stdio.texi

一个C源代码分析器

@node I/O on Streams, Low-Level I/O, I/O Overview, Top
@chapter Input/Output on Streams

This chapter describes the functions for creating streams and performing
input and output operations on them.  As discussed in @ref{I/O
Overview}, a stream is a fairly abstract, high-level concept
representing a communications channel to a file, device, or process.

@menu
* Streams::                     About the data type representing a stream.
* Standard Streams::            Streams to the standard input and output 
                                 devices are created for you.
* Opening Streams::             How to create a stream to talk to a file.
* Closing Streams::             Close a stream when you are finished with it.
* Simple Output::               Unformatted output by characters and lines.
* Character Input::             Unformatted input by characters and words.
* Line Input::                  Reading a line or a record from a stream.
* Unreading::                   Peeking ahead/pushing back input just read.
* Formatted Output::            @code{printf} and related functions.
* Customizing Printf::          You can define new conversion specifiers for
                                 @code{printf} and friends.
* Formatted Input::             @code{scanf} and related functions.
* Block Input/Output::          Input and output operations on blocks of data.
* EOF and Errors::              How you can tell if an I/O error happens.
* Binary Streams::              Some systems distinguish between text files
                                 and binary files.
* File Positioning::            About random-access streams.
* Portable Positioning::        Random access on peculiar ANSI C systems.
* Stream Buffering::            How to control buffering of streams.
* Other Kinds of Streams::      Streams that do not necessarily correspond
                                 to an open file. 
@end menu

@node Streams
@section Streams

For historical reasons, the type of the C data structure that represents
a stream is called @code{FILE} rather than ``stream''.  Since most of
the library functions deal with objects of type @code{FILE *}, sometimes
the term @dfn{file pointer} is also used to mean ``stream''.  This leads
to unfortunate confusion over terminology in many books on C.  This
manual, however, is careful to use the terms ``file'' and ``stream''
only in the technical sense.
@cindex file pointer

@pindex stdio.h
The @code{FILE} type is declared in the header file @file{stdio.h}.

@comment stdio.h
@comment ANSI
@deftp {Data Type} FILE
This is the data type used to represent stream objects.  A @code{FILE}
object holds all of the internal state information about the connection
to the associated file, including such things as the file position
indicator and buffering information.  Each stream also has error and
end-of-file status indicators that can be tested with the @code{ferror}
and @code{feof} functions; see @ref{EOF and Errors}.
@end deftp

@code{FILE} objects are allocated and managed internally by the
input/output library functions.  Don't try to create your own objects of
type @code{FILE}; let the library do it.  Your programs should
deal only with pointers to these objects (that is, @code{FILE *} values)
rather than the objects themselves.
@c !!! should say that FILE's have "No user-servicable parts inside."

@node Standard Streams
@section Standard Streams
@cindex standard streams
@cindex streams, standard

When the @code{main} function of your program is invoked, it already has
three predefined streams open and available for use.  These represent
the ``standard'' input and output channels that have been established
for the process.

These streams are declared in the header file @file{stdio.h}.
@pindex stdio.h

@comment stdio.h
@comment ANSI
@deftypevar {FILE *} stdin
The @dfn{standard input} stream, which is the normal source of input for the
program.
@end deftypevar
@cindex standard input stream

@comment stdio.h
@comment ANSI
@deftypevar {FILE *} stdout
The @dfn{standard output} stream, which is used for normal output from
the program.
@end deftypevar
@cindex standard output stream

@comment stdio.h
@comment ANSI
@deftypevar {FILE *} stderr
The @dfn{standard error} stream, which is used for error messages and
diagnostics issued by the program.
@end deftypevar
@cindex standard error stream

In the GNU system, you can specify what files or processes correspond to
these streams using the pipe and redirection facilities provided by the
shell.  (The primitives shells use to implement these facilities are
described in @ref{File System Interface}.)  Most other operating systems
provide similar mechanisms, but the details of how to use them can vary.

In the GNU C library, @code{stdin}, @code{stdout}, and @code{stderr} are
normal variables which you can set just like any others.  For example, to redirect
the standard output to a file, you could do:

@smallexample
fclose (stdout);
stdout = fopen ("standard-output-file", "w");
@end smallexample

Note however, that in other systems @code{stdin}, @code{stdout}, and
@code{stderr} are macros that you cannot assign to in the normal way.
But you can use @code{freopen} to get the effect of closing one and
reopening it.  @xref{Opening Streams}.

@node Opening Streams
@section Opening Streams

@cindex opening a stream
Opening a file with the @code{fopen} function creates a new stream and
establishes a connection between the stream and a file.  This may
involve creating a new file.  

@pindex stdio.h
Everything described in this section is declared in the header file
@file{stdio.h}.

@comment stdio.h
@comment ANSI
@deftypefun {FILE *} fopen (const char *@var{filename}, const char *@var{opentype})
The @code{fopen} function opens a stream for I/O to the file
@var{filename}, and returns a pointer to the stream.

The @var{opentype} argument is a string that controls how the file is
opened and specifies attributes of the resulting stream.  It must begin
with one of the following sequences of characters:

@table @samp
@item r
Open an existing file for reading only.

@item w
Open the file for writing only.  If the file already exists, it is
truncated to zero length.  Otherwise a new file is created.

@item a
Open a file for append access; that is, writing at the end of file only.
If the file already exists, its initial contents are unchanged and
output to the stream is appended to the end of the file.
Otherwise, a new, empty file is created.

@item r+
Open an existing file for both reading and writing.  The initial contents
of the file are unchanged and the initial file position is at the
beginning of the file.

@item w+
Open a file for both reading and writing.  If the file already exists, it
is truncated to zero length.  Otherwise, a new file is created.

@item a+
Open or create file for both reading and appending.  If the file exists,
its initial contents are unchanged.  Otherwise, a new file is created.
The initial file position for reading is at the beginning of the file,
but output is always appended to the end of the file.
@end table

As you can see, @samp{+} requests a stream that can do both input and
output.  The ANSI standard says that when using such a stream, you must
call @code{fflush} (@pxref{Stream Buffering}) or a file positioning
function such as @code{fseek} (@pxref{File Positioning}) when switching
from reading to writing or vice versa.  Otherwise, internal buffers
might not be emptied properly.  The GNU C library does not have this
limitation; you can do arbitrary reading and writing operations on a
stream in whatever order.

The GNU C library defines one additional character for use in
@var{opentype}: the character @samp{x} insists on creating a new
file---if a file @var{filename} already exists, @code{fopen} fails
rather than opening it.  If you use @samp{x} you can are guaranteed that
you will not clobber an existing file.  This is equivalent to the
@code{O_EXCL} option to the @code{open} function (@pxref{Opening and
Closing Files}).

The character @samp{b} in @var{opentype} has a standard meaning; it
requests a binary stream rather than a text stream.  But this makes no
difference in POSIX systems (including the GNU system).  If both
@samp{+} and @samp{b} are specified, they can appear in either order.
@xref{Binary Streams}.

Any other characters in @var{opentype} are simply ignored.  They may be
meaningful in other systems.

If the open fails, @code{fopen} returns a null pointer.
@end deftypefun

You can have multiple streams (or file descriptors) pointing to the same
file open at the same time.  If you do only input, this works
straightforwardly, but you must be careful if any output streams are
included.  @xref{Stream/Descriptor Precautions}.  This is equally true
whether the streams are in one program (not usual) or in several
programs (which can easily happen).  It may be advantageous to use the
file locking facilities to avoid simultaneous access.  @xref{File
Locks}.

@comment stdio.h
@comment ANSI
@deftypevr Macro int FOPEN_MAX
The value of this macro is an integer constant expression that
represents the minimum number of streams that the implementation
guarantees can be open simultaneously.  You might be able to open more
than this many streams, but that is not guaranteed.  The value of this
constant is at least eight, which includes the three standard streams
@code{stdin}, @code{stdout}, and @code{stderr}.  In POSIX.1 systems this
value is determined by the @code{OPEN_MAX} parameter; @pxref{General
Limits}.  In BSD and GNU, it is controlled by the @code{RLIMIT_NOFILE}
resource limit; @pxref{Limits on Resources}.
@end deftypevr

@comment stdio.h
@comment ANSI
@deftypefun {FILE *} freopen (const char *@var{filename}, const char *@var{opentype}, FILE *@var{stream})
This function is like a combination of @code{fclose} and @code{fopen}.
It first closes the stream referred to by @var{stream}, ignoring any
errors that are detected in the process.  (Because errors are ignored,
you should not use @code{freopen} on an output stream if you have
actually done any output using the stream.)  Then the file named by
@var{filename} is opened with mode @var{opentype} as for @code{fopen},
and associated with the same stream object @var{stream}.

If the operation fails, a null pointer is returned; otherwise,
@code{freopen} returns @var{stream}.

@code{freopen} has traditionally been used to connect a standard stream
such as @code{stdin} with a file of your own choice.  This is useful in
programs in which use of a standard stream for certain purposes is
hard-coded.  In the GNU C library, you can simply close the standard
streams and open new ones with @code{fopen}.  But other systems lack
this ability, so using @code{freopen} is more portable.
@end deftypefun


@node Closing Streams
@section Closing Streams

@cindex closing a stream
When a stream is closed with @code{fclose}, the connection between the
stream and the file is cancelled.  After you have closed a stream, you
cannot perform any additional operations on it.

@comment stdio.h
@comment ANSI
@deftypefun int fclose (FILE *@var{stream})
This function causes @var{stream} to be closed and the connection to
the corresponding file to be broken.  Any buffered output is written
and any buffered input is discarded.  The @code{fclose} function returns
a value of @code{0} if the file was closed successfully, and @code{EOF}
if an error was detected. 

It is important to check for errors when you call @code{fclose} to close
an output stream, because real, everyday errors can be detected at this
time.  For example, when @code{fclose} writes the remaining buffered
output, it might get an error because the disk is full.  Even if you
know the buffer is empty, errors can still occur when closing a file if
you are using NFS.

The function @code{fclose} is declared in @file{stdio.h}.
@end deftypefun

If the @code{main} function to your program returns, or if you call the
@code{exit} function (@pxref{Normal Termination}), all open streams are
automatically closed properly.  If your program terminates in any other
manner, such as by calling the @code{abort} function (@pxref{Aborting a
Program}) or from a fatal signal (@pxref{Signal Handling}), open streams
might not be closed properly.  Buffered output might not be flushed and
files may be incomplete.  For more information on buffering of streams,
see @ref{Stream Buffering}.

@node Simple Output
@section Simple Output by Characters or Lines

@cindex writing to a stream, by characters
This section describes functions for performing character- and
line-oriented output.

These functions are declared in the header file @file{stdio.h}.
@pindex stdio.h

@comment stdio.h
@comment ANSI
@deftypefun int fputc (int @var{c}, FILE *@var{stream})
The @code{fputc} function converts the character @var{c} to type
@code{unsigned char}, and writes it to the stream @var{stream}.  
@code{EOF} is returned if a write error occurs; otherwise the
character @var{c} is returned.
@end deftypefun

@comment stdio.h
@comment ANSI
@deftypefun int putc (int @var{c}, FILE *@var{stream})
This is just like @code{fputc}, except that most systems implement it as
a macro, making it faster.  One consequence is that it may evaluate the
@var{stream} argument more than once.  @code{putc} is usually the best
function to use for writing a single character.
@end deftypefun

@comment stdio.h
@comment ANSI
@deftypefun int putchar (int @var{c})
The @code{putchar} function is equivalent to @code{putc} with
@code{stdout} as the value of the @var{stream} argument.
@end deftypefun

@comment stdio.h
@comment ANSI
@deftypefun int fputs (const char *@var{s}, FILE *@var{stream})
The function @code{fputs} writes the string @var{s} to the stream
@var{stream}.  The terminating null character is not written.
This function does @emph{not} add a newline character, either.
It outputs only the chars in the string.

This function returns @code{EOF} if a write error occurs, and otherwise
a non-negative value.

For example:

@smallexample
fputs ("Are ", stdout);
fputs ("you ", stdout);
fputs ("hungry?\n", stdout);
@end smallexample

@noindent
outputs the text @samp{Are you hungry?} followed by a newline.
@end deftypefun