Working with the Unix OS

Processes II
FIFOs
First In, First Out is similar to a pipe (System V). FIFOs are used by the System V line printer.
A FIFO is created by the mknod system call.
int mknod(char *pathname, int mode, int dev); /etc/mknod name p
Pipe/FIFO rules for reading and writing:
- read ask for less data than is in pipe
- returns requested data
- leaves remainder
- ask for more - only return what is available
- no data in pipe - read returns zero - EOF
- writes less than capacity of pipe (4096 bytes) - write is guaranteed to be atomic
- write to a pipe & no read process - SIGPIPE signal
Consider a daemon that uses a FIFO to receive client requests:
- Daemon opens FIFO for read-only & its typical state is waiting in a read system call for a client request.
- Client processes are started and they open the FIFO for writing, write their request, & exit.
- What happens is that the read returns zero to the daemon every time a client process terminates, if no other
clients have FIFO open for writing.
- Daemon has to then open the FIFO again and it waits here until client opens FIFO for writing.
- To avoid this, the daemon opens FIFO two times - once for reading & once for writing.
- File descriptor returned for reading is used to read the client requests & fd for writing is never used.
- By having FIFO always open for writing the reads do not return EOF, but wait for next client request.
Client-Server FIFO example
- mknod to create the FIFOs (may already exist). After fork both processes must open each of 2 FIFOs.
- Parent process remove FIFOs with unlink system call, after waiting for the child to terminate.
- The order of the open calls is important, and avoids a deadlock condition.
- With pipes the client and server had to originate from the same process - no restriction with FIFOs.
Streams and Messages
The data is a stream of bytes with no interpretation done by the system. Many UNIX processes that need to impose
a message structure on top of a stream based IPC facility do it using the newline character to separate messages.
! Name Spaces
The name is how the client and server "connect" to exchange messages
IPC type Name space Identification
pipe (no name) file descriptor
FIFO pathname file descriptor
message queue key_t key identifier
shared memory key_t key identifier
semaphore key_t key identifier
unix socket pathname file descriptor
key_t key
ftok function converts a pathname to a IPC key
#include
#include
key_t ftok(char *pathname, char proj);
Guarantee a unique key
32-bit inode
=> 16-bits
8-bit major device number
8-bit minor device number => 8-bits
8-bit project
=> 8-bits
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