stderr as a File.
Note: The returned File wraps core.stdc.stdio.stderr, and is therefore thread global. Reassigning stderr to a different File must be done in a single-threaded or locked context in order to avoid race conditions.
All writing to stderr automatically locks the file globally, and will cause all other threads calling write to wait until the lock is released.
void main() { stderr.writeln("Write a message to stderr."); }
The standard error stream.