> I have size_with_nul because snprintf man pages say

> The functions snprintf() and vsnprintf() write at most size bytes (including the terminating null byte (‘\0’)) to str.

If 'size' includes the null byte, why do we have to add 1?

Yes, it's a GNU extension, but it's also supported by various BSDs [1][2][3], and yes, Musl has it too. It's present in pretty much any sane C library.

[1] https://man.openbsd.org/man3/printf.3

[2] https://man.netbsd.org/vasprintf.3

[3] https://man.freebsd.org/cgi/man.cgi?query=vasprintf&sektion=...

https://github.com/RhysU/snprintf_realloc/blob/master/snprin...

Worth critically reviewing before using. It's been a while.