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    ftruncate

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

    ftruncate — ファイルを指定した長さに丸める

    説明

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

    ファイルポインタstreamを引数とし、 ファイルを指定した長さ、サイズに丸めます。

    パラメータ

    stream

    ファイルポインタ。

    注意:

    stream は書き込みモードでオープンする必要があります。

    size

    丸める大きさ。

    注意:

    size がファイルのサイズより大きい場合は、 null バイトを用いてファイルを拡大します。

    size がファイルのサイズより小さい場合は、 余分なデータは失われます。

    戻り値

    成功した場合に true を、失敗した場合に false を返します。

    例

    例1 ファイルの丸めの例

    <?php
    $filename     = 'lorem_ipsum.txt';

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

    注意

    注意:

    ファイルポインタは変更 されません。

    参考

    • fopen() - ファイルまたは URL をオープンする
    • fseek() - ファイルポインタを移動する

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    User Contributed Notes 5 notes

    up
    44
    emailfire at gmail dot com
    13 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.
    9 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
    4 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
    13 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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