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PHP : Function Reference : Filesystem Functions : fseek

fseek

Seeks on a file pointer (PHP 4, PHP 5)
int fseek ( resource handle, int offset [, int whence] )

Example 648. fseek() example

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<?php

$fp = fopen('somefile.txt', 'r');

// read some data
$data = fgets($fp, 4096);

// move back to the beginning of the file
// same as rewind($fp);
fseek($fp, 0);

?>

Related Examples ( Source code ) » fseek

Examples ( Source code ) » Reading a Specific Character
Examples ( Source code ) » Reading Specific Data from a File
Examples ( Source code ) » File read by char
Examples ( Source code ) » Moving Around a File with fseek()

Code Examples / Notes » fseek

25-aug-2002 01:12
The following call moves to the end of file (i.e. just after the last byte of the file):
    fseek($fp, 0, SEEK_END);
It can be used to tell the size of an opened file when the file name is unknown and can't be used with the filesize() function:
    fseek($fp, 0, SEEK_END);
    $filesize = ftell($fp);
The following call moves to the begining of file:
    fseek($fp, 0, SEEK_SET);
It is equivalent to:
    rewind($fp);

alan
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);

lucky
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!

phil
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-- ;
   }
}

jeffunk7
If you, like me, need the second to last line from a text file (or some other line near the end that you will know the number of, ie the fourth to last line) then this addition to Jim's code can help you.
//$linefromlast is the linenumber that you need, the last line being 1, the second to last being 2, and so on...
function readlog($file, $linefromlast){
  $fp = @fopen($file, "r");
  $pos = -2;
  $t = " ";
  $linecounter = 1;
       while ($t != "\n" and $linecounter<=$linefromlast) {
               fseek($fp, $pos, SEEK_END);
               $t = fgetc($fp);
               $pos = $pos - 1;
               if ($t == "\n" and $linecounter < $linefromlast) {
                       fseek($fp, $pos, SEEK_END);
                       $t = fgetc($fp);
                       $pos = $pos - 1;
                       $linecounter = $linecounter +1;
               }
        }
   $t = fgets($fp);
   fclose($fp);
   return $t;
}

aspyrine
If you want to go to the end of a socket stream with fseek() you'll get the following error :
"stream does not support seeking"
feof() wont work eiver in a stream (ie. smtp)
You can move the pointer to the end with this command :
while(fgetc($fp)) {}
...so easy :-)

chenganeyou
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;
}
?>

marc dot roe
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.

info
I think the function should be as following to deal any combination of cr & lf, no matter the line ends by cr, lf, cr-lf or lf-cr:
<?php
function getline ($handle) {
      while (!feof($handle)) {
          $char = fgetc($handle);
          if (($char == "\n") or ($char == "\r")) {
              $char2 = fgetc($handle);
              if (($char2 != "\n") && ($char2 != "\r")) {
                  fseek ($handle,-1,SEEK_CUR);
              }
              break;
          }
          else {
              $buffer .= $char;
          }
      }
      return $buffer;
}
?>

mhinks
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;
}

jim
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;
}
?>

mail
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;
?>

dan
For all first-time users of the fseek() function, remember these three things:
1. to use a programming expression, fseek() is "base 0", so to prepare the file for writing at character 1, you'd say fseek($fp,0); and to prepare the file for writing at character $num, you'd say fseek($fp,($num-1));
2. here's the formula for accessing fixed-length records in a file (you need to seek the position of the end of the previous record):
 /* assumes the desired record number is in $rec_num */
 /* assumes the record length is in $rec_len */
 $pos = ( ($rec_num-1) * $rec_len );
 fseek($fp,$pos);
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.
Hope this helps.

16-sep-2002 09:25
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...

lutz l_broedel
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()
?>

memphis
Actually chenganeyou, your function causes the entire file to be read into an array, and then you look at the last element of the array.  While this works fine for a small file, an sizeable file is going to suck down memory and time.  Using a 15 MB file your function took around 2 secs to return.
The function provided by Jim goes directly to the end of the file and only reads in that line.  I had to set the offset ($pos) to -2 for it to work in my case however.  Using the same 15 MB file this function returns immediately.

ekow
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;
}
?>

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