FAQ-1207 Why might code using VA_LIST stop working when I use the RVCT compiler?


FAQ-1207 Why might code using VA_LIST stop working when I use the RVCT compiler?

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Classification:	C++	Category:	Debugging
Created:	01/13/2005	Modified:	01/13/2005
Number:	FAQ-1207
Platform:	Symbian OS v8.1b, Symbian OS v9.0

Question:
I've got code using "..." and VA_LIST, and it's worked fine with Developer Studio, CodeWarrior and GCC. When I compile it using the RealView compiler, the code stops working.

Answer:
The change in behaviour may be due to the change in representation of VA_LIST.

For most of the older compilers, Symbian generically defines VA_LIST to be

typedef TInt8 *VA_LIST[1];

The change in behaviour may be due to the change in representation of VA_LIST.
For newer compilers complying to the ARM Base Platfrom ABI, the definition is:

typedef struct __va_list { void *__ap; } va_list;

These definitions have a different implications for code which passes VA_LIST objects around: the old definition is effectively equivalent to a reference, but the new definition is an object which can be copied and passed by value. This shows up in code like this:

int DoNextArg(char*& aFmt, VA_LIST aList)
{
// code which finds and decodes at the next % thing in aFmt
aFmt = new_ptr;
char* arg = VA_ARG(aList, char*);
if (arg == 0)
return 0; // end of list
// more code ...
return 1;
}

int MyPrintf(const char* aFmt, ...) const
{
VA_LIST list;
VA_START(list, aPath);
char* fmt = aFmt;
int argCount=0;

while (DoNextArg(fmt, list))
{
argCount++;
}

VA_END(list);
return argCount;
}

The above code will work with the GCC compiler, but not with the RVCT compiler. Compiled with the new definition of VA_LIST, the call to DoNextArg will pass a copy of "list", and so the VA_ARG() macro will only update the local copy and the loop will not make any progress.

The fix is simple - change DoNextArg to take a reference to the VA_LIST parameter, and the compiler now understands that you are expecting to update the object as a side-effect.