The construct is a POSIX invention, but it's just so useful, let's also
define it in EFI mode, so that we can use similar constructs in EFI mode
and userspace.
Also, make sure the NUL byte iovec becomes an exported constant too.
This is better than the previous situation where this was a macro
resolving to a compount expression, since the lifetime of the expression
is limited to its invoking scope. By turning this into a proper variable
the lifetime becomes unbounded, which makes it easier to use in various
scenarios, such as "if" blocks.
This avoids the ({}) that IOVEC_MAKE_STRING() so far used and might
cause a memory corruption if the parameter passed in is itself allocated
via a compount initialized array or so.
Also, this makes sure both IOVEC_MAKE_STRING() and IOVEC_MAKE() accept
'const' parameters without this causing a compiler warning.
Even more than with the previous commit, this is not a trivial function
and there's no reason to believe this will actually be inlined nor that
it would be beneficial.
The function isn't necessarily fast (it's O(n)), and there's no reason
to have it defined as inline function, since it's neither fast, nor
entirely trivial.
This is preparation for #28891, which adds a bunch more helpers around
"struct iovec", at which point this really deserves its own .c/.h file.
The idea is that we sooner or later can consider "struct iovec" as an
entirely generic mechanism to reference some binary blob, and is the
go-to type for this purpose whenever we need one.
Mike Yuan
Lennart Poettering
Zbigniew Jędrzejewski-Szmek