Filesystem 函数
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ftruncate

(PHP 4, PHP 5)

ftruncate — 将文件截断到给定的长度

说明

bool ftruncate ( resource $handle , int $size )

接受文件指针 handle 作为参数，并将文件大小截取为 size。成功时返回 TRUE， 或者在失败时返回 FALSE.

Note:

文件只会在 append 模式下改变。在 write 模式下，必须加上 fseek() 操作。

Note:

在 PHP 4.3.3 之前，ftruncate() 在成功时返回一个 integer 值 1，而不是 boolean 的 TRUE。

参见 fopen() 和 fseek()。

参数

handle

The file pointer.

Note:

The handle 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.

返回值

成功时返回 TRUE， 或者在失败时返回 FALSE.

更新日志

版本 说明
PHP 4.3.3 Prior to this release ftruncate() returned an integer value of 1 on success, instead of boolean TRUE.

范例

Example #1 File truncation example

<?php
$filename = 'lorem_ipsum.txt';

$handle = fopen($filename, 'r+');
ftruncate($handle, rand(1, filesize($filename)));
rewind($handle);
echo fread($handle, filesize($filename));
fclose($handle);
?>

注释

Note:

The file pointer is not changed.

参见

Filesystem 函数
在线手册：中文 英文
PHP手册
PHP手册 - N: 将文件截断到给定的长度

用户评论:

emailfire at gmail dot com (16-Jun-2011 10:14)

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);
?>

eurosat7 at yahoo dot de (21-Apr-2011 04:31)

If you want to ftruncate but keep the end:
<?php
    function ftruncatestart($filename,$maxfilesize){
        $size=filesize($filename);
        if ($size<$maxfilesize*1.0) return;
        $maxfilesize=$maxfilesize*0.5; //we don't want to do it too often...
        $fh=fopen($filename,"r+");
        $start=ftell($fh);
        fseek($fh,-$maxfilesize,SEEK_END);
        $drop=fgets($fh);
        $offset=ftell($fh);
        for ($x=0;$x<$maxfilesize;$x++){
            fseek($fh,$x+$offset);
            $c=fgetc($fh);
            fseek($fh,$x);
            fwrite($fh,$c);
        }
        ftruncate($fh,$maxfilesize-strlen($drop));
        fclose($fh);
    }
?>
It will not just cut it but search for a newline so you avoid corrupting your csv or logfiles. But I don't know if you will stress the reading head of your drive. ;)

yetihehe at yetihehe dot com (22-Apr-2010 11:12)

Simple script which will delete spaces from end of php files (suitable in big sites, when you are including many files before sending headers and without output buffering):

<?php
function mapfiles($func,$filename,$level=0) {
  if(is_dir($filename)) {
    if($level>30) return; //limit recurence in case of looped dirs
    $handle=opendir($filename);
    while(($dirname=readdir($handle))!==false) {
      if($dirname=='.'||$dirname=='..') continue;
      mapfiles($func,$filename.'/'.$dirname,$level+1);
    }
    closedir($handle);
  } else {
    $func($filename);
  }
}

function scanfile($filename) {
  if(substr($filename,-4)!=".php") return;
  $ff=fopen($filename,"r+");
  fseek($ff, -10, SEEK_END);
  $str=fread($ff,12);
  $matches=array();
  if(preg_match('/\?>(\s+)$/',$str,$matches)) {
    $fsize=filesize($filename);
    ftruncate($ff, $fsize-strlen($matches[1]));
  }
  fclose($ff);
}

mapfiles("scanfile",".");
?>

rc at opelgt dot org (06-Jan-2008 06:49)

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";

$dh = fopen($datei,"w");
fwrite($dh, $str1);
fclose($dh);

$dh = fopen ($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>";
$str = fread($dh, 6);
echo "content: $str<br>in ASCII: ";
for($i = 0; $i < 6; $i++)
 echo ord($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.

mike at mikeleigh dot com (04-Jan-2007 08:30)

I have produced a number of tests below which walk through my findings of the ftruncate function.  For the impatient among you ftruncate can be used to increase the size of the file and will fill the rest of the file with CHR 0 or ASCII NULL.  It can be used as a very convenient way of making a 1Mb file for instance.

Test 1
<?php
/*
  Test 1: Write "some text" to a file.
  Result: The text "some text" should be present in test_1.txt
*/
$fp = fopen('test_1.txt', 'w+');
fwrite($fp, 'some text');
?>
The first test is only here to make sure that a file can be written with some text.

Test 2
<?php
/*
  Test 2: Write "some text" to a file and ftruncate the file to 4 bytes.
  Result: The text "some" should be present in test_2.txt as the file will have been truncated to 4 bytes.
*/
$fp = fopen('test_2.txt', 'w+');
fwrite($fp, 'some text');
ftruncate($fp, 4);
?>
As expected the file has been truncated to 4 bytes.

Test 3
<?php
/*
  Test 3: Write "some text" to a file and ftruncate the file to 40 bytes.
  Result: The text "some text" should be present in test_3.txt as the file will have been truncated to 40 bytes.
*/
$fp = fopen('test_3.txt', 'w+');
fwrite($fp, 'some text');
ftruncate($fp, 40);
?>
Interestingly the file has increased from 9 bytes to 40 bytes.  The remaining 31 bytes of the file are ASCII code 0 or NULL though.

Further notes can be found here http://mikeleigh.com/links/ftruncate