CODEWITHSUNDEEP
c
Gem Taylor
Gem Taylor

Reputation: 5635

How to create an args list for vscanf / vprinf directly?

I know how to call vprintf, etc from an implementation of printf, correspondingly.

But what if I want to create an argument list dynamically in code, especially for vsscanf, where I know they are all pointers?

Is there some reasonably portable way that I can convert my dynamically allocated and filled array of pointers (actually a std::vector) into a va_list?

This is all so that I can add a %n argument to the end and detect how much input was actually consumed, in order to provide an answer to ftell in my own FILE-like wrapper.

Other than doing something hairy with a pipe; or writing my own implementation; or splitting the string and doing one argument at a time; I don't see how else I can detect the input stream usage, especially for sscanf. So other ideas to that end might be interesting.

Upvotes: 1

Views: 141

Answers (2)

Gem Taylor
Gem Taylor

Reputation: 5635

X-Y answer: In the end I used fopencookie and vfscanf on that, so I didn't have to process the arguments nor process the format string.

Upvotes: 0

Arhadthedev
Arhadthedev

Reputation: 239

Argument lists, be they fixed or variable sized, are static compile-time entities. So in runtime you can read them, but not create. Use conventional pointers instead.

The sole purpose of va_list is an iteration through arguments because nothing else provides durable access to them. So treat it as a roundabout for a specific feature of a language (a possibility to pass more arguments than specified in a prototype), not intended to be used anywhere else.

Upvotes: 1

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