PHP 8.3.4 Released!

ftruncate

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

ftruncate将文件截断到指定的长度

说明

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

接受文件指针 stream 作为参数,并将文件大小截取为 size

参数

stream

文件指针。

注意:

stream 必须打开写入。

size

截断的大小。

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

示例

示例 #1 文件截取示例

<?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:

文件指针不会改变。

参见

add a note

User Contributed Notes 5 notes

up
43
emailfire at gmail dot com
12 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
20
Julien B.
8 years ago
You MUST use rewind() after ftruncate() to replace file content
up
9
rc at opelgt dot org
16 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
2
Masoud
3 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.
up
1
eurosat7 at yahoo dot de
12 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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