стандартные потоки ввода / вывода (standard I/O streams)
Имя (Name)
stdin, stdout, stderr - standard I/O streams
Синопсис (Synopsis)
#include <stdio.h>
extern FILE *stdin;
extern FILE *stdout;
extern FILE *stderr;
Описание (Description)
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).
On program startup, 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>. (Applying freopen(3) to one of these streams can
change the file descriptor number associated with the stream.)
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(3), 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 nonportable. 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.
Стандарты (Conforming to)
The stdin, stdout, and stderr macros conform to C89 and this
standard also stipulates that these three streams shall be open
at program startup.
Примечание (Note)
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).
Смотри также (See also)
csh(1), sh(1), open(2), fopen(3), stdio(3)
