Working with the Unix OS

Processes I
dup (and other similar calls) copy a file descriptor to
- the designated fd entry
- the lowest free fd entry
dup2 does the former;
dup2 - used for redirecting I/O (stdin, stdout) of a process to/from:
- a file (implements '')
- a pipe (1mplements '|')
NOTE: stdin defined as 0, so identifying fd1 of open file table.
int pfd [2];
pipe (pfd);
if (fork()!= 0) { /* parent */
close (pfd [0]); /* close the reading end */
if (fork()!= 0) { /* parent still */
close(pfd[l]); /* parent closes the writing end */
} else { /* 2nd child */
close(l); /* close stdout */
dup2(pfd[l],l); /* copy the writing end over stdout */
close(pfd[l]); /* close the original writing end */
exec(foo); /* execute the utility writing to stdout */
}
} else { /* first child */
close(pfd[l]); /* close the writing end */
close(0); /* close stdin */
dup2(pfd[0], 0); /* copy reading end over stdin */
close(pfd[0]); /* close the original reading end */
exec (utility); /* utility will read from stdin */
}
Bi-directional pipes ?
two results:
- short circuit (Pl will read back from pfd[0] its own data just written to pfd[l])
- possibility of deadlock or looping (both processes):
while not eof pfd[0] {
read pfd[0];
process data;
}
close pfd[l];
Solution: Use 2 pipes, treat them as simplex channels.
File Subsystem
- Manages files
- Allocates file space
- Administers free space
- Controls access to files
- Retrieves data for users
Processes interact with F.S. by a set of system calls
- open
- close
- read
- write
- stat (Query attributes of a file)
- chrnod (change access permissions)
Process control subsystem
- Process synchronization
- Inter-process communication
- Memory management
- Process scheduling
P.C.S system calls
- fork
- exec
- exit
- wait
- brk
- signal
Memory Management Module
- Controls allocation of memory
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