How to construct a string where every char is doubled?

Question

TL;DR:

I am asking you to tell me what would be the most efficient approach to double my strings and print them out?

Full story:

I had trouble with the title, and the actual problem may be a bit different than you expect.

• Imagine I have a main buffer.
• At some index determined by the program, I want to insert a string.
• But every char in that string needs to be doubled.

So "abc", inserted at index 10 of buffer[999], needs to be "aabbcc".

Now, the second part of the problem - this needs to be as efficient as possible. I could make this easily, but I need the fastest option.

I thought I had devised several approaches, but it boils down to:

• fill buffer(1000) with single chars and double the chars when printing (pushing to stdout)
• fill buffer(2000) with double chars and print like normal

The variations to the second approach would be When to double the chars (when copying or generating "aabbcc" from the start and copy the full thing).

The first approach would be the most intuitive, but I fear I would need to devise a low level char-doubling function because putc and printf and any large amount of function calls will have much overhead. (There are allegedly very efficient functions in libc with bitshifting and pointer magic, but I couldn't find them. I can only find the very dissappointing versions where fgets() is just a wrapper for getc() - which can't be efficient.)

The second approach obviously wastes a lot of memory and requires a lot of copying, but it could probably put everything into stdout more efficiently as a chunk without the overhead of copying single chars.

I am unsure if under everything there is just a system write call, and I also lack the knowledge how it works. I am just going with my research that says that fgets is about 12 times faster than fgetc for equal data. And so I assume it is with all the single-char vs line functions.

So in conclusion, I am asking you to tell me what would be the most efficient approach to double my strings and print them out?