CODEWITHSUNDEEP
cvariadic-functions
Cyker
Cyker

Reputation: 10894

wrapper for a function with variable arguments in C

Recently I wanted to implement a printf wrapper. After some search I found the vprintf is good for this need:

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

But is it possible to implement such a wrapper for printf or any other similar functions with variable arguments instead of va_list?

(I mean, what if they don't provide a v version?)

Since some commenter didn't fully capture my idea, I'd better elaborate it.

Suppose you have a plain printf function as in the C library.

Some one gives you a fmt string "%d %u %f", and corresponding inputs.

Now you want to write a function similar to printf, but with all %f replaced by %.2f.

Of course you can use two statements to finish the task:

replace(fmt, "%f", "%.2f");
printf(fmt, inputs);

But if you use this function many times, probably you want to have a wrapper to save some time.

A macro can finish this task, of course. But is it possible without a macro, like:

void myprintf(fmt, ...)
{
    replace(fmt, "%f", "%.2f");
    printf(fmt, inputs);
}

The problem here is that you don't know how to feed the inner printf with the arguments ... of myprintf.

Hope this clarifies.

Upvotes: 2

Views: 5365

Answers (3)

AvirukBasak
AvirukBasak

Reputation: 148

If the variadic function was created by you and you need the wrapper to be a function as well (as in my case; I needed a pointer to the wrapper), then you can move the implementation of that function into a macro that takes (fmt, args) where args is va_list.

You can then use this macro to implement wrapper functions.

Upvotes: 0

Jens Gustedt
Jens Gustedt

Reputation: 78903

If you just want to use this to prepend an string to the output or so, you could use a variadic macro.

#define MYPRINT(...) printf("toto has: " __VA_ARGS__)

in that simple example this suppposes that format that you pass in is a string literal, so this is a bit restricted. But I hope you see the pattern how to use such simple macro wrappers.

Upvotes: 3

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

There is no portable way to construct a call to a variadic function when the API don't offer you functions with va_list parameter.

However, you could [ab]use the libffi for that purpose, by constructing the variadic call and doing it.

Upvotes: 2

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