1STDIN(3) Linux Programmer's Manual STDIN(3)
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6 stdin, stdout, stderr - standard I/O streams
7
9 #include <stdio.h>
10 extern FILE *stdin;
12 extern FILE *stdout;
13 extern FILE *stderr;
14
16 Under normal circumstances every UNIX program has three streams opened
17 for it when it starts up, one for input, one for output, and one for
18 printing diagnostic or error messages. These are typically attached to
19 the user's terminal (see tty(4)) but might instead refer to files or
20 other devices, depending on what the parent process chose to set up.
21 (See also the "Redirection" section of sh(1).)
22 The input stream is referred to as "standard input"; the output stream
24 is referred to as "standard output"; and the error stream is referred
25 to as "standard error". These terms are abbreviated to form the sym‐
26 bols used to refer to these files, namely stdin, stdout, and stderr.
27 Each of these symbols is a stdio(3) macro of type pointer to FILE, and
29 can be used with functions like fprintf(3) or fread(3).
30 Since FILEs are a buffering wrapper around UNIX file descriptors, the
32 same underlying files may also be accessed using the raw UNIX file
33 interface, that is, the functions like read(2) and lseek(2).
34 On program startup, the integer file descriptors associated with the
36 streams stdin, stdout, and stderr are 0, 1, and 2, respectively. The
37 preprocessor symbols STDIN_FILENO, STDOUT_FILENO, and STDERR_FILENO are
38 defined with these values in <unistd.h>. (Applying freopen(3) to one
39 of these streams can change the file descriptor number associated with
40 the stream.)
41 Note that mixing use of FILEs and raw file descriptors can produce
43 unexpected results and should generally be avoided. (For the masochis‐
44 tic among you: POSIX.1, section 8.2.3, describes in detail how this
45 interaction is supposed to work.) A general rule is that file descrip‐
46 tors are handled in the kernel, while stdio is just a library. This
47 means for example, that after an exec(3), the child inherits all open
48 file descriptors, but all old streams have become inaccessible.
49 Since the symbols stdin, stdout, and stderr are specified to be macros,
51 assigning to them is nonportable. The standard streams can be made to
52 refer to different files with help of the library function freopen(3),
53 specially introduced to make it possible to reassign stdin, stdout, and
54 stderr. The standard streams are closed by a call to exit(3) and by
55 normal program termination.
56
58 The stdin, stdout, and stderr macros conform to C89 and this standard
59 also stipulates that these three streams shall be open at program
60 startup.
61
63 The stream stderr is unbuffered. The stream stdout is line-buffered
64 when it points to a terminal. Partial lines will not appear until
65 fflush(3) or exit(3) is called, or a newline is printed. This can pro‐
66 duce unexpected results, especially with debugging output. The buffer‐
67 ing mode of the standard streams (or any other stream) can be changed
68 using the setbuf(3) or setvbuf(3) call. Note that in case stdin is
69 associated with a terminal, there may also be input buffering in the
70 terminal driver, entirely unrelated to stdio buffering. (Indeed, nor‐
71 mally terminal input is line buffered in the kernel.) This kernel
72 input handling can be modified using calls like tcsetattr(3); see also
73 stty(1), and termios(3).
74
76 csh(1), sh(1), open(2), fopen(3), stdio(3)
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79 This page is part of release 4.16 of the Linux man-pages project. A
80 description of the project, information about reporting bugs, and the
81 latest version of this page, can be found at
82 https://www.kernel.org/doc/man-pages/.
83Linux 2017-09-15 STDIN(3)