> I am writing a library of functions to include in my programs
 > with similar tasks to those in CONIO.H but using direct video
 > output.

Like cprintf()?

 > I have struck problems in trying to write a version of printf().
 > My idea was to write a function called dprintf() which
 > accepted the arguments, used sprintf() to format them and store
 > in a buffer and then output the resulting string to the screen.
 > I don't know how the variable argument system works that printf()
 > uses so I am unable to pass the values on to sprintf().

There's a generic interface called v?printf() which will probably help.

To provide a brief example:

#include 
#include 

int dprintf(char const * fmt, ...)
{
    int rc;
    va_list argp;
    char buf[1024];

    va_start(argp, fmt);
    rc = vsprintf(buf, fmt, argp);
    va_end(argp);
    output(buf);            // Output the string
    return rc;
}

ISO will soon to introducing a new interface to the v?printf() family
called v?nprintf(), where you can specify the destination buffer size and
therefore avoid buffer overflow - something that you can only guard against
currently by defining large buffers. No compilers that I am currently aware
of include it (yet).


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