Is there any way to determine how many characters will be written by sprintf?

Question

I'm working in C++.

I want to write a potentially very long formatted string using sprintf (specifically a secure counted version like _snprintf_s, but the idea is the same). The approximate length is unknown at compile time so I'll have to use some dynamically allocated memory rather than relying on a big static buffer. Is there any way to determine how many characters will be needed for a particular sprintf call so I can always be sure I've got a big enough buffer?

My fallback is I'll just take the length of the format string, double it, and try that. If it works, great, if it doesn't I'll just double the size of the buffer and try again. Repeat until it fits. Not exactly the cleverest solution.

It looks like C99 supports passing NULL to snprintf to get the length. I suppose I could create a module to wrap that functionality if nothing else, but I'm not crazy about that idea.

Maybe an fprintf to "/dev/null"/"nul" might work instead? Any other ideas?

EDIT: Alternatively, is there any way to "chunk" the sprintf so it picks up mid-write? If that's possible it could fill the buffer, process it, then start refilling from where it left off.

Answer 1

The man page for snprintf says:

   Return value
       Upon  successful  return,  these  functions return the number of
       characters printed (not including the trailing '\0' used to  end
       output to strings).  The functions snprintf and vsnprintf do not
       write more than size bytes (including the  trailing  '\0').   If
       the output was truncated due to this limit then the return value
       is the number of characters (not including  the  trailing  '\0')
       which  would  have  been  written  to the final string if enough
       space had been available. Thus, a return value of size  or  more
       means  that  the  output  was  truncated.  (See also below under
       NOTES.)  If an output error is encountered, a negative value  is
       returned.

What this means is that you can call snprintf with a size of 0. Nothing will get written, and the return value will tell you how much space you need to allocate to your string:

int how_much_space = snprintf(NULL, 0, fmt_string, param0, param1, ...);