A file with associated buffering is called a stream and is declared to be
a pointer to a defined type FILE. The fopen() function shall
create certain descriptive data for a stream and return a pointer to designate
the stream in all further transactions. Normally, there are three open streams
with constant pointers declared in the <stdio.h> header and
associated with the standard open files.
At program start-up, three streams shall be predefined and need not be opened
explicitly: standard input (for reading conventional input),
standard output (for writing conventional output), and standard
error (for writing diagnostic output). When opened, the standard error
stream is not fully buffered; the standard input and standard output streams
are fully buffered if and only if the stream can be determined not to refer to
an interactive device.
The following symbolic values in <unistd.h> define the file
descriptors that shall be associated with the C-language stdin,
stdout, and stderr when the application is started:
Standard input value, stdin. Its value is 0.
Standard output value, stdout. Its value is 1.
Standard error value, stderr. Its value is 2.
The stderr stream is expected to be open for reading and writing.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html