Variables exposed by a C library can be declared inside a
lib declaration using a global-variable-like declaration:
lib C $errno : Int32 end
Then it can be get and set:
C.errno # => some value C.errno = 0 C.errno # => 0
A variable can be marked as thread local with an attribute:
lib C @[ThreadLocal] $errno : Int32 end
Refer to the type grammar for the notation used in external variables types.
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