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FILE * stderr;
Standard error stream
The standard error stream is the default destination for error messages and other diagnostic warnings. Like stdout, it is usually also directed by default to the text console (generally, on the screen).
stderr can be used as an argument for any function that takes an argument of type FILE* expecting an output stream, like fputs or fprintf.
Although in many cases both stdout and stderr are associated with the same output device (like the console), applications may differentiate between what is sent to stdout and what to stderr for the case that one of them is redirected. For example, it is frequent to redirect the regular output of a console program (stdout) to a file while expecting the error messages to keep appearing in the console.
It is also possible to redirect stderr to some other destination from within a program using the freopen function.
stderr is is never fully buffered on startup. It is library-dependent whether the stream is line buffered or not buffered by default (see setvbuf).