The standard error stream
#include <stdio.h> FILE * stderr;
This global variable defines the standard error stream. It's set to the console by default, but you can redirect it by calling freopen().
STDERR_FILENO, which is defined in <unistd.h>, defines the file descriptor that corresponds to stderr.
assert(), err(), errx(), getopt(), herror(), perror(), stdin, stdout, strerror(), strerror_r(), verr(), verrx(), vwarn(), vwarnx(), warn(), warnx()