PHP: fflush - Manual

User Contributed Notes
fflush

draco at lomag dot net
12-Nov-2004 03:50


If you are write to a file, fflush() it and then hope to re-read in the data, you probably need to use http://www.php.net/manual/en/function.clearstatcache.php



Because of PHP's caching, it caches the original file when the script is first executed, so if you reread the file you wrote to, it will look like it was when it first executed. It wont contain any of the information you wrote to it.

jzho327 at cse dot unsw dot edu dot au
27-Jan-2003 02:27


Sorry, the note I previously added regarding fflush() has a bug!



when you read file by calling 

$fp = fopen("myfile", "a+");



and you wish to read the content, you have to call 

rewind($fp) before you attemp to read it. (Silly me) Or else if you tried something like

$fp = fopen("myfile", "a+");

$content = fread($fp);



// show that content is in fact null

if ($content == NULL)

 print "but it must be first put to rewind<br>\n";



will result



but it must be first put to rewind.

[new line]



in fact, if you are not careful (like me), and you opened it with

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

$content = fread($fp);

..

// do something with content

..

then you tried to again

$content = fread($fp);

without rewind, you'd get NULL again. This sounds trivial, but if you work with several file resources, with freads and fopens everywhere, you can get easily lost. So becareful.

jzho327 at cse dot unsw dot edu dot au
27-Jan-2003 01:42


The use of fflush() seems to be useful when a resource handler is opened via 'r+' or 'a+', whence the user wish to read and write with one fopen command. Normally in such situation, for some reason everyone do



$fileName = "myfile";

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

..

// read the content

..

fclose($fp);

$fp = fopen($fileName, "a");

..

// write some new content

..

fclose($fp);



Now, it's probably sensible if you are writing completely new contents but if you're appending a few lines of text to a large file, to me this seems waste of resource to read in the content, then wipe the file, and then rewrite with almost the same content. In such case, maybe a single fopen with "a+" will suffice, provided you call fflush() before you actually write to the file. 



Example,



suppose 'textfile' contain

line1: blah blah blah

line2: eni mani mu

line3: I can't believe it's not Java

[new line]



and you did 

$fp = fopen("textfile", "a+");

fwrite($fp, "line4: at least it's not C#\n");

fclose($fp);



you will get



line1: blah blah blah

line2: eni mani mu

line3: I can't believe it's not Java

line1: blah blah blah

line2: eni mani mu

line3: I can't believe it's not Java

line4: as long as it's not C#\n

[new line]

 

instead, you should do

$fp = fopen("textfile", "a+");

fflush($fp);

fwrite($fp, "line4: as long as it's not C#\n");

// or maybe even 

// $newtext = "line4: as long as it's not C#\n"; 

// fwrite($fp, $newtext, strlen($newtext));

fclose($fp);



which will give you the desired result,

line1: blah blah blah

line2: eni mani mu

line3: I can't believe it's not Java

line4: as long as it's not C#\n

[new line]



Now, I am new to PHP (really started coding yesterday or so) so please don't flame me if there is anything terribly wrong. I do however like to see someone explain why everyone fopen(..,"r"); and then fopen(..,"a"); and hardly anyone use fopen(..,"a+"); because it can be done with the help of fflush().



...Or maybe fflush() is just as wasteful.

fflush

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