2004-11-03 13:51:07 +00:00
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.\" From dholland@burgundy.eecs.harvard.edu Tue Mar 24 18:08:15 1998
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.\"
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.\" This man page was written in 1998 by David A. Holland
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.\" and placed in the Public Domain. Polished a bit by aeb.
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2005-06-16 16:10:05 +00:00
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.\" 2005-06-16 mtk, mentioned freopen()
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2004-11-03 13:51:07 +00:00
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.\"
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.Dd March 24, 1998
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.Dt STDIN 3
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.Os "Linux 2.0"
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.Sh NAME
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.Nm stdin ,
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.Nm stdout ,
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.Nm stderr
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.Nd standard I/O streams
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.Sh SYNOPSIS
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.Fd #include <stdio.h>
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.Fd extern FILE *stdin;
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.Fd extern FILE *stdout;
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.Fd extern FILE *stderr;
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.Sh DESCRIPTION
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Under normal circumstances every Unix program has three streams opened
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for it when it starts up, one for input, one for output, and one for
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printing diagnostic or error messages. These are typically attached to
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the user's terminal (see
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.Xr tty 4 )
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but might instead refer to files or other devices, depending on what
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the parent process chose to set up. (See also the ``Redirection'' section of
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.Xr sh 1 .)
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.Pp
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The input stream is referred to as ``standard input''; the output stream is
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referred to as ``standard output''; and the error stream is referred to
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as ``standard error''. These terms are abbreviated to form the symbols
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used to refer to these files, namely
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.Nm stdin ,
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.Nm stdout ,
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and
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.Nm stderr .
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.Pp
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Each of these symbols is a
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.Xr stdio 3
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macro of type pointer to FILE, and can be used with functions like
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.Xr fprintf 3
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or
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.Xr fread 3 .
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.Pp
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Since FILEs are a buffering wrapper around Unix file descriptors, the
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same underlying files may also be accessed using the raw Unix file
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interface, that is, the functions like
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.Xr read 2
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and
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.Xr lseek 2 .
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.Pp
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On program startup, the integer file descriptors
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associated with the streams
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2004-11-03 13:51:07 +00:00
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.Nm stdin ,
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.Nm stdout ,
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and
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.Nm stderr
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are 0, 1, and 2, respectively.
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The preprocessor symbols STDIN_FILENO,
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STDOUT_FILENO, and STDERR_FILENO are defined with these values in
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<unistd.h>.
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(Applying
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.Xr freopen 3
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to one of these streams can change the file descriptor number
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associated with the stream.)
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2004-11-03 13:51:07 +00:00
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.Pp
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Note that mixing use of FILEs and raw file descriptors can produce
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unexpected results and should generally be avoided.
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(For the masochistic among you: POSIX.1, section 8.2.3, describes
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in detail how this interaction is supposed to work.)
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A general rule is that file descriptors are handled in the kernel,
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while stdio is just a library. This means for example, that after an
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2006-04-30 21:11:57 +00:00
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.Fn exec ,
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2005-10-20 15:11:10 +00:00
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the child inherits all open file descriptors, but all old streams
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2004-11-03 13:51:07 +00:00
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have become inaccessible.
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.Pp
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Since the symbols
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.Nm stdin ,
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.Nm stdout ,
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and
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.Nm stderr
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are specified to be macros, assigning to them is non-portable.
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The standard streams can be made to refer to different files
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with help of the library function
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.Xr freopen 3 ,
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specially introduced to make it possible to reassign
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.Nm stdin ,
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.Nm stdout ,
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and
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.Nm stderr .
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The standard streams are closed by a call to
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.Xr exit 3
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and by normal program termination.
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.Sh SEE ALSO
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.Xr sh 1 ,
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.Xr csh 1 ,
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.Xr open 2 ,
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.Xr fopen 3 ,
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.Xr stdio 3
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.Sh CONSIDERATIONS
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The stream
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.Nm stderr
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is unbuffered. The stream
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.Nm stdout
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is line-buffered when it points to a terminal. Partial lines will not
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appear until
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.Xr fflush 3
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or
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.Xr exit 3
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is called, or a newline is printed. This can produce unexpected
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results, especially with debugging output.
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The buffering mode of the standard streams (or any other stream)
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can be changed using the
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.Xr setbuf 3
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or
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.Xr setvbuf 3
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call.
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Note that in case
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.Nm stdin
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is associated with a terminal, there may also be input buffering
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in the terminal driver, entirely unrelated to stdio buffering.
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(Indeed, normally terminal input is line buffered in the kernel.)
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This kernel input handling can be modified using calls like
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.Xr tcsetattr 3 ;
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see also
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.Xr stty 1 ,
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and
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.Xr termios 3 .
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.Sh "CONFORMING TO"
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The
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.Nm stdin ,
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.Nm stdout ,
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and
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.Nm stderr
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macros conform to
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.St -ansiC ,
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and this standard also stipulates that these three
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streams shall be open at program startup.
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