va_list help

[John Guo]

Hi all,

I am trying to nest two functions both with variable length arguments.
(The only reason I nested them is that I called the outer function in
too many places, I don't want to replace every calls with the new one).

void outerFunction(const int arg1, const int arg2, ...)
{
int arg3;
va_list ap;
va_start(ap, arg2);
innerFunction(arg3, ap);
va_end(ap);
}
void innerFunction(int arg, ...)
{
va_list ap;
va_start(ap, arg);
char a[255];
strcpy(a, va_arg(ap,char*));
int x = va_arg(ap,int);
cout<<a<<endl;
cout<<x<<endl;
va_end(ap);
}

Say if I call outerfunction with

outFunction(1,2, "ProcessName", 5)

As I debug this, it seems that ap before passed in is ok to read using
va_arg(), but in innerfunction, it doesn't work, read out some garbage
in a and x.

Could someone give some comments on this? Any help is greatly
appreciated.

John

 

[Pete Becker]

void innerFunction(int arg, ...)

Make that:

void innerFunction(int arg, va_list ap)

 

[John Guo]

That's similar to the prototype of vsprintf (i.e., take arguent va_list
ap).

If I do that, can the call to innerfunction directly still work?
e.g. innerFunction(1, 'sldafkdlasf", 2)

 

[Victor Bazarov]

I am trying to nest two functions both with variable length arguments.

That's generally impossible.

(The only reason I nested them is that I called the outer function in
too many places, I don't want to replace every calls with the new
one).

void outerFunction(const int arg1, const int arg2, ...)
{
int arg3;
va_list ap;
va_start(ap, arg2);
innerFunction(arg3, ap);
va_end(ap);
}
void innerFunction(int arg, ...)
{
va_list ap;
va_start(ap, arg);
char a[255];
strcpy(a, va_arg(ap,char*));
int x = va_arg(ap,int);
cout<<a<<endl;
cout<<x<<endl;
va_end(ap);
}

Say if I call outerfunction with

outFunction(1,2, "ProcessName", 5)

As I debug this, it seems that ap before passed in is ok to read using
va_arg(), but in innerfunction, it doesn't work, read out some garbage
in a and x.

Could someone give some comments on this? Any help is greatly
appreciated.

If you need some common code called from both functions, you will need
to defined a function that takes va_args (take a look at vfprintf in
the source for your standard library, or on the web somewhere). Make
both your functions that have ellipsis call that function (like the
'fprintf' does with 'vfprintf')

Good luck!

V

 

[Pete Becker]

That's similar to the prototype of vsprintf (i.e., take arguent va_list
ap).

Yup. Same problem, same solution.

If I do that, can the call to innerfunction directly still work?
e.g. innerFunction(1, 'sldafkdlasf", 2)

No. innerFunction now takes two arguments, one of type int and one of
type va_list.

 

[Pete Becker]

No. innerFunction now takes two arguments, one of type int and one of
type va_list.

I should add: often the best solution to splitting a function is not to
make two functions, but three.