stdout(3)

1stdin(3)                   Library Functions Manual                   stdin(3)
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NAME

6       stdin, stdout, stderr - standard I/O streams
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <stdio.h>
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14       extern FILE *stdin;
15       extern FILE *stdout;
16       extern FILE *stderr;
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DESCRIPTION

19       Under  normal circumstances every UNIX program has three streams opened
20       for it when it starts up, one for input, one for output,  and  one  for
21       printing diagnostic or error messages.  These are typically attached to
22       the user's terminal (see tty(4)) but might instead refer  to  files  or
23       other  devices,  depending  on what the parent process chose to set up.
24       (See also the "Redirection" section of sh(1).)
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26       The input stream is referred to as "standard input"; the output  stream
27       is  referred  to as "standard output"; and the error stream is referred
28       to as "standard error".  These terms are abbreviated to form  the  sym‐
29       bols used to refer to these files, namely stdin, stdout, and stderr.
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31       Each  of these symbols is a stdio(3) macro of type pointer to FILE, and
32       can be used with functions like fprintf(3) or fread(3).
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34       Since FILEs are a buffering wrapper around UNIX file  descriptors,  the
35       same  underlying files may also be accessed using the raw UNIX file in‐
36       terface, that is, the functions like read(2) and lseek(2).
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38       On program startup, the integer file descriptors  associated  with  the
39       streams  stdin,  stdout, and stderr are 0, 1, and 2, respectively.  The
40       preprocessor symbols STDIN_FILENO, STDOUT_FILENO, and STDERR_FILENO are
41       defined  with  these values in <unistd.h>.  (Applying freopen(3) to one
42       of these streams can change the file descriptor number associated  with
43       the stream.)
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45       Note  that mixing use of FILEs and raw file descriptors can produce un‐
46       expected results and should generally be avoided.  (For the masochistic
47       among  you: POSIX.1, section 8.2.3, describes in detail how this inter‐
48       action is supposed to work.)  A general rule is that  file  descriptors
49       are  handled  in the kernel, while stdio is just a library.  This means
50       for example, that after an exec(3), the child inherits  all  open  file
51       descriptors, but all old streams have become inaccessible.
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53       Since the symbols stdin, stdout, and stderr are specified to be macros,
54       assigning to them is nonportable.  The standard streams can be made  to
55       refer  to different files with help of the library function freopen(3),
56       specially introduced to make it possible to reassign stdin, stdout, and
57       stderr.   The  standard  streams are closed by a call to exit(3) and by
58       normal program termination.
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STANDARDS

61       C11, POSIX.1-2008.
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63       The standards also stipulate that these three streams shall be open  at
64       program startup.
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HISTORY

67       C89, POSIX.1-2001.
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NOTES

70       The  stream  stderr  is unbuffered.  The stream stdout is line-buffered
71       when it points to a terminal.  Partial  lines  will  not  appear  until
72       fflush(3) or exit(3) is called, or a newline is printed.  This can pro‐
73       duce unexpected results, especially with debugging output.  The buffer‐
74       ing  mode  of the standard streams (or any other stream) can be changed
75       using the setbuf(3) or setvbuf(3) call.  Note that in case stdin is as‐
76       sociated with a terminal, there may also be input buffering in the ter‐
77       minal driver, entirely unrelated to stdio buffering.  (Indeed, normally
78       terminal input is line buffered in the kernel.)  This kernel input han‐
79       dling can be modified using calls like tcsetattr(3); see also  stty(1),
80       and termios(3).
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