DEC C defines three file pointers that allow you to perform I/O to and from the logical devices usually associated with your terminal (for interactive jobs) or a batch stream (for batch jobs). In the OpenVMS environment, the three permanent process files SYS$INPUT, SYS$OUTPUT, and SYS$ERROR perform the same functions for both interactive and batch jobs. Terminal I/O refers to both terminal and batch stream I/O. The file pointers stdin, stdout, and stderr are defined when you include the <stdio.h> header file using the #include preprocessor directive.
The stdin file pointer is associated with the terminal to perform input. This file is equivalent to SYS$INPUT. The stdout file pointer is associated with the terminal to perform output. This file is equivalent to SYS$OUTPUT. The stderr file pointer is associated with the terminal to report run-time errors. This file is equivalent to SYS$ERROR.
There are three file descriptors that refer to the terminal. The file descriptor 0 is equivalent to SYS$INPUT, 1 is equivalent to SYS$OUTPUT, and 2 is equivalent to SYS$ERROR.
When performing I/O at the terminal, you can use Standard I/O functions and macros (specifying the pointers stdin, stdout, or stderr as arguments), you can use UNIX I/O functions (giving the corresponding file descriptor as an argument), or you can use the Terminal I/O functions and macros. There is no functional advantage to using one type of I/O over another; the Terminal I/O functions might save keystrokes since there are no arguments.