PHP - Manual: ftruncate

2024-04-27

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ftruncate

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

ftruncate — 将文件截断到给定的长度

说明

ftruncate(resource $handle, int $size): bool

接受文件指针 handle 作为参数，并将文件大小截取为 size。

参数

handle: 文件指针。

注意:
The handle must be open for writing.
size: The size to truncate to.

注意:
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
4.3.3	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`。

范例

示例 #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

注意:
The file pointer is not changed.

参见

fopen() - 打开文件或者 URL
fseek() - 在文件指针中定位

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User Contributed Notes 5 notes

down

emailfire at gmail dot com ¶

11 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);
?>

down

Julien B. ¶

7 years ago


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

down

rc at opelgt dot org ¶

14 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";

$dh = fopen($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.

down

eurosat7 at yahoo dot de ¶

11 years ago


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. ;)

down

Masoud ¶

1 year 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.

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官方地址：https://www.php.net/manual/en/function.ftruncate.php

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ftruncate

说明

参数

返回值

更新日志

范例

注释

参见

User Contributed Notes 5 notes