1. CS 3214 Computer Systems
Lecture 13
Godmar Back

2. Announcements Project 3 milestone due Oct 8
Have still not heard from everybody regarding SVN
Exercise 6 handed out
CS 3214 Fall 2010

3. Part 1
Threads and Processes
CS 3214 Fall 2010

4. A Context Switch Scenario
Timer interrupt: P1 is preempted, context switch to P2
I/O device interrupt: P2’s I/O complete switch back to P2
Process 1
Process 2
user mode
kernel mode
System call: (trap): P2 starts I/O operation, blocks context switch to process 1
Timer interrupt: P2 still has
time left, no context switch
Kernel
CS 3214 Fall 2010

5. Bottom Up View: Exceptions
An exception is a transfer of control to the OS in response to some event (i.e., change in processor state)
User Process OS
event
current
exception
next
exception processing
by exception handler
exception
return (optional)
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6. Reasoning about Processes: Process States
RUNNING
READY
BLOCKED
Process
must wait
for event
Event arrived
Scheduler
picks process
preempted
Only 1 process (per CPU) can be in RUNNING state
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7. User View
If process’s lifetimes overlap, they are said to execute concurrently
Else they are sequential
Default assumption is concurrently
Exact execution order is unpredictable
Programmer should never make any assumptions about it
Any interaction between processes must be carefully synchronized
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8. fork() #include <unistd.h> #include <stdio.h> int main() {
int x = 1;
if (fork() == 0) {
// only child executes this
printf("Child, x = %d\n", ++x);
} else {
// only parent executes this
printf("Parent, x = %d\n", --x); }
// parent and child execute this
printf("Exiting with x = %d\n", x);
return 0;
fork()
Child, x = 2
Exiting with x = 2
Parent, x = 0
Exiting with x = 0
CS 3214 Fall 2010

9. The fork()/join() paradigm
After fork(), parent & child execute in parallel
Unlike a fork in the road, here we take both roads
Used in many contexts
In Unix, ‘join()’ is called wait()
Purpose:
Launch activity that can be done in parallel & wait for its completion
Or simply: launch another program and wait for its completion (shell does that)
Parent:
fork()
Parent
process
executes
Child
process
executes
Child
process
exits
Parent: join()
OS notifies
CS 3214 Fall 2010

10. fork() #include <sys/types.h> #include <unistd.h>
#include <stdio.h>
int main(int ac, char *av[]) {
pid_t child = fork();
if (child < 0)
perror(“fork”), exit(-1);
if (child != 0) {
printf ("I'm the parent %d, my child is %d\n",
getpid(), child);
wait(NULL); /* wait for child (“join”) */
} else {
printf ("I'm the child %d, my parent is %d\n",
getpid(), getppid());
execl("/bin/echo", "echo", "Hello, World", NULL); }
CS 3214 Fall 2010

11. fork() vs. exec() fork(): exec():
Clone most state of parent, including memory
Inherit some state, e.g. file descriptors
Keeps program, changes process
Called once, returns twice
exec():
Overlays current process with new executable
Keeps process, changes program
Called once, does not return (if successful)
CS 3214 Fall 2010

12. exit(3) vs. _exit(2) exit(3) destroys current processes
OS will free resources associated with it
E.g., closes file descriptors, etc. etc.
Can have atexit() handlers
_exit(2) skips them
Exit status is stored and can be retrieved by parent
Single integer
Convention: exit(EXIT_SUCCESS) signals successful execution, where EXIT_SUCCESS is 0
CS 3214 Fall 2010

13. wait() vs waitpid() int wait(int *status)
Blocks until any child exits
If status != NULL, will contain value child passed to exit()
Return value is the child pid
Can also tell if child was abnormally terminated
int waitpid(pid_t pid, int *status, int options)
Can say which child to wait for
CS 3214 Fall 2010

14. If multiple children completed, wait() returns them in arbitrary order
Can use macros WIFEXITED and WEXITSTATUS to get information about exit status
void fork10() {
pid_t pid[N];
int i;
int child_status;
for (i = 0; i < N; i++)
if ((pid[i] = fork()) == 0)
exit(100+i); /* Child */
for (i = 0; i < N; i++) {
pid_t wpid = wait(&child_status);
if (WIFEXITED(child_status))
printf("Child %d terminated with exit status %d\n",
wpid, WEXITSTATUS(child_status));
else
printf("Child %d terminate abnormally\n", wpid); }
Wait Example
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15. Observations on fork/exit/wait
Process can have many children at any point in time
Establishes a parent/child relationship
Resulting in a process tree
Zombies: processes that have exited, but their parent hasn’t waited for them
“Reaping a child process” – call wait() so that zombie’s resources can be destroyed
Orphans: processes that are still alive, but whose parent has already exited (without waiting for them)
Become the child of a dedicated process (“init”) who will reap them when they exit
“Run Away” processes: processes that (unintentionally) execute an infinite loop and thus don’t call exit() or wait()
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16. Unix File Descriptors Unix provides a file descriptor abstraction
File descriptors are
Small integers that have a local meaning within one process
Can be obtained from kernel
Several functions create them, e.g. open()
Can refer to various kernel objects (not just files)
Can be passed to a standard set of functions:
read, write, close, lseek, (and more)
Can be inherited when a process forks a child
CS 3214 Fall 2010

17. Examples 0-2 are initially assigned int fd = open(“file”, O_RDONLY);
0 – stdin
1 – stdout
2 – stderr
But this assignment is not fixed – process can change it via syscalls
int fd = open(“file”, O_RDONLY);
int fd = creat(“file”, 0600);
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18. Implementing I/O Redirection
dup and dup2() system call
pipes: pipe(2)
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19. dup2 #include <stdio.h> #include <stdlib.h>
// redirect stdout to a file
int
main(int ac, char *av[]) {
int c;
int fd = creat(av[1], 0600);
if (fd == -1)
perror("creat"), exit(-1);
if (dup2(fd, 1) == -1)
perror("dup2"), exit(-1);
while ((c = fgetc(stdin)) != EOF)
fputc(c, stdout); }
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20. The Big Picture user view kernel view Process 1 1 2 Terminal Device 31 2
Terminal
Device 3
Open File
File
Descriptor x
open(“x”) 4
File
Descriptor
open(“x”)
close(4)
Process 2 1 2 3
dup2(3,0)
CS 3214 Fall 2010

21. Reference Counting
Multiple file descriptors may refer to same open file
Within the same process:
fd = open(“file”); fd2 = dup(fd);
Across anchestor processes:
fd = open(“file”); fork();
But can also open a file multiple times:
fd = open(“file”); fd2 = open(“file”);
In this case, fd and fd2 have different read/write offsets
In both cases, closing fd does not affect fd2
Reference Counting at 2 Levels:
Kernel keeps track of how many processes refer to a file descriptor –fork() and dup() may add refs
And keeps track of how many file descriptors refer to open file
close(fd) removes reference in current process
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22. Practical Implications
Number of simultaneously open file descriptors per process is limited
1024 on current Linux, for instance
Must make sure fd’s are closed
Else ‘open()’ may fail
Number space is reused
“double-close” error may inadvertently close a new file descriptor assigned the same number
CS 3214 Fall 2010

23. IPC via “pipes”
Fixed Capacity Buffer
write()
read()
A bounded buffer providing a stream of bytes flowing through
Properties
Writer() can put data in pipe as long as there is space
If pipe() is full, writer blocks until reader reads()
Reader() drains pipe()
If pipe() is empty, readers blocks until writer writes
Classic abstraction
Decouples reader & writer
Safe – no race conditions
Automatically controls relative progress – if writer produces data faster than reader can read it, it blocks – and OS will likely make CPU time available to reader() to catch up. And vice versa.
CS 3214 Fall 2010

24. pipe Note: there is no race condition in
int main() {
int pipe_ends[2];
if (pipe(pipe_ends) == -1)
perror("pipe"), exit(-1);
int child = fork();
if (child == -1)
perror("fork"), exit(-1);
if (child == 0) {
char msg[] = { "Hi" };
close(pipe_ends[0]);
write(pipe_ends[1], msg, sizeof msg);
} else {
char bread, pipe_buf[128];
close(pipe_ends[1]);
printf("Child said "); fflush(stdout);
while ((bread = read(pipe_ends[0], pipe_buf, sizeof pipe_buf)) > 0)
write(1, pipe_buf, bread); }
pipe
Note: there is no race condition in
this code. No matter what the scheduling order is, the message sent
by the child will reach the parent.
CS 3214 Fall 2010

25. esh – extensible shell Open-ended assignment
Encourage collaborative learning
Run each other’s plug-ins
Does not mean collaboration on your implementation
Secondary goals:
Exposure to yacc/lex and exposure to OO-style programming in C
CS 3214 Fall 2010

26. Using the list implementation
list_entry(e, struct esh_command, elem)
struct esh_pipeline: ….
struct list commands
struct esh_command: ….
struct list_elem elem;
struct list_elem *next;
struct list_elem *prev;
struct esh_command: ….
struct list_elem elem;
struct list_elem *next;
struct list_elem *prev;
struct list_elem head
struct list_elem tail
struct list_elem *next;
struct list_elem *prev;
struct list_elem *next;
struct list_elem *prev;
Key features: “list cell” – here call ‘list_elem’ is embedded in each object being kept in list
Means you need 1 list_elem per list you want to keep an object in
CS 3214 Fall 2010