ftruncate

(PHP 4, PHP 5)

ftruncate — Reduz um arquivo a um tamanho especificado

Descrição

bool ftruncate ( resource $handle , int $size )

Pega o ponteiro de arquivo handle e reduz o arquivo ao comprimento size .

Parâmetros

handle: O ponteiro do arquivo.

Nota: O ponteiro handle deve estar aberto para escrita.
size: O tamanho pro qual reduzir.

Nota: Se size for maior que o arquivo, o arquivo será estendido com bytes nulos.
Se size for menor, os dados extras serão perdidos.

Valor Retornado

Retorna TRUE em caso de sucesso ou FALSE em falhas.

Histórico

Versão	Descrição
PHP 4.3.3	Antes desta versão, ftruncate() retornava integer no valor 1 no caso de sucesso, ao invés de boolean `TRUE`.

Notas

Nota: O ponteiro do arquivo não é alterado.

Veja Também

fopen() - Abre um arquivo ou URL
fseek() - Procura (seeks) em um ponteiro de arquivo

add a note User Contributed Notes
ftruncate

rc at opelgt dot org
06-Jan-2008 06:49


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.

mike at mikeleigh dot com
04-Jan-2007 08:30


I have produced a number of tests below which walk through my findings of the ftruncate function.  For the impatient among you ftruncate can be used to increase the size of the file and will fill the rest of the file with CHR 0 or ASCII NULL.  It can be used as a very convenient way of making a 1Mb file for instance.



Test 1

<?php

/*

  Test 1: Write "some text" to a file.

  Result: The text "some text" should be present in test_1.txt

*/

$fp = fopen('test_1.txt', 'w+');

fwrite($fp, 'some text');

?>

The first test is only here to make sure that a file can be written with some text.



Test 2

<?php

/*

  Test 2: Write "some text" to a file and ftruncate the file to 4 bytes.

  Result: The text "some" should be present in test_2.txt as the file will have been truncated to 4 bytes.

*/

$fp = fopen('test_2.txt', 'w+');

fwrite($fp, 'some text');

ftruncate($fp, 4);

?>

As expected the file has been truncated to 4 bytes.



Test 3

<?php

/*

  Test 3: Write "some text" to a file and ftruncate the file to 40 bytes.

  Result: The text "some text" should be present in test_3.txt as the file will have been truncated to 40 bytes.

*/

$fp = fopen('test_3.txt', 'w+');

fwrite($fp, 'some text');

ftruncate($fp, 40);

?>

Interestingly the file has increased from 9 bytes to 40 bytes.  The remaining 31 bytes of the file are ASCII code 0 or NULL though.



Further notes can be found here http://mikeleigh.com/links/ftruncate

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ftell

Last updated: Fri, 06 Nov 2009