writing data to a file: concurent fopen()s

davepass

i am using fopen, fwrite and fclose to write data to a file. The data is obtained from a web page via an html form (<textareea>).

Suppose one user submits the form (which will open the file, write the data and close it) and a split second after that another does the same? Will PHP allow more than 1 instance of the file to be opened? If not what happens to the 2nd user? If so, is there any way to prevent one instance from overwriting the other?

or is there a way to test to see if a file is opened by another user and if it is to wait until it is closed?

drew010

here is what you can do to prevent the file from being opened for writing by more than one script.

$fp = fopen("file.txt", "w");
flock($fp, LOCK_EX); //exclusive lock
//....writing etc
flock($fp, LOCK_UN); //unlock file
fclose($fp);

or for reading:

$fp = fopen("file.txt", "r");
flock($fp, LOCK_SH); //shared read lock
//...reading
flock($fp, LOCK_UN); //unlock
fclose($fp);