PHP的feof()方法需要注意的地方

自己曾写过这样一段代码:

if(file_exists($pmr_config["datasetfile"])){
	$tmp_counter = 0;
	$hd = fopen($pmr_config["datasetfile"], "r");
	if($hd !== FALSE){
		while (!feof($hd)) {
			$buffer = fgets($hd);
                        if($tmp_counter >= $seq){
				$result[] = $buffer;
			}
                $tmp_counter++;

                if($tmp_counter >=$seq + $size){
                    break;
                }
	}
	}else{
		echo "warning:open file {$pmr_config["datasetfile"]} failed!PHP_EOL";
	}
}else{
	echo "warning:file {$pmr_config["datasetfile"]} does not exsits!PHP_EOL";
}

其中当读取行数包括文件结尾的时候,$result数组中总会比期望的内容多出来一个元素:

(boolean)false

按说,如果读取到最后一行,feof函数会返回TRUE,然后while循环就退出了,为什么不是呢?

while (!feof($hd)) {

事情原来是这样子的:

<?php
// if file can not be read or doesn't exist fopen function returns FALSE
$file = @fopen("no_such_file", "r");

// FALSE from fopen will issue warning and result in infinite loop here
while (!feof($file)) {
}

fclose($file);
?>

feof() is, in fact, reliable.  However, you have to use it carefully in conjunction with fgets().  A common (but incorrect) approach is to try something like this:

<?
$fp = fopen("myfile.txt", "r");
while (!feof($fp)) {
  $current_line = fgets($fp);
  // do stuff to the current line here
}
fclose($fp);
?>

The problem when processing plain text files is that feof() will not return true after getting the last line of input.  You need to try to get input _and fail_ before feof() returns true.  You can think of the loop above working like this:

* (merrily looping, getting lines and processing them)
* fgets used to get 2nd to last line
* line is processed
* loop back up -- feof returns false, so do the steps inside the loop
* fgets used to get last line
* line is processed
* loop back up -- since the last call to fgets worked (you got the last line), feof still returns false, so you do the steps inside the loop again
* fgets used to try to get another line (but there's nothing there!)
* your code doesn't realize this, and tries to process this non-existent line (typically by doing the same actions again)
* now when your code loops back up, feof returns true, and your loop ends

There's two ways to solve this:

1. You can put an additional test for feof() inside the loop
2. You can move around your calls to fgets() so that the testing of feof() happens in a better location

Here's solution 1:

<?
$fp = fopen("myfile.txt", "r");
while(!feof($fp)) {
  $current_line = fgets($fp);
  if (!feof($fp)) {
    // process current line
  }
}
fclose($fp);
?>


And here's solution 2 (IMHO, more elegant):

<?
$fp = fopen("myfile.txt", "r");
$current_line = fgets($fp);
while (!feof($fp)) {
  // process current line
  $current_line = fgets($fp);
}
fclose($fp);
?>


FYI, the eof() function in C++ works the exact same way, so this isn't just some weird PHP thing...

Ref:  http://cn.php.net/manual/en/function.feof.php

Codefor
posted @ 2011-07-07 16:18  Codefor  阅读(3950)  评论(0编辑  收藏  举报