vsprintf or printf variable arguments

[Patrick Schluter via Digitalmars-d-learn]

On Friday, 5 August 2016 at 08:32:42 UTC, kink wrote:
> On Thursday, 4 August 2016 at 21:03:52 UTC, Mark "J" Twain 
> wrote:
>> How can I construct a va_list for vsprintf when all I have is 
>> the a list of pointers to the data, without their type info?
>>
>> A va_list seems to be a packed struct of values and/or 
>> pointers to the data. While I could construct such a list, 
>> theoretically, I don't always know when I should store an 
>> element as a pointer or a value.
>>
>> e.g., ints, floats, and other value types seems to get stored 
>> directly in the list, while strings and *other* stuff get 
>> stored as pointers.
>>
>> I would be nice if a printf variant listed takes only a 
>> pointer list to all the values, does anything like this exist?
>>
>> If not, is there any standardization of what is a value in the 
>> list and what is a pointer so I can attempt to build the list 
>> properly?
>
> This has absolutely nothing to do with D as these are C 
> functions, so you'd be better off asking this in another forum. 
> Anyway, I have no idea why you'd want to construct the va_list 
> manually. These vprintf() functions only exist so that other 
> variadic functions can forward THEIR varargs - e.g.,
>
> extern(C) void myOldschoolPrintf(in char* format, ...)
> {
>   doSomethingSpecial();
>   va_list myVarargs = va_start(format);
>   vprintf(format, myVarargs);
> }

Just a side note, a C nitpicker that I am, you forgot the va_end 
after the call to vprintf(). On a lot of platforms it's not a 
problem, on Linux/x86_64 (i.e. a very common platform) it's very 
likely a memory leak (as the ABI allows to pass parameters in 
register even for vararg functions, these registers have to be 
saved somewhere to be usable in a va_list).
>
> Note that va_list is highly platform-dependent, so filling the 
> struct manually is a very bad idea.

Yes indeed, and one of the most common platforms is the very 
complex one. It's probably the most common issue when porting C 
programs to Linux/x86_64 for the first time.