mirror of https://github.com/mkerrisk/man-pages
152 lines
3.9 KiB
Groff
152 lines
3.9 KiB
Groff
.\" 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 mtk, mentioned freopen()
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.\"
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.\" 2007-12-08, mtk, Converted from mdoc to man macros
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.\"
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.TH STDIN 3 2007-12-28 "Linux" "Linux Programmer's Manual"
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.SH NAME
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stdin, stdout, stderr \- standard I/O streams
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.SH SYNOPSIS
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.nf
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.B #include <stdio.h>
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.BI "extern FILE *" stdin ;
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.BI "extern FILE *" stdout ;
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.BI "extern FILE *" stderr ;
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.fi
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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.
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These are typically attached to
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the user's terminal (see
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.BR 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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.BR 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".
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These terms are abbreviated to form the symbols
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used to refer to these files, namely
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.IR stdin ,
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.IR stdout ,
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and
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.IR stderr .
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Each of these symbols is a
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.BR stdio (3)
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macro of type pointer to
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.IR FILE ,
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and can be used with functions like
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.BR fprintf (3)
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or
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.BR fread (3).
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.PP
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Since
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.IR FILE s
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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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.BR read (2)
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and
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.BR 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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.IR stdin ,
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.IR stdout ,
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and
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.I stderr
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are 0, 1, and 2, respectively.
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The preprocessor symbols
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.BR STDIN_FILENO ,
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.BR STDOUT_FILENO ,
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and
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.B STDERR_FILENO
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are defined with these values in
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\fI<unistd.h>\fP.
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(Applying
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.BR 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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.PP
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Note that mixing use of
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.IR FILE s
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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.
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This means for example, that after an
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.BR exec (3),
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the child inherits all open file descriptors, but all old streams
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have become inaccessible.
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.PP
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Since the symbols
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.IR stdin ,
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.IR stdout ,
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and
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.I 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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.BR freopen (3),
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specially introduced to make it possible to reassign
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.IR stdin ,
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.IR stdout ,
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and
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.IR stderr .
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The standard streams are closed by a call to
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.BR exit (3)
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and by normal program termination.
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.SH CONSIDERATIONS
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The stream
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stderr is unbuffered.
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The stream
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stdout is line-buffered when it points to a terminal.
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Partial lines will not
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appear until
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.BR fflush (3)
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or
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.BR exit (3)
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is called, or a newline is printed.
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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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.BR setbuf (3)
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or
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.BR setvbuf (3)
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call.
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Note that in case
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stdin 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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.BR tcsetattr (3);
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see also
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.BR stty (1),
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and
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.BR termios (3).
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.SH "CONFORMING TO"
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The
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.IR stdin ,
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.IR stdout ,
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and
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.I stderr
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macros conform to C89
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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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.SH SEE ALSO
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.BR sh (1),
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.BR csh (1),
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.BR open (2),
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.BR fopen (3),
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.BR stdio (3)
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