Interprocess Communication



Anonymous Pipes
Named Pipes (FIFOs)
popen() / pclose()
InterProcess Communication
(IPC)
Anonymous Pipe
 FIFO queues
 Message queues
 Shared memory with Semaphores
 Sockets
 STREAMS
See table on page 496 for which types of IPC
are supported on which platforms

IPC


The uni-directional (half duplex) pipe
and named pipe (FIFOs) are supported
on all flavors of UNIX
All of these forms of IPC except for
sockets and STREAMS require both
processes to be on the same machine
Unnamed Pipes



Oldest form of IPC in UNIX
Historically they are half duplex (unidirectional)
Unnamed pipes can only be used
between processes with a common
ancestor

Typically a process forks after creating an
unnamed pipe
Unnamed Pipes
int pipe(int filedes[2]);
 filedes[0] opened for reading
 filedes[1] opened for writing
 Writes to filedes[1] appear at filedes[0]
for reading
filedes[0]
pipe
filedes[1]
Unnamed Pipes

Calling fork after calling pipe gives us
parent
child
filedes[0]
filedes[0]
pipe
filedes[1]
filedes[1]
Unnamed Pipes


We must choose whether a pipe sends
messages from the parent to the child or
from the child to the parent
Messages from parent to child



Parent closes filedes[0]
Child closes filedes[1]
Messages from child to parent


Parent closes filedes[1]
Child closes filedes[0]
Unnamed Pipes


Black arrow – parent to child
Red arrow – child to parent
parent
child
filedes[0]
filedes[0]
pipe
filedes[1]
filedes[1]
Unnamed Pipes


Reading from an empty pipe that has its write
end closed returns 0 to indicate EOF
Writing to a pipe when the read end has been
closed generates SIGPIPE. Ignoring the
signal, or returning from the handler causes
the corresponding write to return an error
with errno set to EPIPE
FIFOs (named pipes)
int mkfifo(const char *pathname, mode_t mode);



pathname is a valid pathname in the
filesystem
mode same as for open function
This only creates the pipe, we must still
call the open function on it
FIFOs (named pipes)

If O_NONBLOCK is specified



Open for read only returns immediately
Open for write only returns -1 with errno set to
ENXIO if no process has the FIFO open for reading
If O_NONBLOCK not specified


Open for read only blocks until another process
opens the FIFO for writing
Open for write only blocks until another process
opens the FIFO for reading
FIFOs (named pipes)

On most systems mkfifo calls mknod
int mknod(const char *pathname, mode_t mode, dev_t dev);

can create new files of type





S_IFIFO, S_IFBLK, S_IFCHR
Only root can create device files
Anyone can create FIFOs
dev_t contains major and minor numbers of
device file. This 3rd parameter is ignored for FIFOs
Ex: mknod(“myfifo”, S_IFIFO | 0666, 0);
popen / pclose
FILE *popen(const char *command, const char *type);
int pclose(FILE *stream);

popen



Creates a pipe, forks, closes un-needed
ends of pipe, execs a shell to run
command and waits for command to finish
type can be either “r” to read from child’s
stdout or “w” to write to child’s stdin
FILE* returned is the created pipe
popen / pclose

pclose closes standard I/O stream,
waits for the command to terminate
and returns the exit status of the shell