charts: clamp %n length to buffer size

It appears I overlooked that %n returns the length that would have
been printed regardless of the destination buffer size, not what
was actually written.  The man page is misleading here:

 n      The  number  of characters written so far is stored into the
        integer pointed to  by  the  corresponding  argument.   That
        argument  shall  be  an int *, or variant whose size matches
        the (optionally) supplied integer length modifier.  No argu‐
        ment  is converted.  (This specifier is not supported by the
        bionic C library.)  The behavior is undefined if the conver‐
        sion  specification  includes any flags, a field width, or a
        precision.

In testing, it isn't the count of what's actually written.  It's
oblivious of truncated output scenarios where the output buffer has
been exhausted before reaching the %n.  The man page should be
clarified here.

This commit does the simplest thing and simply clamps the length to
the destination buffer - 1 (for the \0).  %n is being used to avoid
needing an strlen() in this somewhat hot path, but it might make
sense to instead use the snprintf return value similarly clamped
instead of %n since %n isn't doing what was expected.

0 files changed, 0 insertions, 0 deletions