fseek

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

fseek — Procura (seeks) em um ponteiro de arquivo

Descrição

fseek(resource $stream, int $offset, int $whence = SEEK_SET): int

Modifica o indicador de posição do arquivo referenciado por stream. A nova posição, medida em bytes a partir do início do arquivo, é obtida ao adicionar offset à posição especificada por whence.

Geralmente é permitido procurar (seek) além do fim-do-arquivo; se algum dado for então escrito, leituras além de qualquer região escrita entre o fim-do-arquivo e a região desejada irão trazer bytes com valor 0. Entretanto, alguns fluxos (streams) podem não suportar esse comportamento, especialmente quando têm definido um tamanho de armazenamento fixo.

Parâmetros

stream

Um resource de ponteiro do sistema de arquivos que normalmente é criado usando fopen().

offset

A posição.

Para mover até uma posição anterior ao fim-do-arquivo, você precisa passar um valor negativo no offset e definir o parâmetro whence para SEEK_END.

whence

Valores de whence são:

SEEK_SET - Define a posição igual ao offset bytes.
SEEK_CUR - Define a posição para a atual localização mais offset.
SEEK_END - Define a posição para o final do arquivo mais offset.

Valor Retornado

Em caso de sucesso, retorna 0; caso contrário, retorna -1.

Exemplos

Exemplo #1 Exemplo de fseek()

<?php

$fp = fopen('arquivo.txt');

// lê alguns dados
$data = fgets($fp, 4096);

// move de volta para o inicio do arquivo
// o mesmo que rewind($fp);
fseek($fp, 0);

?>

Notas

Nota:
Se você abriu o arquivo em modo de adição (a ou a+), quaisquer dados que você escreva no arquivo serão sempre acrescentados ao final, independente da posição do arquivo, e o resultado da chamada de fseek() será undefined.

Nota:
Nem todos os fluxos (streams) suportam procura (seek). Para aqueles que não suportam, procuras além da posição atual podem ser feitas através da leitura e descarte de dados; outras formas de procura vão falhar.

Veja Também

ftell() - Retorna a posição atual do ponteiro de leitura/gravação do arquivo
rewind() - Retrocede a posição de um ponteiro de arquivos

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Notas Enviadas por Usuários (em inglês) 25 notes

down

seeker at example com ¶

16 years ago

JUST TO QUOTE AND POINT THIS OUT:

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
3. if you're using fseek() to write data to a file, remember to open the file in "r+" 
mode, example:

  $fp=fopen($filename,"r+");

DON'T open the file in mode "a" (for append), because it puts
 the file pointer at the end of the file and doesn't let you 
fseek earlier positions in the file (it didn't for me!). Also, 
don't open the file in mode "w" -- although this puts you at 
the beginning of the file -- because it wipes out all data in 
the file. 

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Took me half a day to figure :/

down

Anonymous ¶

13 years ago

The official docs indicate that not all streams are seekable.
You can try to seek anyway and handle failure:

<?php
if (fseek($stream, $offset, SEEK_CUR) === -1) {
  // whatever
} 
?>

Or, you can use the stream_get_meta_data function:
http://php.net/stream_get_meta_data

<?php 
function fseekable($stream) {
  $meta = stream_get_meta_data($stream);
  return $meta['seekable'];
}
?>

down

lorenzo dot stanco at gmail dot com ¶

11 years ago

I want to give my contribution about the "read last lines from a file" topic. I've done some researches (starting from here, really) and run many tests for different algorithms and scenarios, and came up with this:

What is the best way in PHP to read last lines from a file?
http://stackoverflow.com/a/15025877/995958

In that mini-article I tried to analyze all different methods and their performance over different files.

Hope it helps.

down

synnus at gmail dot com ¶

10 years ago

Write Dummy File 4GB in Php 32bits (X86)
if you want write more GB File (>4GB), use Php(X64) .
this file is created in 0.0041329860687256 second

CreatFileDummy('data_test.txt',4294967296);

FUNCTION CreatFileDummy($file_name,$size) {    
// 32bits 4 294 967 296 bytes MAX Size 
    $f = fopen($file_name, 'wb');
    if($size >= 1000000000)  {
        $z = ($size / 1000000000);        
        if (is_float($z))  { 
            $z = round($z,0);
            fseek($f, ( $size - ($z * 1000000000) -1 ), SEEK_END);
            fwrite($f, "\0");
        }        
        while(--$z > -1) {
            fseek($f, 999999999, SEEK_END);
            fwrite($f, "\0");
        }
    } 
    else {
        fseek($f, $size - 1, SEEK_END);
        fwrite($f, "\0");
    }
    fclose($f);

Return true;
}

Synx

down

marc dot roe at gmail dot com ¶

18 years ago

I tried to improve and modify (mail at ulf-kosack dot de)'s function. Actually it is very fast, i.e. requires much less time than to get the last five, ten or whatever lines of a file using file() ore file_get_contents().

function read_file($file, $lines)
{
       $handle = fopen($file, "r");
       $linecounter = $lines;
       $pos = -2;
       $beginning = false;
       $text = array();
       while ($linecounter > 0) {
         $t = " ";
         while ($t != "\n") {
           if(fseek($handle, $pos, SEEK_END) == -1) {
$beginning = true; break; }
           $t = fgetc($handle);
           $pos --;
         }
         $linecounter --;
         if($beginning) rewind($handle);
         $text[$lines-$linecounter-1] = fgets($handle);
         if($beginning break;
       }
       fclose ($handle);
       return array_reverse($text); // array_reverse is optional: you can also just return the $text array which consists of the file's lines. 
}

The good thing now is, that you don't get an error when your requesting more lines than the file contains. In this case the function will just return the whole file content.

down

Anonymous ¶

16 years ago

To:seeker at example com
Be careful, though. 
You can freely position you pointer if you open a file in (r+) mode, but it will "overwrite" the data, not "append it".

Tested this:

<?php
// file.txt content:
// "You can contribute your notes to the PHP manual from the comfort of your browser!"

$handler = fopen("file.txt", "r+");
fseek($handler, 0);
fwrite($handler, "want to add this");
?>
New contents of the file.txt will be like this:
"want to add thiste your notes to the PHP manual from the comfort of your browser!".

If you really want to append at the beginning, you have to first get all the contents, save it, clear the file, put the new contents and append the saved contents at the end.

down

Lutz ( l_broedel at gmx dot net ) ¶

19 years ago

Based on the function below, provided by info at o08 dot com (thanks), the following should enable you to read a single line from a file, identified by the line number (starting with 1):

<?
    function readLine ($linenum,$fh) {
        $line = fgets ($fh, 4096);
        $pos = -1;
        $i = 0;

        while (!feof($fh) && $i<($linenum-1)) {
            $char = fgetc($fh);
            if ($char != "\n" && $char != "\r") {
                fseek($fh, $pos, SEEK_SET);
                $pos ++;
            }
            else $i ++; 
        }
        $line = fgets($fh);
        return $line;
    } //readLine()
?>

down

ekow[at]te.ugm.ac.id ¶

18 years ago

A little correction for code to read last line from chenganeyou at eyou dot com.
$linenumber = sizeof($file)-1;
should be
$linenumber = sizeof($contents)-1;
because sizeof will count array element, not file size.
<?php
function readlastline($file)
{
       $linecontent = " ";
       $contents = file($file);
       $linenumber = sizeof($contents)-1;
       $linecontet = $contents[$linenumber];
       unset($contents,$linenumber);
       return $linecontent;
}
?>

down

me at php dot net ¶

11 years ago

how to read BIG files using fseek (above 2GB+, upto any size like 4GB+, 100GB+, 100 terabyes+, any file size, 100 petabytes, max limit is php_float_max ) ?

// seek / set file pointer to 50 GB
my_fseek($fp, floatval(50000000000),1);

function my_fseek($fp,$pos,$first=0) {

// set to 0 pos initially, one-time
if($first) fseek($fp,0,SEEK_SET);

// get pos float value
$pos=floatval($pos);

 // within limits, use normal fseek
if($pos<=PHP_INT_MAX)
  fseek($fp,$pos,SEEK_CUR);
 // out of limits, use recursive fseek
else {
  fseek($fp,PHP_INT_MAX,SEEK_CUR);
  $pos -= PHP_INT_MAX;
  my_fseek($fp,$pos);
}

}

hope this helps.

down

Anonymous ¶

22 years ago

Don't use filesize() on files that may be accessed and updated by parallel processes or threads (as the filesize() return value is maintained in a cache).
Instead lock the opened file and use fseek($fp,0,SEEK_END) and ftell($fp) to get the actual filesize if you need to perform a fread() call to read the whole file...

down

Tom Pittlik ¶

15 years ago

The tail example functions below will return a PHP memory limit error when trying to open large files. Since tail is convenient for opening large logs, here is a function that lets you (provided you have permission):



<?php



function unix_tail($lines,$file)

{

    shell_exec("tail -n $lines $file > /tmp/phptail_$file");

    $output = file_get_contents("/tmp/phptail_$file");

    unlink("/tmp/phptail_$file");

    return $output;

}



?>

down

marin at sagovac dot com ¶

8 years ago

Seek to a line of code than break from while to improve performance. Seek to specific line using SEEK_SET and get a specific line. If $range is '0' than will show seeked line. If set to '2' it will show current line + 2 lines above + 2 lines below.

Useful for get a content of a file in very huge file to get lines range. To improve performance a while loop breaks from iteration than go for seeking.

I've created a function that read a file and count lines and store into arrays each lines bytes to seek. If maximum specified by `linenum` is set, it will break from while to keep performance than in a new loop function to seek a position in bytes to get a content of file.

    function readFileSeek($source, $linenum = 0, $range = 0)
    {
        $fh = fopen($source, 'r');
        $meta = stream_get_meta_data($fh);
        
        if (!$meta['seekable']) {
            throw new Exception(sprintf("A source is not seekable: %s", print_r($source, true)));
        }
        
        $pos = 2;
        $result = null;
        
        if ($linenum) {
            $minline = $linenum - $range - 1;
            $maxline = $minline+$range+$range;
        }
        
        $totalLines = 0;
        while (!feof($fh)) {
        
            $char = fgetc($fh);
            
            if ($char == "\n" || $char == "\r") {
                ++$totalLines;
            } else {
                $result[$totalLines] = $pos;   
            }
            $pos++;
            
            if ($maxline+1 == $totalLines) {
                // break from while to not read entire file
                break;
            }
        }
        
        $buffer = '';
        
        for ($nr=$minline; $nr<=$maxline; $nr++) {
            
            if (isset($result[$nr])) {
            
                fseek($fh, $result[$nr], SEEK_SET);

                while (!feof($fh)) {
                    $char = fgetc($fh);

                    if ($char == "\n" || $char == "\r") {
                        $buffer .= $char;
                        break;
                    } else {
                        $buffer .= $char;
                    }
                }
            
            }
        }
        
        return $buffer;
    }

Test results (1.3 GB file, 100000000 lines of codes, seek to 300000 line a code):

    string(55) "299998_abc
    299999_abc
    300000_abc
    300001_abc
    300002_abc
    "
    
    Time: 612 ms, Memory: 20.00Mb
    
    $ ll -h /tmp/testfile
    -rw-rw-r-- 1  1,3G /tmp/testfile

down

kavoshgar3 at gmail dot com ¶

13 years ago

sometimes we want read file from last line to beginning of file.I use the following.
<?php
function read_backward_line($filename, $lines, $revers = false)
{
    $offset = -1;
    $c = '';
    $read = '';
    $i = 0;
    $fp = @fopen($filename, "r");
    while( $lines && fseek($fp, $offset, SEEK_END) >= 0 ) {
        $c = fgetc($fp);
        if($c == "\n" || $c == "\r"){
            $lines--;
            if( $revers ){
                $read[$i] = strrev($read[$i]);
                $i++;
            }
        }
        if( $revers ) $read[$i] .= $c;
        else $read .= $c;
        $offset--;
    }
    fclose ($fp);
    if( $revers ){
        if($read[$i] == "\n" || $read[$i] == "\r")
            array_pop($read);
        else $read[$i] = strrev($read[$i]);
        return implode('',$read);
    }
    return strrev(rtrim($read,"\n\r"));
}
//if $revers=false function return->
//line 1000: i am line of 1000
//line 1001: and i am line of 1001
//line 1002: and i am last line
//but if $revers=true function return->
//line 1002: and i am last line
//line 1001: and i am line of 1001
//line 1000: i am line of 1000
?>
Enjoy! Mail me if it works!  ;-)

down

necudeco at gmail dot com ¶

14 years ago

This a tail php script example for windows system.

<?php
$n = ( isset($_REQUEST['n']) == true )? $_REQUEST['n']:20;

$offset = -$n * 120;

$rs = fopen('C:/wamp/logs/apache_error.log','r');
if ( $rs === false )
    die("No se pudo abrir el archivo de log");

fseek($rs,$offset,SEEK_END);

fgets($rs);
while(!feof($rs))
{
    $buffer = fgets($rs);
    echo $buffer;
    echo "<hr />";
}

fclose($rs);
?>

down

chenganeyou at eyou dot com ¶

19 years ago

I use the following codes to read the last line of a file.
Compared to jim at lfchosting dot com, it should be more efficient.

<?php
function readlastline($file)
{
       $linecontent = " ";
       $contents = file($file);
       $linenumber = sizeof($file)-1;
       $linecontet = $contents[$linenumber];
       unset($contents,$linenumber);
       return $linecontent;
}
?>

down

alan at peaceconstitution.com ¶

19 years ago

Thanks to Dan, whose above comment provided a key to solve the issue of how to append to a file. 
     After, using phpinfo(); I made sure my installation of PHP had the requisite settings mentioned in the text to the manual entry for fopen(), I was puzzled as to why my use of fopen() with the append option 'a' (append option) didn't work. Then I  read a comment contributed to Appendix L (http://us2.php.net/manual/en/wrappers.php) that the append option 'a' for fopen() doesn't work as expected. The writer suggested using the 'w' option instead, which I found did work. But the 'w' option (write option) overwrites everything in the file.
     The question remained how to accomplish appending. Following Dan's suggestion about the 'r+' option, I tried this, which works fine:
       $string = "Message to write to log";
       $filehandle = fopen ("/home/name/sqllogs/phpsqlerr.txt", 'r+');
    fseek ( $filehandle,0, SEEK_END);
    fwrite ( $filehandle, $string."\n" );
    fclose ($filehandle);

down

phil at NOSPAM dot blisswebhosting dot com ¶

19 years ago

In order to read a text file from end->beginning e.g display the most recent contents of a log file first.  I use the following.

It basically just uses fseek to find the end of the file, ftell to find the byte count for a counter, then iterates backwards through the file using fgetc to test for the newline charater.

$i=0 ;
$lines=500 ;
$fp = fopen($log,"r") ;
if(is_resource($fp)){
    fseek($fp,0,SEEK_END) ;
    $a = ftell($fp) ;
    while($i <= $lines){
        if(fgetc($fp) == "\n"){
            echo (fgets($fp));
            $i++ ;
        }
    fseek($fp,$a) ;
    $a-- ;
    }
}

down

jim at lfchosting dot com ¶

21 years ago

Here is a function that returns the last line of a file.  This should be quicker than reading the whole file till you get to the last line.  If you want to speed it up a bit, you can set the $pos = some number that is just greater than the line length.  The files I was dealing with were various lengths, so this worked for me.



<?php

function readlastline($file)

{

        $fp = @fopen($file, "r");

        $pos = -1;

        $t = " ";

        while ($t != "\n") {

              fseek($fp, $pos, SEEK_END);

              $t = fgetc($fp);

              $pos = $pos - 1;

        }

        $t = fgets($fp);

        fclose($fp);

        return $t;

}

?>

down

-1

ben at nullcreations dot net ¶

16 years ago

easier tail() function for php:



<?php

function tail($file, $num_to_get=10)

{

  $fp = fopen($file, 'r');

  $position = filesize($file);

  fseek($fp, $position-1);

  $chunklen = 4096;

  while($position >= 0)

  {

    $position = $position - $chunklen;

    if ($position < 0) { $chunklen = abs($position); $position=0;}

    fseek($fp, $position);

    $data = fread($fp, $chunklen). $data;

    if (substr_count($data, "\n") >= $num_to_get + 1)

    {

       preg_match("!(.*?\n){".($num_to_get-1)."}$!", $data, $match);

       return $match[0];

    }

  }

  fclose($fp);

  return $data;

}

?>

down

-1

Anonymous ¶

12 years ago

I needed to stream a txt file (here big xml file) to get nodes blockwise. I couldn't find a shorter way. So i did write this class.

Function: streams a complete file and returns the content between two search strings with their search strings (multi byte safe)

Hope it helps anyone.

PS: It lacks any boolean checks / exception handling for non existing files / read errors.

<?php
/**
 * Reads txt-files blockwise
 * Usage:
    $c_streamFileTxt = new streamFileTxt;
    $_args = array(

        'file' => 'temporary.xml',
        'start_string' => '<Product>',
        'stop_string' => '</Product>',
        'block_size' => '8192'
    );
    
    $c_streamFileTxt->setArgs($_args);

    while ($txt_block = $c_streamFileTxt->getNextBlock())
    {
        // use $txt_block for something
    }
 */

class streamFileTxt
{
    private $handle;
    private $file;
    private $file_offset;
    private $block;
    private $start_string;
    private $stop_string;
    private $block_size;

    /**
     * Sett class arguments
     * @param array $_args
     */
    public function setArgs($_args)
    {
        $this->file = $_args['file'];
        $this->start_string = $_args['start_string'];
        $this->stop_string = $_args['stop_string'];
        $this->block_size = $_args['block_size'];
    }

    /**
     * Get next textblock within a file
     * @param void
     * @return string $textblock
     */
    public function getNextBlock()
    {
        $this->openFile();

        fseek($this->handle, $this->file_offset);
        $start_string_found = false;
        $stop_string_found = false;
        while (!feof($this->handle))
        {
            $txt_block = fread($this->handle, $this->block_size);

            if (!$start_string_found) // while not start start snippet found
            {
                $strpos = mb_strpos($txt_block, $this->start_string);
                if ($strpos !== false)
                {
                    // cut of first left chunk
                    $txt_block = mb_substr($txt_block, $strpos, $this->block_size);
                    $start_string_found = true;
                }
            }

            if ($start_string_found && !$stop_string_found) // start snipped found, looking for stop snippet
            { 
                $strpos = mb_strpos($txt_block, $this->stop_string);
                if ($strpos !== false)
                {
                    $removed_block_size = mb_strlen($txt_block) - $strpos;
                    $txt_block = mb_substr($txt_block, 0, $strpos + mb_strlen($this->stop_string)); 
                    $stop_string_found = true;
                    $this->setFileOffset($removed_block_size);
                }
            }

            if ($stop_string_found) // stop-snippet found, keep file offset, return 
            {
                $this->closeFile();
                return $txt_block;
            }
        }

        $this->closeFile();
        return false;
    }
    
    /**
     * Set current file offset and consider the removed block size
     * current file position = current file offset - removed block size
     * @param int $removed_block_size
     */
    private function setFileOffset($removed_block_size)
    {
        $this->file_offset = ftell($this->handle) - $removed_block_size;
    }
    
    /**
     * close current file
     * @param void
     * @return void
     */
    private function openFile()
    {
        $this->handle = fopen($this->file, 'r');
    }
    
    /**
     * open file
     * @param void
     * @return void
     */
    private function closeFile()
    {
        fclose($this->handle);
    }
}

down

-2

lexica98 at gmail dot com ¶

10 years ago

Opening the file in a+ mode doesn't work with fseek either, unfortunately. If you want a file created and then want be able to go to any position in the file, you'll have to use append mode when opening the file, then close it, then open it again in r+.

down

-3

lucky at somnius dot com dot ar ¶

18 years ago

Jim's (jim at lfchosting dot com) code for the last-line issue is perfect if the file is not empty, or moreover if it has more than one line. However if the file you're using cotains no new-line character at all (i.e. it is empty or it's got one line and only one) the while loop will stuck indefinitely.

I know this script is meant for big files which would always contain at least several lines, but it would be clever to make the script error-proof.

Thus, here's a little modification to his code.

<?php
function readLastLine ($file) {
    $fp = @fopen($file, "r");

    $pos = -1;
    $t = " ";
    while ($t != "\n") {
        if (!fseek($fp, $pos, SEEK_END)) { // *** - fseek returns 0 if successfull, and -1 if it has no succes as in seeking a byte outside the file's range
            $t = fgetc($fp);
            $pos = $pos - 1;
        } else { // ***
            rewind($fp); // ***
            break; // ***
        } // ***
    }
    $t = fgets($fp);
    fclose($fp);
    return $t;
}
?>

Lines added and/or modified have been marked with "// ***". I hope this helps!

Regards!

down

-2

steve at studio831 dot com ¶

15 years ago

Modified @ben's function to work for files larger than PHP_INT_MAX bytes.



<?php

function longTail($file, $numLines = 100)

{

    $fp = fopen($file, "r");

    $chunk = 4096;

    $fs = sprintf("%u", filesize($file));

    $max = (intval($fs) == PHP_INT_MAX) ? PHP_INT_MAX : filesize($file);



    for ($len = 0; $len < $max; $len += $chunk) {

        $seekSize = ($max - $len > $chunk) ? $chunk : $max - $len;



        fseek($fp, ($len + $seekSize) * -1, SEEK_END);

        $data = fread($fp, $seekSize) . $data;



        if (substr_count($data, "\n") >= $numLines + 1) {

            preg_match("!(.*?\n){".($numLines)."}$!", $data, $match);

            fclose($fp);

            return $match[0];

        }

    }

    fclose($fp);

    return $data;

}

?>

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-2

mhinks at gmail dot com ¶

17 years ago

Here's a function I wrote to binary search for a line of text within a file, particularly useful when the file is too large to read into memory at once and you want a faster search than linear.

function binary_search_in_file($filename, $search) {

    //Open the file
    $fp = fopen($filename, 'r');

    //Seek to the end
    fseek($fp, 0, SEEK_END);

    //Get the max value
    $high = ftell($fp);
    
    //Set the low value
    $low = 0;

    while ($low <= $high) { 
        $mid = floor(($low + $high) / 2);  // C floors for you

        //Seek to half way through
        fseek($fp, $mid);

        if($mid != 0){
            //Read a line to move to eol
            $line = fgets($fp);
        }
        
        //Read a line to get data
        $line = fgets($fp);
        

        if ($line == $search) {
            fclose($fp);
            return $line;
        }
        else {
            if ($search < $line) {
                $high = $mid - 1;
            }
            else {
                $low = $mid + 1;
            }
        }
    }

    //Close the pointer
    fclose($fp);

    return FALSE;

}

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-2

mail at ulf-kosack dot de ¶

18 years ago

Here a little extension for the code of ekow.
If you want to read more than one line and more than one file. Some times the last five ore ten lines are interesting in.

You only have to submit a array with filenames and optionally a number of lines you want to read.

<?php
  function read_logfiles($files, $lines=5) 
  { 
    foreach($files as $file_num => $file) {
      if (file_exists ($file) ) {
        $handle = fopen($file, "r");
        $linecounter = $lines;
        $pos = -2; 
        $t = " ";  
        $text[$file_num] = "";
        while ($linecounter > 0) {
          while ($t != "\n") {
            fseek($handle, $pos, SEEK_END);
            $t = fgetc($handle);
            $pos --;
          }
          $t = " ";
          $text[$file_num] .= fgets($handle);
          $linecounter --;
        }
        fclose ($handle);
      } else {
        $text[$file_num] = "The file doesn't exist.";
      }
    }
    
    return $text;
?>

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