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Pipes and Named Pipes (FIFO)

Pipes are the classical method of interprocess communication.

For example

# ls –l | more

The symbol | indicates a pipe and the shell in the above example runs the processes ls and more which are linked with a pipe. ls writes data to the pipe and more reads it.

Named pipes otherwise called as the FIFO (First in First Out) is the other variant of pipe.

They can be created like this

# mkfifo pathname

Example

# mkfifo hello

# ls –l hello

prw-r- -r- - 1 temp users 0 Aug 28 10.45 hello |

there are many similarities between the pipes and the FIFO, but the inode specification for the both are more or less the same.

following is the inode specification for the pipe

struct pipe_inode_info

{

wait_queue_head_t wait; //wait queue

char * base; //address of the FIFO buffer

unsigned int readers; //no of processes reading at this moment

unsigned int writers; //no of processes writing at this moment

unsigned int waiting_readers; //no of blocked process reading at this moment

unsigned int waiting_writers; //no of blocked processes writing at this moment

unsigned int r_counter; //no of read processes that have opened

unsigned int w_counter; //no of write processes that have opened

};

The length of the area of the pipe is managed in the i_size field. The system call pipe creates a pipe which involves setting up a temporary inode and allocating a page of memory which is decided by the architecture dependent model.

Opening a FIFO

    Blocking Non Blocking
For reading No Writing Processes Block Open FIFO
  Writing Processes Open FIFO Open FIFO
For Writing No Reading Processes Block Error ENXIO
  Reading Process Open FIFO Open FIFO
For Reading and Writing   Open FIFO Open FIFO

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