ftruncate()

(PHP 4, PHP 5, PHP 7)

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。

Note:

The$handlemust be open for writing.

$size

The size to truncate to.

Note:

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

If$sizeis 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

范例

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 isnotchanged.

参见

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

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);
?>

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

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.

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. ;)