The PHP Function Fseek

There are a lot of ways to process files in PHP. Classic functions like fread might serve you well much of the time, but sometimes you might want to work more intimately with the file itself. The php function fseek can help you do just that.

Fseek() is a function that allows you to move the file pointer for the specified file stream in a number of different ways. The first parameter is the file pointer, and the second one is the offset that you wish to seek to. The third is the whence, which can be SEEK_SET (default), SEEK_CUR, or SEEK_END.

In the example below, we're reading a text file with lines of a fixed length. We're using fseek() to skip ahead by a fixed number of characters, which might be more efficient than simply reading the entire file and parsing it with fscanf, or even just splitting it and echoing it.

The next step in the example is to use rewind() to move the file pointer back to the beginning of the file, and then we'll call fwrite(), which will replace that first letter "n" in the middle of the line with an "o". Finally, fseek() is called again with 2 as the offset and SEEK_CUR, which moves us two bytes ahead in the file.

It's worth noting that this method of working with files might not work with all streams - for example, any streams that were opened in append ("a" or "a+") mode will give undefined results when used with fseek().

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fseek example


    protected function _isMboxFile($file, $fileIsString = true)
    {
        if ($fileIsString) {
            $file = @fopen($file, 'r');
            if (!$file) {
                return false;
            }
        } else {
            fseek($file, 0);
        }

        $result = false;

        $line = fgets($file);
        if (strpos($line, 'From ') === 0) {
            $result = true;
        }

        if ($fileIsString) {
            @fclose($file);
        }
return $assoc;
    }

    /** * @param resource $resource */
    private function seek($resource, int $offset): void
    {
        $currentOffset = ftell($resource);
        if ($currentOffset !== $offset) {
            fseek($resource, $offset);
        }
    }

    private function setOffset(int $offset): void
    {
        $this->offset = $offset;
    }
}
if ($body instanceof File) {
            $path = $body->getPath();
            if ((is_file($path) && !is_readable($path)) || is_dir($path)) {
                throw new InvalidArgumentException(sprintf('Path "%s" is not readable.', $path));
            }
        }

        $this->body = $body;
        $this->charset = $charset;
        $this->subtype = $subtype;
        $this->seekable = \is_resource($body) ? stream_get_meta_data($body)['seekable'] && 0 === fseek($body, 0, \SEEK_CUR) : null;

        if (null === $encoding) {
            $this->encoding = $this->chooseEncoding();
        } else {
            if ('quoted-printable' !== $encoding && 'base64' !== $encoding && '8bit' !== $encoding) {
                throw new InvalidArgumentException(sprintf('The encoding must be one of "quoted-printable", "base64", or "8bit" ("%s" given).', $encoding));
            }
            $this->encoding = $encoding;
        }
    }

    
// Detect ending of header list             if (preg_match('/(\r\n\r\n|\n\n)/', $headers, $match)) {
                $headersPosEnd = strpos($headers, $headerBodySeparator = $match[0]);

                break;
            }
        }

        $headers = $this->getMessageHeaders(trim(substr($headers, 0, $headersPosEnd)));

        fseek($stream, $headersPosEnd + \strlen($headerBodySeparator));

        return new SMimePart($this->getStreamIterator($stream), $type, $subtype, $this->getParametersFromHeader($headers['content-type']));
    }

    protected function getStreamIterator($stream): iterable
    {
        while (!feof($stream)) {
            yield str_replace("\n", "\r\n", str_replace("\r\n", "\n", fread($stream, 16372)));
        }
    }

    

        // Header Object: (mandatory, one only)         // Field Name Field Type Size (bits)         // Object ID GUID 128 // GUID for header object - GETID3_ASF_Header_Object         // Object Size QWORD 64 // size of header object, including 30 bytes of Header Object header         // Number of Header Objects DWORD 32 // number of objects in header object         // Reserved1 BYTE 8 // hardcoded: 0x01         // Reserved2 BYTE 8 // hardcoded: 0x02
        $info['fileformat'] = 'asf';

        $this->fseek($info['avdataoffset']);
        $HeaderObjectData = $this->fread(30);

        $thisfile_asf_headerobject['objectid']      = substr($HeaderObjectData, 0, 16);
        $thisfile_asf_headerobject['objectid_guid'] = $this->BytestringToGUID($thisfile_asf_headerobject['objectid']);
        if ($thisfile_asf_headerobject['objectid'] != GETID3_ASF_Header_Object) {
            unset($info['fileformat'], $info['asf']);
            return $this->error('ASF header GUID {'.$this->BytestringToGUID($thisfile_asf_headerobject['objectid']).'} does not match expected "GETID3_ASF_Header_Object" GUID {'.$this->BytestringToGUID(GETID3_ASF_Header_Object).'}');
        }
        $thisfile_asf_headerobject['objectsize']    = getid3_lib::LittleEndian2Int(substr($HeaderObjectData, 16, 8));
        $thisfile_asf_headerobject['headerobjects'] = getid3_lib::LittleEndian2Int(substr($HeaderObjectData, 24, 4));
        $thisfile_asf_headerobject['reserved1']     = getid3_lib::LittleEndian2Int(substr($HeaderObjectData, 28, 1));
        
$flattened = [];
            $this->flatten($value, $flattened, $keySeparator, '', $escapeFormulas);
            $value = $flattened;
        }
        unset($value);

        $headers = array_merge(array_values($headers), array_diff($this->extractHeaders($data), $headers));
        $endOfLine = $context[self::END_OF_LINE] ?? $this->defaultContext[self::END_OF_LINE];

        if (!($context[self::NO_HEADERS_KEY] ?? $this->defaultContext[self::NO_HEADERS_KEY])) {
            fputcsv($handle, $headers, $delimiter, $enclosure, $escapeChar);
            if ("\n" !== $endOfLine && 0 === fseek($handle, -1, \SEEK_CUR)) {
                fwrite($handle, $endOfLine);
            }
        }

        $headers = array_fill_keys($headers, '');
        foreach ($data as $row) {
            fputcsv($handle, array_replace($headers, $row), $delimiter, $enclosure, $escapeChar);
            if ("\n" !== $endOfLine && 0 === fseek($handle, -1, \SEEK_CUR)) {
                fwrite($handle, $endOfLine);
            }
        }

        
try {
                    if (!$chunk->isTimeout() && $chunk->isFirst()) {
                        $this->response->getStatusCode(); // ignore 3/4/5xx                     }
                } catch (ExceptionInterface $e) {
                    trigger_error($e->getMessage(), \E_USER_WARNING);

                    return false;
                }
            }

            if (0 !== fseek($this->content, $this->offset ?? 0)) {
                return false;
            }

            if ('' !== $data = fread($this->content, $count)) {
                fseek($this->content, 0, \SEEK_END);
                $this->offset += \strlen($data);

                return $data;
            }
        }

        
                    for ($i = 0; $i < $p_len; $i++) {
                        $this->_readBlock();
                    }
                } else {
                    if ($this->_compress_type == 'lzma2') {
                        // ----- Replace missing xztell() and xzseek()                         for ($i = 0; $i < $p_len; $i++) {
                            $this->_readBlock();
                        }
                    } else {
                        if ($this->_compress_type == 'none') {
                            @fseek($this->_file, $p_len * 512, SEEK_CUR);
                        } else {
                            $this->_error(
                                'Unknown or missing compression type ('
                                . $this->_compress_type . ')'
                            );
                        }
                    }
                }
            }
        }
        return true;
    }
public static function hash_data($file, $offset, $end, $algorithm) {
        if (!self::intValueSupported($end)) {
            return false;
        }
        if (!in_array($algorithm, array('md5', 'sha1'))) {
            throw new getid3_exception('Invalid algorithm ('.$algorithm.') in self::hash_data()');
        }

        $size = $end - $offset;

        $fp = fopen($file, 'rb');
        fseek($fp, $offset);
        $ctx = hash_init($algorithm);
        while ($size > 0) {
            $buffer = fread($fp, min($size, getID3::FREAD_BUFFER_SIZE));
            hash_update($ctx, $buffer);
            $size -= getID3::FREAD_BUFFER_SIZE;
        }
        $hash = hash_final($ctx);
        fclose($fp);

        return $hash;
    }

    

    public function Analyze() {
        $info = &$this->getid3->info;

        // http://www.volweb.cz/str/tags.htm
        if (!getid3_lib::intValueSupported($info['filesize'])) {
            $this->warning('Unable to check for Lyrics3 because file is larger than '.round(PHP_INT_MAX / 1073741824).'GB');
            return false;
        }

        $this->fseek((0 - 128 - 9 - 6), SEEK_END);          // end - ID3v1 - "LYRICSEND" - [Lyrics3size]         $lyrics3offset = null;
        $lyrics3version = null;
        $lyrics3size   = null;
        $lyrics3_id3v1 = $this->fread(128 + 9 + 6);
        $lyrics3lsz    = (int) substr($lyrics3_id3v1, 0, 6); // Lyrics3size         $lyrics3end    = substr($lyrics3_id3v1,  6,   9); // LYRICSEND or LYRICS200         $id3v1tag      = substr($lyrics3_id3v1, 15, 128); // ID3v1
        if ($lyrics3end == 'LYRICSEND') {
            // Lyrics3v1, ID3v1, no APE
            
if (!getid3_lib::intValueSupported($info['filesize'])) {
            $this->warning('Unable to check for APEtags because file is larger than '.round(PHP_INT_MAX / 1073741824).'GB');
            return false;
        }

        $id3v1tagsize     = 128;
        $apetagheadersize = 32;
        $lyrics3tagsize   = 10;

        if ($this->overrideendoffset == 0) {

            $this->fseek(0 - $id3v1tagsize - $apetagheadersize - $lyrics3tagsize, SEEK_END);
            $APEfooterID3v1 = $this->fread($id3v1tagsize + $apetagheadersize + $lyrics3tagsize);

            //if (preg_match('/APETAGEX.{24}TAG.{125}$/i', $APEfooterID3v1)) {             if (substr($APEfooterID3v1, strlen($APEfooterID3v1) - $id3v1tagsize - $apetagheadersize, 8) == 'APETAGEX') {

                // APE tag found before ID3v1                 $info['ape']['tag_offset_end'] = $info['filesize'] - $id3v1tagsize;

            //} elseif (preg_match('/APETAGEX.{24}$/i', $APEfooterID3v1)) {             } elseif (substr($APEfooterID3v1, strlen($APEfooterID3v1) - $apetagheadersize, 8) == 'APETAGEX') {

                
/** * Tests that uploading files exceeding maximum size are handled correctly. */
  public function testUploadFileExceedingMaximumFileSize() {
    $session = $this->getSession();

    // Create a test file that exceeds the maximum POST size with 1 kilobyte.     $post_max_size = Bytes::toNumber(ini_get('post_max_size'));
    $invalid_file = 'public://exceeding_post_max_size.bin';
    $file = fopen($invalid_file, 'wb');
    fseek($file, $post_max_size + 1024);
    fwrite($file, 0x0);
    fclose($file);

    // Go to the node creation form and try to upload the test file.     $this->drupalGet('node/add/article');
    $page = $session->getPage();
    $page->attachFileToField("files[field_file_0]", $this->fileSystem->realpath($invalid_file));

    // An error message should appear informing the user that the file exceeded     // the maximum file size. The error message includes the actual file size     // limit which depends on the current environment, so we check for a part

        public function read( $bytes ) {
            return fread( $this->_f, $bytes );
        }

        /** * @param int $pos * @return bool */
        public function seekto( $pos ) {
            if ( -1 === fseek( $this->_f, $pos, SEEK_SET ) ) {
                return false;
            }
            $this->_pos = $pos;
            return true;
        }

        /** * @return bool */
        public function is_resource() {
            return is_resource( $this->_f );
        }
/** * @param int $min_data * * @return bool */
    private function EnsureBufferHasEnoughData($min_data=1024) {
        if (($this->current_offset - $this->EBMLbuffer_offset) >= ($this->EBMLbuffer_length - $min_data)) {
            $read_bytes = max($min_data, $this->getid3->fread_buffer_size());

            try {
                $this->fseek($this->current_offset);
                $this->EBMLbuffer_offset = $this->current_offset;
                $this->EBMLbuffer        = $this->fread($read_bytes);
                $this->EBMLbuffer_length = strlen($this->EBMLbuffer);
            } catch (getid3_exception $e) {
                $this->warning('EBML parser: '.$e->getMessage());
                return false;
            }

            if ($this->EBMLbuffer_length == 0 && $this->feof()) {
                return $this->error('EBML parser: ran out of file at offset '.$this->current_offset);
            }
        }


    public function testAskMultilineResponseWithWithCursorInMiddleOfSeekableInputStream()
    {
        $input = <<<EOD This is some input EOD;
        $response = $this->getInputStream($input);
        fseek($response, 8);

        $dialog = new QuestionHelper();

        $question = new Question('Write an essay');
        $question->setMultiline(true);

        $this->assertSame("some\ninput", $dialog->ask($this->createStreamableInputInterfaceMock($response), $this->createOutputInterface(), $question));
        $this->assertSame(8, ftell($response));
    }

    /** * @dataProvider getAskConfirmationData */
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