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154 lines
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HTML
154 lines
7.0 KiB
HTML
4 years ago
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>fopen (The Red Hat newlib C Library)</title>
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<meta name="description" content="fopen (The Red Hat newlib C Library)">
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<meta name="keywords" content="fopen (The Red Hat newlib C Library)">
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<link href="Document-Index.html#Document-Index" rel="index" title="Document Index">
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<link href="Document-Index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Stdio.html#Stdio" rel="up" title="Stdio">
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</head>
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<body lang="en">
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<a name="fopen"></a>
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<div class="header">
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<p>
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Next: <a href="fopencookie.html#fopencookie" accesskey="n" rel="next">fopencookie</a>, Previous: <a href="fmemopen.html#fmemopen" accesskey="p" rel="prev">fmemopen</a>, Up: <a href="Stdio.html#Stdio" accesskey="u" rel="up">Stdio</a> [<a href="Document-Index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Document-Index.html#Document-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="fopen_002d_002d_002dopen-a-file"></a>
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<h3 class="section">4.17 <code>fopen</code>—open a file</h3>
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<a name="index-fopen"></a>
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<a name="index-_005ffopen_005fr"></a>
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<p><strong>Synopsis</strong>
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</p><div class="example">
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<pre class="example">#include <stdio.h>
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FILE *fopen(const char *<var>file</var>, const char *<var>mode</var>);
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FILE *_fopen_r(struct _reent *<var>reent</var>,
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const char *<var>file</var>, const char *<var>mode</var>);
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</pre></div>
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<p><strong>Description</strong><br>
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<code>fopen</code> initializes the data structures needed to read or write a
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file. Specify the file’s name as the string at <var>file</var>, and the kind
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of access you need to the file with the string at <var>mode</var>.
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</p>
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<p>The alternate function <code>_fopen_r</code> is a reentrant version.
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The extra argument <var>reent</var> is a pointer to a reentrancy structure.
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</p>
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<p>Three fundamental kinds of access are available: read, write, and append.
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<code>*<var>mode</var></code> must begin with one of the three characters ‘<code>r</code>’,
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‘<code>w</code>’, or ‘<code>a</code>’, to select one of these:
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</p>
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<dl compact="compact">
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<dt><code>r</code></dt>
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<dd><p>Open the file for reading; the operation will fail if the file does
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not exist, or if the host system does not permit you to read it.
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</p>
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</dd>
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<dt><code>w</code></dt>
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<dd><p>Open the file for writing <em>from the beginning</em> of the file:
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effectively, this always creates a new file. If the file whose name you
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specified already existed, its old contents are discarded.
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</p>
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</dd>
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<dt><code>a</code></dt>
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<dd><p>Open the file for appending data, that is writing from the end of
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file. When you open a file this way, all data always goes to the
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current end of file; you cannot change this using <code>fseek</code>.
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</p></dd>
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</dl>
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<p>Some host systems distinguish between “binary” and “text” files.
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Such systems may perform data transformations on data written to, or
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read from, files opened as “text”.
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If your system is one of these, then you can append a ‘<code>b</code>’ to any
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of the three modes above, to specify that you are opening the file as
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a binary file (the default is to open the file as a text file).
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</p>
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<p>‘<code>rb</code>’, then, means “read binary”; ‘<code>wb</code>’, “write binary”; and
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‘<code>ab</code>’, “append binary”.
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</p>
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<p>To make C programs more portable, the ‘<code>b</code>’ is accepted on all
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systems, whether or not it makes a difference.
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</p>
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<p>Finally, you might need to both read and write from the same file.
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You can also append a ‘<code>+</code>’ to any of the three modes, to permit
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this. (If you want to append both ‘<code>b</code>’ and ‘<code>+</code>’, you can do it
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in either order: for example, <code>"rb+"</code> means the same thing as
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<code>"r+b"</code> when used as a mode string.)
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</p>
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<p>Use <code>"r+"</code> (or <code>"rb+"</code>) to permit reading and writing anywhere in
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an existing file, without discarding any data; <code>"w+"</code> (or <code>"wb+"</code>)
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to create a new file (or begin by discarding all data from an old one)
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that permits reading and writing anywhere in it; and <code>"a+"</code> (or
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<code>"ab+"</code>) to permit reading anywhere in an existing file, but writing
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only at the end.
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</p>
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<br>
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<p><strong>Returns</strong><br>
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<code>fopen</code> returns a file pointer which you can use for other file
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operations, unless the file you requested could not be opened; in that
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situation, the result is <code>NULL</code>. If the reason for failure was an
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invalid string at <var>mode</var>, <code>errno</code> is set to <code>EINVAL</code>.
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</p>
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<br>
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<p><strong>Portability</strong><br>
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<code>fopen</code> is required by ANSI C.
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</p>
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<p>Supporting OS subroutines required: <code>close</code>, <code>fstat</code>, <code>isatty</code>,
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<code>lseek</code>, <code>open</code>, <code>read</code>, <code>sbrk</code>, <code>write</code>.
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</p>
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<br>
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<hr>
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<div class="header">
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<p>
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Next: <a href="fopencookie.html#fopencookie" accesskey="n" rel="next">fopencookie</a>, Previous: <a href="fmemopen.html#fmemopen" accesskey="p" rel="prev">fmemopen</a>, Up: <a href="Stdio.html#Stdio" accesskey="u" rel="up">Stdio</a> [<a href="Document-Index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Document-Index.html#Document-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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