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<HTML><HEAD><TITLE>Files and Streams</TITLE></HEAD><BODY><H1><A NAME="Files and Streams">Files and Streams</A></H1><HR><P><B><A HREF="#Text and Binary Streams">Text and Binary Streams</A>· <A HREF="#Controlling Streams">Controlling Streams</A>· <A HREF="#Stream States">Stream States</A></B></P><HR><P>A program communicates with the target environment by reading and writing<B><A NAME="files">files</A></B> (ordered sequences of bytes). A file canbe, for example, a data set that you can read and write repeatedly(such as a disk file), a stream of bytes generated by a program (suchas a pipeline), or a stream of bytes received from or sent to a peripheraldevice (such as the keyboard or display). The latter two are<B><A NAME="interactive files">interactive files</A></B>.Files are typically the principal means by which to interact with aprogram.</P><P>You manipulate all these kinds of files in much the same way-- by calling library functions. You include the standard header<CODE><stdio.h></CODE> to declare most of these functions.</P><P>Before you can perform many of the operations on a file, thefile must be<B><A NAME="file open">opened</A></B>.Opening a file associates it with a<B><A NAME="stream">stream</A></B>, a data structure withinthe Standard C library that glosses over many differencesamong files of various kinds.The library maintains the state of each stream in an object of type<A HREF="stdio.html#FILE"><CODE>FILE</CODE></A>.</P><P>The target environment opens three files prior to<A HREF="lib_over.html#program startup">program startup</A>.You can open a file by calling the library function<A HREF="stdio.html#fopen"><CODE>fopen</CODE></A> withtwo arguments. The first argument is a<A HREF="lib_over.html#filename">filename</A>, a<A HREF="lib_over.html#multibyte string">multibyte string</A>that the target environment uses to identify which file youwant to read or write. The second argument is a<A HREF="lib_over.html#C string">C string</A> that specifies:</P><UL><LI>whether you intend to read data from the file or write datato it or both</LI><LI>whether you intend to generate new contents for the file (orcreate a file if it did not previously exist) or leave the existingcontents in place</LI><LI>whether writes to a file can alter existing contents or shouldonly append bytes at the end of the file</LI><LI>whether you want to manipulate a<A HREF="#text stream">text stream</A> or a<A HREF="#binary stream">binary stream</A></LI></UL><H2><A NAME="Text and Binary Streams">Text and Binary Streams</A></H2><P>A<B><A NAME="text stream">text stream</A></B> consists of one or more<B><A NAME="text lines">lines</A></B> of textthat can be written to a text-oriented display so that they canbe read. When reading from a text stream, the program reads an<CODE><I>NL</I></CODE> (newline) at the end of each line.When writing to a text stream, the program writes an<CODE><I>NL</I></CODE> to signal the end of a line. To matchdiffering conventions among target environments for representing textin files, the library functions can alter the number and representationsof characters transmitted between the program and a text stream.</P><P>Thus, positioning within a text stream is limited.You can obtain the current<A HREF="#file-position indicator">file-position indicator</A>by calling <CODE><A HREF="stdio.html#fgetpos">fgetpos</A></CODE> or<CODE><A HREF="stdio.html#ftell">ftell</A></CODE>.You can position a text stream at a position obtained this way,or at the beginning or end of the stream, by calling<CODE><A HREF="stdio.html#fsetpos">fsetpos</A></CODE> or<CODE><A HREF="stdio.html#fseek">fseek</A></CODE>.Any other change of position might well be not supported.</P><P>For maximum portability, the program should not write:</P><UL><LI>empty files</LI><LI><CODE><I>space</I></CODE> characters at the end of a line</LI><LI>partial lines (by omitting the <CODE><I>NL</I></CODE>at the end of a file)</LI><LI>characters other than the printable characters,<CODE><I>NL</I></CODE>, and <CODE><I>HT</I></CODE> (horizontal tab)</LI></UL><P>If you follow these rules, the sequence of characters you readfrom a text stream (either as byte or multibyte characters)will match the sequence of characters you wrote to the text streamwhen you created the file. Otherwise, the library functions can removea file you create if the file is empty when you close it. Or theycan alter or delete characters you write to the file.</P><P>A<B><A NAME="binary stream">binary stream</A></B> consists ofone or more bytes of arbitrary information.You can write the value stored in an arbitrary objectto a (byte-oriented) binary stream and read exactly what was storedin the object when you wrote it. The library functions do not alterthe bytes you transmit between the program and a binary stream. Theycan, however, append an arbitrary number of null bytes to the filethat you write with a binary stream. The program must deal with theseadditional null bytes at the end of any binary stream.</P><P>Thus, positioning within a binary stream is well defined,except for positioning relative to the end of the stream.You can obtain and alter the current<A HREF="#file-position indicator">file-position indicator</A>the same as for a <A HREF="#text stream">text stream</A>.Moreover, the offsets used by<CODE><A HREF="stdio.html#ftell">ftell</A></CODE> and<CODE><A HREF="stdio.html#fseek">fseek</A></CODE>count bytes from the beginning of the stream (which is byte zero),so integer arithmetic on these offsets yields predictable results.</P><P>A<B><A NAME="byte stream">byte stream</A></B>treats a file as a sequence of bytes. Within the program,the stream looks like the same sequence of bytes, exceptfor the possible alterations described above.</P><H2><A NAME="Controlling Streams">Controlling Streams</A></H2><P><A HREF="stdio.html#fopen"><CODE>fopen</CODE></A>returns the address of an object of type<A HREF="stdio.html#FILE"><CODE>FILE</CODE></A>.You use this address as the <CODE>stream</CODE> argument to several libraryfunctions to perform various operations on an open file. For a bytestream, all input takes place as if each character is read by calling<A HREF="stdio.html#fgetc"><CODE>fgetc</CODE></A>,and all output takes place as if each character is written by calling<A HREF="stdio.html#fputc"><CODE>fputc</CODE></A>.</P><P>You can<B><A NAME="file close">close</A></B> a file by calling<A HREF="stdio.html#fclose"><CODE>fclose</CODE></A>,after which the address of the<A HREF="stdio.html#FILE"><CODE>FILE</CODE></A> object is invalid.</P><P>A <A HREF="stdio.html#FILE"><CODE>FILE</CODE></A>object stores the state of a stream, including:</P><UL><LI>an <B><A NAME="error indicator">error indicator</A></B> --set nonzero by a function that encounters a read or write error</LI><LI>an <B><A NAME="end-of-file indicator">end-of-file indicator</A></B> --set nonzero by a function thatencounters the end of the file while reading</LI><LI>a <B><A NAME="file-position indicator">file-position indicator</A></B> --specifies the next byte in the stream to read or write,if the file can support positioning requests</LI><LI>a <B><A HREF="#Stream States">stream state</A></B> --specifies whether the stream will accept reads and/or writes</LI><LI>a <B><A NAME="file buffer">file buffer</A></B> --specifies the address and size of an array objectthat library functions can use to improve the performanceof read and write operations to the stream</LI></UL><P>Do not alter any value stored in a<A HREF="stdio.html#FILE"><CODE>FILE</CODE></A> object or ina file buffer that you specify for use with that object.You cannot copy a<A HREF="stdio.html#FILE"><CODE>FILE</CODE></A> objectand portably use the address of the copyas a <CODE>stream</CODE> argument to a library function.</P><H2><A NAME="Stream States">Stream States</A></H2><P>The valid states, and state transitions, for a stream areshown in the diagram.</P><P><IMG SRC="stream.gif"></P><P>Each of the circles denotes a stablestate. Each of the lines denotes a transition that can occur as theresult of a function call that operates on the stream.(The ``wide'' states are never entered in the Embedded C++ library.)Three groups of functions can cause state transitions.All are declared in<A HREF="stdio.html"><CODE><stdio.h></CODE></A>:</P><UL><LI>the <B><A NAME="byte read functions">byte read functions</A></B> --<A HREF="stdio.html#fgetc"><CODE>fgetc</CODE></A>,<A HREF="stdio.html#fgets"><CODE>fgets</CODE></A>,<A HREF="stdio.html#fread"><CODE>fread</CODE></A>,<A HREF="stdio.html#fscanf"><CODE>fscanf</CODE></A>,<A HREF="stdio.html#getc"><CODE>getc</CODE></A>,<A HREF="stdio.html#getchar"><CODE>getchar</CODE></A>,<A HREF="stdio.html#gets"><CODE>gets</CODE></A>,<A HREF="stdio.html#scanf"><CODE>scanf</CODE></A>,and<A HREF="stdio.html#ungetc"><CODE>ungetc</CODE></A></LI><LI>the <B><A NAME="byte write functions">byte write functions</A></B> --<A HREF="stdio.html#fprintf"><CODE>fprintf</CODE></A>,<A HREF="stdio.html#fputc"><CODE>fputc</CODE></A>,<A HREF="stdio.html#fputs"><CODE>fputs</CODE></A>,<A HREF="stdio.html#fwrite"><CODE>fwrite</CODE></A>,<A HREF="stdio.html#printf"><CODE>printf</CODE></A>,<A HREF="stdio.html#putc"><CODE>putc</CODE></A>,<A HREF="stdio.html#putchar"><CODE>putchar</CODE></A>,<A HREF="stdio.html#puts"><CODE>puts</CODE></A>,<A HREF="stdio.html#vfprintf"><CODE>vfprintf</CODE></A>, and<A HREF="stdio.html#vprintf"><CODE>vprintf</CODE></A></LI><LI>the <B><A NAME="position functions">position functions</A></B> --<A HREF="stdio.html#fflush"><CODE>fflush</CODE></A>,<A HREF="stdio.html#fseek"><CODE>fseek</CODE></A>,<A HREF="stdio.html#fsetpos"><CODE>fsetpos</CODE></A>, and<A HREF="stdio.html#rewind"><CODE>rewind</CODE></A></LI></UL><P>The state diagram shows that you must call one of the positionfunctions between most write and read operations:</P><UL><LI>You cannot call a read function if the last operation on thestream was a write.</LI><LI>You cannot call a write function if the last operation on thestream was a read, unless that read operation set the<A HREF="#end-of-file indicator">end-of-file indicator</A>.</LI></UL><P>Finally, the state diagram shows that a position operation never<I>decreases</I> the number of valid function calls that can follow.</P><HR><P>See also the<B><A HREF="index.html#Table of Contents">Table of Contents</A></B> and the<B><A HREF="_index.html">Index</A></B>.</P><P><I><A HREF="crit_pb.html">Copyright</A> © 1989-2002by P.J. Plauger and Jim Brodie. All rights reserved.</I></P><!--V4.01:1125--></BODY></HTML>⌨️ 快捷键说明
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