snprintf() always writes a null terminator as well, so I think you can safely pass the full buffer size when calling it (no -1).

Here's an example showcasing the issue:

#include <Rcpp.h>using namespace Rcpp;// [[Rcpp::export]]std::string test() {    return Rcpp::internal::coerce_to_string<RAWSXP>(0xaa);}/*** Rtest()*/

Running with this PR, you'll see:

> test()[1] "a"

Given that this change doesn't cause any failures I guess this isn't a commonly-exercised code path.

Whoops. Scrambled egg on my face.

snprintf() always writes a null terminator as well, so I think you can safely pass the full buffer size when calling it (no -1).

I was a little fuzzy / unaware on this one butdo recall seeingmany use of -1 in the wild. Including in the same source file a few lines above:

but then not here (so I changed it in the PR -- the link shown is from the master branch)

What is a good rule about when 'n == n' is safe and when it is not?

Rebased and updated. Let me know what you think@kevinushey

What is a good rule about when 'n == n' is safe and when it is not?

Good question... it's complicated and comes down to whether the format / print routine writes a null terminator for you or not. Sincesnprintf() always writes a null terminator, it's safe to usen == n where the buffer includes space for the null terminator, but for other formatting functions the documentation needs to be checked to be sure.

Another wrinkle forsnprintf() is that, if you're using it to figure out the buffer size when printing, you need to be careful. Fromhttps://en.cppreference.com/w/cpp/io/c/fprintf:

4. Writes the results to a character string buffer. At most buf_size - 1 characters are written. The resulting character string will be terminated with a null character, unless buf_size is zero. If buf_size is zero, nothing is written and buffer may be a null pointer, however the return value (number of bytes that would be written not including the null terminator) is still calculated and returned.

So you might write code like:

// figure out size of formatted string// NOTE: does _not_ include null terminator!int n = snprintf(nullptr, 0, "%s\n", "code");// create a buffer, adding 1 for the null terminatorstd::vector<char> buffer(n + 1);// format string into buffer, again adding 1 for terminatorsnprintf(&buffer[0], n + 1, "%s\n", "code");

tl;dr: it's unfortunately complicated and depends on whether the formatter writes a null terminator for you, and in what conditions it writes the null terminator.

This is also why I am so happy to havefmt.

Thanks for the comment above. Reading it I realize I once knew only n-1 may be copied to leave room. So yes, 'it depends'.

Anyway, we made a nice improvement and you rightly took me to school as there shouldn't be a PR without a test under it. All good.