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ftruncate

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

ftruncateTrunca um arquivo a um tamanho especificado

Descrição

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

Toma o ponteiro de arquivo stream e trunca o arquivo para o comprimento definido em size.

Parâmetros

stream

O ponteiro do arquivo.

Nota:

O fluxo stream deve estar aberto para escrita.

size

O tamanho para o qual será truncado.

Nota:

Se o parâmetro size for maior que o arquivo, o arquivo será estendido com bytes nulos.

Se o parâmetro size for menor que o arquivo, o arquivo será truncado para este tamanho.

Valor Retornado

Retorna true em caso de sucesso ou false em caso de falha.

Exemplos

Exemplo #1 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);
?>

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 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
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
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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