[Bug 196666] write(2) maximum I/O size too small / inadequately documented in man page

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=196666

            Bug ID: 196666
           Summary: write(2) maximum I/O size too small / inadequately
                    documented in man page
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: emaste at freebsd.org

write(2) claims at most INT_MAX bytes may be written at once, and EINVAL is
returned upon an attempt to exceed this:

     [EINVAL]           The value nbytes is greater than INT_MAX.

The limit depends on the debug.iosize_max_clamp:

joule% sysctl -d debug.iosize_max_clamp
debug.iosize_max_clamp: Clamp max i/o size to INT_MAX

in 11-CURRENT this defaults to 0 so the limit is actually SSIZE_MAX, as POSIX
wants, and the man page is correct

in 10-STABLE it defaults to 1 so the man page is correct but the limit is too
small

See also https://sourceforge.net/p/elftoolchain/tickets/478/ for the sort of
issue probably caused by this

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