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

参数

handle

文件指针。

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
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 booleanTRUE

范例

Example #1 File truncation example

<?php
$filename 
'lorem_ipsum.txt';

$handle fopen($filename'r+');
ftruncate($handlerand(1filesize($filename)));
rewind($handle);
echo 
fread($handlefilesize($filename));
fclose($handle);
?>

注释

Note:

The file pointer is not changed.

参见

add a note add a note

User Contributed Notes 3 notes

up
3
rc at opelgt dot org
6 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.
up
1
emailfire at gmail dot com
2 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);
?>
up
0
eurosat7 at yahoo dot de
2 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. ;)
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