C file input/output: Difference between revisions

Revision as of 09:13, 1 February 2007

Revision as of 09:26, 1 February 2007

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In [[C (programming language)|C programming]], [[File system|file]] [[input/output|input and output]] is controlledinitiated and terminated by the the <code>'''fopen'''</code> and <code>'''fclose'''</code> [[Subroutine|functions]], respectively, which are defined in the [[stdio.h]] [[C standard library]]. Typically, they are used in a sequence:

#A file is opened for reading/writing/appending, using <code>fopen</code>;

#The file is processed;

#The file is closed, using <code>fclose</code>.

==Opening a file using <tt>fopen</tt>==

A file is opened using <code>'''fopen'''</code>, which returns an I/O [[stream]] attached to the specified file or other device from which reading and writing can be done. If the function fails, it returns 0. Because the functionality is so useful, many languages derived from C provide functions of the same name, with the same or similar function: for example, [[PHP]]. <code>fopen</code> is considered higher-level than the <code>open</code> [[system call]] of UNIX operating systems. The related C library function '''<code>freopen</code>''' performs the same operation after first closing any open stream associated with its parameter.

They are defined as

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!colspan=3| mode || description || starts..

|-

|<tt>r</tt> || <tt>rb</tt> || || open for reading || beginning

|-

|<tt>w</tt> || <tt>wb</tt> || || open for writing (creates file if it doesn't exist). Deletes content and overwrites the file. || beginning

|-

|<tt>a</tt> || <tt>ab</tt> || || open for appending (creates file if it doesn't exist) || end

|-

|<tt>r+</tt> || <tt>rb+</tt> || <tt>r+b</tt> || open for reading and writing || beginning

|-

|<tt>w+</tt> || <tt>wb+</tt> || <tt>w+b</tt> || open for reading and writing. Deletes content and overwrites the file. || beginning

|-

|<tt>a+</tt> || <tt>ab+</tt> || <tt>a+b</tt> || open for reading and writing (append if file exists) || end

|}

The '<tt>b</tt>' stands for binary'''b'''inary. The C standard gives two kinds of files -— [[text filesfile]]s and [[binary filesfile]]s -— although operating systems may or may not distinguish between the two. A ''text file'' is a file consisting of text arranged in lines with some sort of distinguishing end-of-line character or sequence (in [[Unix]], a bare linefeed character; in the [[Apple Macintosh|Macintosh]] OS, a bare carriage return; on [[DOS]] and [[Microsoft Windows]], a carriage return followed by a linefeed). When bytes are read in from a text file, an end-of-line sequence is usually mapped to a linefeed for ease in processing. When a text file is written to, a bare linefeed is mapped to the OS-specific end-of-line character sequence before writing. A ''binary file'' is a file where bytes are read in "raw," and delivered "raw," without any kind of mapping.

When a file is opened with update mode ( '<tt>+</tt>' as the second or third character in the mode argument), both input and output may be performed on the associated stream. However, writes cannot be followed by reads without an intervening call to <code>fflush()</code> or to a file positioning function ( <code>[[Fseek|fseek()]]</code>, <code>fsetpos()</code>, or <code>[[Rewind (C)|rewind()]]</code>), and reads cannot be followed by writes without an intervening call to a file positioning function. [http://www.opengroup.org/onlinepubs/009695399/functions/fopen.html]

Writing and appending modes will create a file to write to in the case that the file name doesn't already exist. However, the operation of fopen is undefined if the filename doesn't follow requirements by the OS. For example, if the filename contains illegal characters , the program might crash. For example, in windows <tt>\ /: * ? &gt; &lt;</tt> and <tt>|</tt> cannot be part of a file name.

==Closing a file using <tt>fclose</tt>==

:<code>int fclose(FILE *file_pointer)</code>