5.3. Standard Input, Standard Output, and Standard Error

Three streams are predefined and automatically available to a process: standard input, standard output, and standard error. These streams refer to the same files as the file descriptors STDIN_FILENO, STDOUT_FILENO, and STDERR_FILENO, which we mentioned in Section 3.2.

These three standard I/O streams are referenced through the predefined file pointers stdin, stdout, and stderr. The file pointers are defined in the <stdio.h> header.