PHP: ftruncate - Manual (original) (raw)

(PHP 4, PHP 5, PHP 7, PHP 8)

ftruncate — Truncates a file to a given length

Description

Parameters

stream

The file pointer.

Note:

The stream must be open for writing.

size

The size to truncate to.

Note:

If size is larger than the file then the file is extended with null bytes.

If size is smaller than the file then the file is truncated to that size.

Return Values

Returns [true](reserved.constants.php#constant.true) on success or [false](reserved.constants.php#constant.false) on failure.

Examples

Example #1 File truncation example

<?php <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>f</mi><mi>i</mi><mi>l</mi><mi>e</mi><mi>n</mi><mi>a</mi><mi>m</mi><mi>e</mi><msup><mo>=</mo><mo mathvariant="normal" lspace="0em" rspace="0em">′</mo></msup><mi>l</mi><mi>o</mi><mi>r</mi><mi>e</mi><msub><mi>m</mi><mi>i</mi></msub><mi>p</mi><mi>s</mi><mi>u</mi><mi>m</mi><mi mathvariant="normal">.</mi><mi>t</mi><mi>x</mi><msup><mi>t</mi><mo mathvariant="normal" lspace="0em" rspace="0em">′</mo></msup><mo separator="true">;</mo></mrow><annotation encoding="application/x-tex">filename = 'lorem_ipsum.txt';</annotation></semantics></math>filename=′loremipsum.txt′;handle = fopen($filename, 'r+'); ftruncate($handle, rand(1, filesize($filename))); rewind($handle); echo fread($handle, filesize($filename)); fclose($handle); ?>

Notes

Note:

The file pointer is not changed.

Found A Problem?

emailfire at gmail dot com ¶

14 years ago

If you want to empty a file of it's contents bare in mind that opening a file in w mode truncates the file automatically, so instead of doing...

<?php
$fp = fopen("/tmp/file.txt", "r+");
ftruncate($fp, 0);
fclose($fp);
?>

You can just do...

<?php
$fp = fopen("/tmp/file.txt", "w");
fclose($fp);
?>

Julien B. ¶

10 years ago

You MUST use rewind() after ftruncate() to replace file content

rc at opelgt dot org ¶

17 years ago

Writing after ftruncate

I didnt expect that I can write in the middle of nowhere. I thought that I would write at the beginning of the file but the first 4 bytes were filled automatically with NULLs followed by "56":

<?php
$str1  = 1234;
$str2  =   56;
$datei = "test.txt";
 <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>d</mi><mi>h</mi><mo>=</mo><mi>f</mi><mi>o</mi><mi>p</mi><mi>e</mi><mi>n</mi><mo stretchy="false">(</mo></mrow><annotation encoding="application/x-tex">dh = fopen(</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.6944em;"></span><span class="mord mathnormal">d</span><span class="mord mathnormal">h</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord mathnormal">o</span><span class="mord mathnormal">p</span><span class="mord mathnormal">e</span><span class="mord mathnormal">n</span><span class="mopen">(</span></span></span></span>datei,"w");
fwrite($dh, $str1);
fclose($dh);
 <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>d</mi><mi>h</mi><mo>=</mo><mi>f</mi><mi>o</mi><mi>p</mi><mi>e</mi><mi>n</mi><mo stretchy="false">(</mo></mrow><annotation encoding="application/x-tex">dh = fopen (</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.6944em;"></span><span class="mord mathnormal">d</span><span class="mord mathnormal">h</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord mathnormal">o</span><span class="mord mathnormal">p</span><span class="mord mathnormal">e</span><span class="mord mathnormal">n</span><span class="mopen">(</span></span></span></span>datei,"r+");
echo "content: ".fread($dh, filesize($datei))."<br>";
echo "pointer after fread at: ".ftell($dh)."<br>";
ftruncate($dh, 0);
echo "pointer after truncate at: ".ftell($dh)."<br>";
fwrite($dh, $str2);
echo "pointer after fwrite at: ".ftell($dh)."<br>";
rewind($dh);
echo "pointer after rewind at: ".ftell($dh)."<br>"; <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>s</mi><mi>t</mi><mi>r</mi><mo>=</mo><mi>f</mi><mi>r</mi><mi>e</mi><mi>a</mi><mi>d</mi><mo stretchy="false">(</mo></mrow><annotation encoding="application/x-tex">str = fread(</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.6151em;"></span><span class="mord mathnormal">s</span><span class="mord mathnormal">t</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord mathnormal">re</span><span class="mord mathnormal">a</span><span class="mord mathnormal">d</span><span class="mopen">(</span></span></span></span>dh, 6);
echo "content: $str<br>in ASCII: ";
for($i = 0; <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>i</mi><mo>&lt;</mo><mn>6</mn><mo separator="true">;</mo></mrow><annotation encoding="application/x-tex">i &lt; 6; </annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.6986em;vertical-align:-0.0391em;"></span><span class="mord mathnormal">i</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">&lt;</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:0.8389em;vertical-align:-0.1944em;"></span><span class="mord">6</span><span class="mpunct">;</span></span></span></span>i++)
 echo ord($str{$i})."-";
fclose($dh);

/*
   OUTPUT:
   content: 1234
   pointer after fread at: 4
   pointer after truncate at: 4
   pointer after fwrite at: 6
   pointer after rewind at: 0
   content: 56
   in ASCII: 0-0-0-0-53-54
*/
?>

So not only ftruncate is filling an empty file up with NULLs as in the note before. Fread is filling leading space with NULLs too.

Masoud ¶

5 years ago

The problem that rc at opelgt dot org mentioned seems completely logical.

When pointer is at offset 4 and you truncate file, the pointer is still at offset 4.

So when you write(), the first 4 bytes are filled with null byte by Operating System - There is nothing wrong by PHP. And it's filled with null byte, because there is data on disk and that needs to be cleared with zero bits.

Even though this is a Operating System's gotcha, to avoid data corruption, PHP Docs should mention it clearly. Also it would be nice if PHP automatically sets the pointer's offset to SEEK_END  after truncating to an smaller size to fool-proof it.

eurosat7 at yahoo dot de ¶