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stdin, stdout, stderr - standard I/O streams
extern FILE *stdin;
extern FILE *stdout;
extern FILE *stderr;
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). 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
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, 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 non-portable. 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.
sh(1), csh(1), open(2), fopen(3), stdio(3)
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).
The stdin, stdout, and stderr macros conform to ANSI X3.159-1989
(“ANSI C”), and this standard also stipulates that these three streams
shall be open at program startup.