PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

78 Chapter 2 Storing and Retrieving Data
Locking Files
Imagine a situation in which two customers are trying to order a product at the same
time. (This situation is not uncommon, especially when your website starts to get any
kind of traffic volume.) What if one customer calls fopen() and begins writing, and then
the other customer calls fopen() and also begins writing? What will be the final contents
of the file? Will it be the first order followed by the second order, or vice versa?
Will it be one order or the other? Or will it be something less useful, such as the two
orders interleaved somehow? The answer depends on your operating system but is often
impossible to know.
To avoid problems like this, you can use file locking.You use this feature in PHP by
using the flock() function.This function should be called after a file has been opened
but before any data is read from or written to the file.
The prototype for flock() is
bool flock (resource fp, int operation [, int &wouldblock])
You need to pass it a pointer to an open file and a constant representing the kind of lock
you require. It returns true if the lock was successfully acquired and false if it was not.
The optional third parameter will contain the value true if acquiring the lock would
cause the current process to block (that is, have to wait).
The possible values for operation are shown in Table 2.2.The possible values
changed at PHP 4.0.1, so both sets of values are shown in the table.
Table 2.2 flock() Operation Values
Value of Operation Meaning
LOCK_SH (formerly 1)
LOCK_EX (formerly 2)
LOCK_UN (formerly 3)
LOCK_NB (formerly 4)
Reading lock.The file can be shared with other readers.
Writing lock.This operation is exclusive; the file cannot be shared.
The existing lock is released.
Blocking is prevented while you are trying to acquire a lock.
If you are going to use flock(), you need to add it to all the scripts that use the file;
otherwise, it is worthless.
Note that flock() does not work with NFS or other networked file systems. It also
does not work with older file systems that do not support locking, such as FAT. On
some operating systems, it is implemented at the process level and does not work correctly
if you are using a multithreaded server API.
To use it with the order example, you can alter processorder.php as follows:
$fp = fopen(“$DOCUMENT_ROOT/../orders/orders.txt”, ‘ab’);
flock($fp, LOCK_EX); // lock the file for writing
fwrite($fp, $outputstring);
flock($fp, LOCK_UN); // release write lock
fclose($fp);