CSE4413

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====== Our Own Perl Wiki ======






===== Differences from Java =====
  * Type: Variables are either **scalars** prefixed by **$**, **arrays** of scalars prefixed by **@**, or **hashes** of scalar pairs prefixed by **%**.

  * Operators: same as Java except **eq**, **gt**, **lt**, ... for string comparisons.
 
  * Control Structures (selection and iteration): same as Java plus **foreach** for hashes.

  * Cool stuff: interpolation, here-docs, regex, and web goodies such as param and %ENV.

----

===== File I/O in Perl =====
**Default file handlers:**
  * STDIN: Standard input from console
  * STDOUT: Standard output to console
  * STDERR: Standard error output to console

**Opening a file**
<code perl>
open(FILEHANDLE, filename)
</code>
Where FILEHANDLE specifes the name for the file handler in which you will refer to your file and filename specifies the location of your file.

In the file name one can specify the various read/write modes.
^ Example      ^ Description       ^
| open(FILEHANDLE, <filename)      | opens the file for reading    | 
| open(FILEHANDLE, >filename)     | opens the file for writing    |
| open(FILEHANDLE, >>filename)     | opens the file for appending    |
| open(FILEHANDLE, +>filename)     | opens the file for reading and writing    |
| open(FILEHANDLE, +<filename)     | opens the file for reading and writing    |
| open(FILEHANDLE, +>>filename)     | opens the file for reading and writing    |

File locking for I/O is provided using the **flock()** function. It is suggested to do the lock on a seperate file to prevent race conditions.
<code perl>
open(LOCK, ">write.lock")
flock(LOCK, LOCK_EX);
open(DATA, myFile);
#...do work with the DATA file...
close LOCK;
close DATA;
</code>
Closing the **LOCK** handler automatically unlocks the semaphore. One can also use **flock(LOCK, LOCK_UN)** to do the unlocking specifically. The flock function takes the following operation parameters:
^ Operation      ^ Description       ^
| LOCK_SH      | requests a shared lock for a file, will block until lock is obtained    | 
| LOCK_EX      | requests an exclusive lock for a file, will block until lock is obtained   | 
| LOCK_UN      | unlocks the previously locked file    | 
| LOCK_NB      | should be bitwise 'or'-ed with either LOCK_SH or LOCK_EX and will result in a non-blocking lock    |