Originální popis anglicky:
stdin,
stdout,
stderr —
standard I/O streams
Návod, kniha: Library Functions Manual
#include
<stdio.h>
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
<unistd.h>.
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 C89”), and this standard also stipulates
that these three streams shall be open at program startup.
