Named Pipes

1. Named Pipes

2. Kinds of IPC • Mutexes/Conditional Variables/Semaphores • Pipes • Named pipes • Signals • Shared memory • Messages • Sockets

3. Named Pipes • A pipe is a unidirectional buffer between two processes handled by the kernel • A pipe has a file descriptor, just like a file • Unnamed pipes can only be used between processes with a common ancestor • Named pipes do not require any relationship • Named pipes are special files within the file system • you can use ‘ls’ and see them

4. Creating a FIFO • C – Library function called mkfifo() • Synopsis #include <sys/types.h> #include <sys/stat.h> int mkfifo(const char *pathname, mode_t mode);

5. Creating a FIFO file #include <sys/stat.h> #include <sys/types.h> #include <unistd.h> #include <iostream> using namespace std; int main(){ if(access("my_fifo",F_OK) == 0) cout << "FIFO already exists" << endl; else { int res = mkfifo("my_fifo", 0777); if (res==0) cout << "FIFO created" << endl; else cout << "FIFO not created" << endl; } }

6. Working with FIFOs • use open and close as with ”normal” files • both ends of the pipe need to be opened! • Note: open returns a file descriptor • A process opening a name pipe blocks until some process opens the FIFO at "the other end" • close doesn’t need to block • A reading process blocks if the FIFO is empty • reader and writer become synchronized

7. Opening a FIFO pipe file • Named pipes are opened like other files • For example, by using the open system call • flags specify whether to read or write, and whether to block • Opening a FIFO with the O_NONBLOCK flag makes access to the pipe non-blocking: • for O_READ, open returns immediately • even if the writing end has not been opened by another process • for O_WRITE, open returns immediately, but returns -1 if the reading end is not open • Also subsequent read and write operations become nonblocking!

8. Reading and writing to FIFOs • Like normal files • using low-level I/O: open, read, write, close • using high level C++ I/O: <fstream> • For FIFOs in blocking mode • reading from an empty fifo blocks • writing to a full FIFO blocks • size of FIFO buffer PIPE_BUF = 4096 KB • For FIFOs opened with O_NONBLOCK • reading from an empty FIFO succeeds with 0 bytes of data • writing to a full or nearly full FIFO may fail, or may write onlysome of the data into the FIFO