Read a file backwards line by line using fseek

How do I read a file backwards line by line using fseek?

code can be helpful. must be cross platform and pure php.

many thanks in advance

regards

Jera

If you are going to read the entire file in anyways, just use file() to read the file in as an array (each line is each element in the array) and then use array_reverse() to flip the array backwards and loop through that. Or just do a reverse for loop where you start at the end and decrement on each loop.

$file = file("test.txt");
$file = array_reverse($file);
foreach($file as $f){
    echo $f."<br />";
}

The question is asking using fseek, so can only assume that performance is an issue and file() is not the solution. Here is a simple approach using fseek:

My file.txt

#file.txt
Line 1
Line 2
Line 3
Line 4
Line 5

And the code:

<?php

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

$pos = -2; // Skip final new line character (Set to -1 if not present)

$lines = array();
$currentLine = '';

while (-1 !== fseek($fp, $pos, SEEK_END)) {
    $char = fgetc($fp);
    if (PHP_EOL == $char) {
            $lines[] = $currentLine;
            $currentLine = '';
    } else {
            $currentLine = $char . $currentLine;
    }
    $pos--;
}

$lines[] = $currentLine; // Grab final line

var_dump($lines);

Output:

array(5) {
   [0]=>
   string(6) "Line 5"
   [1]=>
   string(6) "Line 4"
   [2]=>
   string(6) "Line 3"
   [3]=>
   string(6) "Line 2"
   [4]=>
   string(6) "Line 1"
}

You don't have to append to the $lines array like I am, you can print the output straight away if that is the purpose of your script. Also it is easy to introduce a counter if you want to limit the number of lines.

$linesToShow = 3;
$counter = 0;
while ($counter <= $linesToShow && -1 !== fseek($fp, $pos, SEEK_END)) {
   // Rest of code from example. After $lines[] = $currentLine; add:
   $counter++;
}

<?php

class ReverseFile implements Iterator
{
    const BUFFER_SIZE = 4096;
    const SEPARATOR = "\n";

    public function __construct($filename)
    {
        $this->_fh = fopen($filename, 'r');
        $this->_filesize = filesize($filename);
        $this->_pos = -1;
        $this->_buffer = null;
        $this->_key = -1;
        $this->_value = null;
    }

    public function _read($size)
    {
        $this->_pos -= $size;
        fseek($this->_fh, $this->_pos);
        return fread($this->_fh, $size);
    }

    public function _readline()
    {
        $buffer =& $this->_buffer;
        while (true) {
            if ($this->_pos == 0) {
                return array_pop($buffer);
            }
            if (count($buffer) > 1) {
                return array_pop($buffer);
            }
            $buffer = explode(self::SEPARATOR, $this->_read(self::BUFFER_SIZE) . $buffer[0]);
        }
    }

    public function next()
    {
        ++$this->_key;
        $this->_value = $this->_readline();
    }

    public function rewind()
    {
        if ($this->_filesize > 0) {
            $this->_pos = $this->_filesize;
            $this->_value = null;
            $this->_key = -1;
            $this->_buffer = explode(self::SEPARATOR, $this->_read($this->_filesize % self::BUFFER_SIZE ?: self::BUFFER_SIZE));
            $this->next();
        }
    }

    public function key() { return $this->_key; }
    public function current() { return $this->_value; }
    public function valid() { return ! is_null($this->_value); }
}

$f = new ReverseFile(__FILE__);
foreach ($f as $line) echo $line, "\n";

To completely reverse a file:

$fl = fopen("\some_file.txt", "r");
for($x_pos = 0, $output = ''; fseek($fl, $x_pos, SEEK_END) !== -1; $x_pos--) {
    $output .= fgetc($fl);
    }
fclose($fl);
print_r($output);

Of course, you wanted line-by-line reversal...

$fl = fopen("\some_file.txt", "r");
for($x_pos = 0, $ln = 0, $output = array(); fseek($fl, $x_pos, SEEK_END) !== -1; $x_pos--) {
    $char = fgetc($fl);
    if ($char === "\n") {
        // analyse completed line $output[$ln] if need be
        $ln++;
        continue;
        }
    $output[$ln] = $char . ((array_key_exists($ln, $output)) ? $output[$ln] : '');
    }
fclose($fl);
print_r($output);

Really though, Jonathan Kuhn has the best answer IMHO above. The only cases you'd not use his answer that I know of is if file or like functions are disabled via php.ini, yet the admin forgot about fseek, or when opening a huge file just get the last few lines of contents would magically save memory this way.

Note: Error handling not included. And, PHP_EOL didn't cooperate, so I used "\n" to denote end of line instead. So, above may not work in all cases.

You cannot fseek line by line, because you do not know how long the lines are until you read them.

You should either read the whole file into a list of lines, or if the file is too big for that and you only need the last lines, read fixed-sized chunks from the end of the file and implement a bit more complicated logic which detects lines from such data.