Ubuntu Manpage: stdin, stdout, stderr

NAME

       stdin, stdout, stderr - standard I/O streams

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <stdio.h>

       extern FILE *stdin;
       extern FILE *stdout;
       extern FILE *stderr;

DESCRIPTION

Under normal circumstances every UNIX program has three streams opened for it when it starts up, one for
input, one for output, and one for printing diagnostic or error messages. These are typically attached
to the user's terminal (see tty(4)) but might instead refer to files or other devices, depending on what
the parent process chose to set up. (See also the "Redirection" section of sh(1).)

The input stream is referred to as "standard input"; the output stream is referred to as "standard
output"; and the error stream is referred to as "standard error". These terms are abbreviated to form
the symbols used to refer to these files, namely stdin, stdout, and stderr.

Each of these symbols is a stdio(3) macro of type pointer to FILE, and can be used with functions like
fprintf(3) or fread(3).

Since FILEs are a buffering wrapper around UNIX file descriptors, the same underlying files may also be
accessed using the raw UNIX file interface, that is, the functions like read(2) and lseek(2).

On program startup, the integer file descriptors associated with the streams stdin, stdout, and stderr
are 0, 1, and 2, respectively. The preprocessor symbols STDIN_FILENO, STDOUT_FILENO, and STDERR_FILENO
are defined with these values in <unistd.h>. (Applying freopen(3) to one of these streams can change the
file descriptor number associated with the stream.)

Note that mixing use of FILEs and raw file descriptors can produce unexpected results and should
generally be avoided. (For the masochistic among you: POSIX.1, section 8.2.3, describes in detail how
this interaction is supposed to work.) A general rule is that file descriptors are handled in the
kernel, while stdio is just a library. This means for example, that after an exec(3), the child inherits
all open file descriptors, but all old streams have become inaccessible.

Since the symbols stdin, stdout, and stderr are specified to be macros, assigning to them is nonportable.
The standard streams can be made to refer to different files with help of the library function
freopen(3), specially introduced to make it possible to reassign stdin, stdout, and stderr. The standard
streams are closed by a call to exit(3) and by normal program termination.

STANDARDS

       C11, POSIX.1-2008.

       The standards also stipulate that these three streams shall be open at program startup.

HISTORY

       C89, POSIX.1-2001.

NOTES

       The stream stderr is unbuffered.  The stream stdout is  line-buffered  when  it  points  to  a  terminal.
       Partial  lines  will  not appear until fflush(3) or exit(3) is called, or a newline is printed.  This can
       produce unexpected results, especially with debugging output.  The buffering mode of the standard streams
       (or any other stream) can be changed using the setbuf(3) or setvbuf(3) call.  Note that in case stdin  is
       associated  with a terminal, there may also be input buffering in the terminal driver, entirely unrelated
       to stdio buffering.  (Indeed, normally terminal input is line buffered in the kernel.)  This kernel input
       handling can be modified using calls like tcsetattr(3); see also stty(1), and termios(3).