==== perror ====
The perror() function writes str, a ":" followed by a space, an implementation-defined and/or language-dependent error message corresponding to the global variable errno, and a newline to stderr. For example:
If the file called not_found.txt is not found, this code will produce the following output:
Error opening file 'not_found.txt': No such file or directory
If "str" is a null pointer or points to the null byte, only the error message corresponding to errno and a newline are written to stderr.
clearerr - feof - ferror