Communication Via Files (IPC)

This is one of the oldest way of data exchange in which the information is sent as a file.

File needs two type of locking

Mandatory Locking

• Read wirte is blocked during the entire locking period

Advisory locking

• Allows Read and write even after the lock has been set
• processes accessing the file for Read and Write has to lock it and release it again

Locking Entire Files

There are two methods for locking the entire file

First Method is by the use of several system calls

• link – a system call
• create – another system call
• combination of flags like O_CREAT and O_EXEC with open (system call)
• lock a fle with open() function using O_CREAT | O_WRONLY | O_TRUNK, but this option fails under the superuser mode

Second Method

fcntl – A system call to lock the entire file. It is also useful to lock the file areas. In Linux 2.0 it is called flock() which is not advisable to use.

Locking file areas

int sys_fcntl(unsigned int fd, unsigned int cmd, unsigned long arg);

int sys_fcntl64(unsigned int fd, unsigned int cmd, unsigned long arg);

where fd – file descriptor

cmd – includes F_GETLK, F_SETLK, F_SETLKW

Semantics of fcntl locks

Existing Lock	Setting a read lock	Write lock
None	Possible	Possible
More than One	Possible	Not Allowed
A Write Lock	Not Allowed	Not Allowed

Locking files may also lead to deadlock with the following example

Assume there are two processes wanted to read a file,

P1  -> 1 –> 2  (P1 tries to access the 1st block and then 2nd block within a file)

P2 –> 2  -> 1  (P2 tries to access the 2nd block and then 1st block within a file)

Now for getting the 2 and the 1st block, both processes will be in deadlock,

So Communication via files may not be a great option, of course there are some methods to avoid the deadlocks.