an avcall version of the function:

int av_blah (char *a, int b, double c, char **values, int num_of_values)
{
    av_alist alist;
    int i, ret;
    av_start_int (alist, &blah, &ret); //let it know which function
    av_ptr (alist, char*, a); // push values onto stack starting from left
    av_int (alist, b);
    av_double (alist, c);
    for (i=0;i<num_of_values;i++)
    {
        // do what you want with values and add to stack
    }
    av_call (alist);  //call blah()

    return (ret);
}

Now, the function I am using avcall with is:

int read_row (struct some_struct *a, struct another_struct *b[], ...);

And it is used like so:

struct some_struct a;
struct another_struct **b = fill_with_stuff ();

char name[64];
int num;
while (read_row (&a, b, name, &num)==0)
{
    printf ("name=%s, num=%d\n", name, num);
}

But I want to use avcall to capture a certain amount of values from this function and I do not know this information in advance. So I thought I'd just create an array of void pointers and then malloc space according to the type:

char printf_string[64]=""; //need to build printf string inside av_read_row()
void **vals = Calloc (n+1, sizeof (void*)); //wrapper
while (av_read_row (&a, b, vals, n, printf_string) == 0)
{
    // vals should now hold the values i want
    av_printf (printf_string, vals, n);  //get nonsense output from this
    // free the mallocs which each vals[i] is pointing to
    void **ptrs = vals;
    while (*ptrs) {
       free (*ptrs);  //seg faults on first free() ?
       *ptrs=NULL;
       ptrs++;
    }
    //reset printf_string
    printf_string[0]='\0';
    printf ("\n");
}

And av_read_row is just:

int av_read_row (struct some_struct *a, struct another_struct *b[], void **vals, int num_of_args, char *printf_string)
{
    int i, ret;
    av_alist alist;

    av_start_int (alist, &read_row, &ret);
    av_ptr (alist, struct some_struct *, a);
    av_ptr (alist, struct another_struct **, b);

    for (i=0;i<num_of_args;i++)
    {
        switch (type)  //for simplicity
        {
          case INT: {
              vals[i] = Malloc (sizeof (int));
              av_ptr (alist, int*, vals[i]);
              strcat (printf_string, "%d, ");
              break;
          }
          case FLOAT: {
               //Same thing
          }
          //etc
        }
    }

    av_call (alist);
    return (ret);
}

I have been experiencing a bunch of memory corruption errors and it seems as though it doesn't like what I'm doing here. I can't spot anything wrong with the way I did this, can you? At the moment, it doesn't like it when I try to free the mallocs inside the av_read_row while loop. Can anyone see what I'm doing wrong, if anything?

Thanks

Answer 1

The only information I can easily find about avcall is from 2001, but it does suggest POSIX. If you can run your stuff on Linux, valgrind will find your memory faults in a jiffy. It's a terrific tool.