1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- Created by GNU Texinfo 6.1, http://www.gnu.org/software/texinfo/ -->
<head>
<title>GNU Octave: File Positioning</title>
<meta name="description" content="GNU Octave: File Positioning">
<meta name="keywords" content="GNU Octave: File Positioning">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link href="index.html#Top" rel="start" title="Top">
<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="C_002dStyle-I_002fO-Functions.html#C_002dStyle-I_002fO-Functions" rel="up" title="C-Style I/O Functions">
<link href="Plotting.html#Plotting" rel="next" title="Plotting">
<link href="EOF-and-Errors.html#EOF-and-Errors" rel="prev" title="EOF and Errors">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.indentedblock {margin-right: 0em}
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smalllisp {margin-left: 3.2em}
kbd {font-style: oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nolinebreak {white-space: nowrap}
span.roman {font-family: initial; font-weight: normal}
span.sansserif {font-family: sans-serif; font-weight: normal}
ul.no-bullet {list-style: none}
-->
</style>
</head>
<body lang="en">
<a name="File-Positioning"></a>
<div class="header">
<p>
Previous: <a href="EOF-and-Errors.html#EOF-and-Errors" accesskey="p" rel="prev">EOF and Errors</a>, Up: <a href="C_002dStyle-I_002fO-Functions.html#C_002dStyle-I_002fO-Functions" accesskey="u" rel="up">C-Style I/O Functions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="File-Positioning-1"></a>
<h4 class="subsection">14.2.19 File Positioning</h4>
<p>Three functions are available for setting and determining the position of
the file pointer for a given file.
</p>
<a name="XREFftell"></a><dl>
<dt><a name="index-ftell"></a>Built-in Function: <em><var>pos</var> =</em> <strong>ftell</strong> <em>(<var>fid</var>)</em></dt>
<dd><p>Return the position of the file pointer as the number of characters from the
beginning of the file specified by file descriptor <var>fid</var>.
</p>
<p><strong>See also:</strong> <a href="#XREFfseek">fseek</a>, <a href="#XREFfrewind">frewind</a>, <a href="EOF-and-Errors.html#XREFfeof">feof</a>, <a href="Opening-and-Closing-Files.html#XREFfopen">fopen</a>.
</p></dd></dl>
<a name="XREFfseek"></a><dl>
<dt><a name="index-fseek"></a>Built-in Function: <em></em> <strong>fseek</strong> <em>(<var>fid</var>, <var>offset</var>)</em></dt>
<dt><a name="index-fseek-1"></a>Built-in Function: <em></em> <strong>fseek</strong> <em>(<var>fid</var>, <var>offset</var>, <var>origin</var>)</em></dt>
<dt><a name="index-fseek-2"></a>Built-in Function: <em><var>status</var> =</em> <strong>fseek</strong> <em>(…)</em></dt>
<dd><p>Set the file pointer to the location <var>offset</var> within the file <var>fid</var>.
</p>
<p>The pointer is positioned <var>offset</var> characters from the <var>origin</var>,
which may be one of the predefined variables <code><span class="nolinebreak">SEEK_CUR</span></code><!-- /@w --> (current
position), <code><span class="nolinebreak">SEEK_SET</span></code><!-- /@w --> (beginning), or <code><span class="nolinebreak">SEEK_END</span></code><!-- /@w --> (end of
file) or strings <code>"cof"</code>, <code>"bof"</code> or <code>"eof"</code>. If
<var>origin</var> is omitted, <code><span class="nolinebreak">SEEK_SET</span></code><!-- /@w --> is assumed. <var>offset</var> may
be positive, negative, or zero but not all combinations of <var>origin</var> and
<var>offset</var> can be realized.
</p>
<p><code>fseek</code> returns 0 on success and -1 on error.
</p>
<p><strong>See also:</strong> <a href="Line_002dOriented-Input.html#XREFfskipl">fskipl</a>, <a href="#XREFfrewind">frewind</a>, <a href="#XREFftell">ftell</a>, <a href="Opening-and-Closing-Files.html#XREFfopen">fopen</a>.
</p></dd></dl>
<a name="XREFSEEK_005fSET"></a><dl>
<dt><a name="index-SEEK_005fSET"></a>Built-in Function: <em></em> <strong>SEEK_SET</strong> <em>()</em></dt>
<dt><a name="index-SEEK_005fCUR"></a>Built-in Function: <em></em> <strong>SEEK_CUR</strong> <em>()</em></dt>
<dt><a name="index-SEEK_005fEND"></a>Built-in Function: <em></em> <strong>SEEK_END</strong> <em>()</em></dt>
<dd><p>Return the numerical value to pass to <code>fseek</code> to perform one of the
following actions:
</p>
<dl compact="compact">
<dt><code>SEEK_SET</code></dt>
<dd><p>Position file relative to the beginning.
</p>
</dd>
<dt><code>SEEK_CUR</code></dt>
<dd><p>Position file relative to the current position.
</p>
</dd>
<dt><code>SEEK_END</code></dt>
<dd><p>Position file relative to the end.
</p></dd>
</dl>
<p><strong>See also:</strong> <a href="#XREFfseek">fseek</a>.
</p></dd></dl>
<a name="XREFfrewind"></a><dl>
<dt><a name="index-frewind"></a>Built-in Function: <em></em> <strong>frewind</strong> <em>(<var>fid</var>)</em></dt>
<dt><a name="index-frewind-1"></a>Built-in Function: <em><var>status</var> =</em> <strong>frewind</strong> <em>(<var>fid</var>)</em></dt>
<dd><p>Move the file pointer to the beginning of the file specified by file
descriptor <var>fid</var>.
</p>
<p><code>frewind</code> returns 0 for success, and -1 if an error is encountered. It
is equivalent to <code>fseek (<var>fid</var>, 0, SEEK_SET)</code>.
</p>
<p><strong>See also:</strong> <a href="#XREFfseek">fseek</a>, <a href="#XREFftell">ftell</a>, <a href="Opening-and-Closing-Files.html#XREFfopen">fopen</a>.
</p></dd></dl>
<p>The following example stores the current file position in the variable
<code>marker</code>, moves the pointer to the beginning of the file, reads
four characters, and then returns to the original position.
</p>
<div class="example">
<pre class="example">marker = ftell (myfile);
frewind (myfile);
fourch = fgets (myfile, 4);
fseek (myfile, marker, SEEK_SET);
</pre></div>
<hr>
<div class="header">
<p>
Previous: <a href="EOF-and-Errors.html#EOF-and-Errors" accesskey="p" rel="prev">EOF and Errors</a>, Up: <a href="C_002dStyle-I_002fO-Functions.html#C_002dStyle-I_002fO-Functions" accesskey="u" rel="up">C-Style I/O Functions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
</body>
</html>
|
