http://kor.i2p/books/The%20C%20Programming%20Language%20by%20K&R/chapter7.html
When a C program is started, the operating system environment is responsible
for opening three files and providing pointers for them. These files are the
standard input, the standard output, and the standard error; the
corresponding file pointers are called stdin , stdout , and stderr , and are declared in < stdio.h > .