How to use va_list correctly in a sequence of wrapper functions calls?

Question

I have an ethernet library for microcontroller (keil rl-tcpnet for lpc2478). Library uses va_list (standard pointer to arg array macro defined in stdarg.h) in debug output function in such a way:

void __debug__ (const char *fmt, ...)
{
  va_list args;
  va_start (args,fmt);
  vprintf (fmt,args);
  va_end (args);
}

However vprintf sends data to incorrect stream and I need to redirect debug output of library to correct serial port. Library has .c cofigure files and I write c++ code so I use wrapper function pointers to call c++ style code from c code. So I rewrite this function:

extern void (* printDebugMsg)(const char * fmt, ...);    
void __debug__ (const char *fmt, ...)
{
  va_list args;
  printDebugMsg(fmt, args);
}

other cpp file:

void printDebugMsgImplementation(const char * fmt, ...)
{
  va_list args;

  char buffer[100] = { 0 };

  sprintf(buffer, fmt, args);

  debugUart->write(buffer);
}
void (* printDebugMsg)(const char * fmt, ...) = &printDebugMsgImplementation;

The result debug output looks like:

TCP: Init 0 Sockets
IP :  Src. IP : 0.0.14.1668436768
ETH:  Dest.MAC: A1E05E10:A0002B44:5F9F:01:5FF8:33

The text and some numbers (like 0) seems to be correct but most other ones seems to be wrongly formatted (however it can be internal library problem also). I tried to rewrite my code using va_list as it is usually made in examples:

void __debug__ (const char *fmt, ...)
{
  va_list args;
  va_start (args,fmt);
  printDebugMsg(fmt, args);
  va_end (args);
}

and .cpp file:

void printDebugMsgImplementation(const char * fmt, ...)
{
  va_list args;

  char buffer[100] = { 0 };

  va_start(args, fmt);
  vsprintf(buffer, fmt, args);
  va_end(args);

  debugUart->write(buffer);
}

However the result is definetely worse that it was:

TCP: Init -1579131344 Sockets
ARP:  Dest.IP: -1579131400.-1610601474.24805.4
ARP:  Src.MAC: A1E05DF8:A0002BFE:60FF:04:616C:20

What I'm doing wrong? And how to deal with va_list in my case of wrapper functions calls?

Answer 1

You can't pass va_list to a function expecting ..., unless that function (of course) expects a va_list in the variable-arguments part. Your code clearly does not.

You need to explicitly accept a va_list argument into printDebugMsg(), since that's what you're passing. See for instance vprintf() which solves this problem for the printf() family.