stderr

The standard error stream is the default destination for error messages and other diagnostic warnings. Like stdout, it is usually also directed to the output device of the standard console (generally, the screen).

stderr can be used as an argument for any function that expects an output stream as one of its parameters, like fputs or fprintf.

Although generally both stdout and stderr are associated with the same console output, 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 screen.

It is also possible to redirect stderr to some other destination from within a program using the freopen function.