1. Recitation 7 (Oct. 25) Outline Minglong Shao Office hours: Reminders
Exceptions
Process
Signals
Non-local jumps
Reminders
Lab4:
Due Thursday
Minglong Shao
Office hours:
Thursdays 5-6PM
Wean Hall 1315

2. Exceptional control flow (ECF)
Abrupt changes in the control flow
React to changes in system state that are not captured by internal program variables and are not necessarily related to the execution of the program
Happens at all levels of a computer system
Exceptions
Concurrent processes
Signals
Non-local jumps

3. Exceptions Interrupt (asynchronous exceptions) Traps Faults Aborts
I/O interrupt, hardware reset, software reset, etc.
Traps
System calls, breakpoint traps, etc.
Faults
Page fault, protection fault, etc.
Aborts
Parity error, machine check, etc.

4. Process concept An instance of running program
Multiple processes run “concurrently” by time slicing
Context switching
Control flow passes from one process to another
Preemptive scheduler of OS

5. Process IDs & process groups
A process has its own, unique process ID
pid_t getpid();
A process belongs to exactly one process group
pid_t getpgrp();
A new process belongs to which process group?
Its parent’s process group
A process can make a process group for itself and its children
pid_t pid = getpid();
setpgid(0, 0);
getpgrp()

6. Create a new process int fork(void) Test your understanding…
Create a new process that is identical to the parent process
Return 0 to child process
Return child’s pid to the parent process
Call once, return twice
Test your understanding…
Problem 1

7. Problem 1 #include <unistd.h> #include <stdio.h>
int cnt = 0;
int main(void) {
if (fork() == 0){
cnt ++;
fork();
cnt++; }
printf("%d", cnt);
return 0;
Possible output:
133
313
331

8. Reaping child process Child process becomes zombie when terminates
Still consume system resources
Parent performs reaping on terminated child
pid_t wait(int *status)
pid_t waitpid(pid_t pid, int *status, int options)
Straightforward for reaping a single child
Tricky for Shell implementation!
Multiple child processes
Both foreground and background
Practice time again: problem 2

9. Problem 2 int main(){ int status; int counter = 1; if (fork() == 0){
printf("%d", counter);
}else {
printf("9");
counter --;
exit(0);
if (wait(&status) > 0){
printf("6"); }
printf("8");
Answers:
A. Y
B. N
C. Y
D. N
E. Y

10. Signals Section 8.5 in text
Read at least twice … really!
A signal tells our program that some event has occurred
Can we use signals to count events? No
Why? Signals not queued!!

11. Important signals (Fig 8.23)
SIGINT
Interrupt signal from terminal (ctrl-c)
SIGTSTP
Stop signal from terminal (ctrl-z)
SIGCHLD
A child process has stopped or terminated

12. Signals: sending other events OS Kernel blocked pending OS procedure
Process 1
Process 2
blocked
kill(pid, SIGINT)
pending 1
OS procedure
other events
divide by zero: SIGFPE
ctrl-c: SIGINT
child process exit: SIGCHLD
OS Kernel

13. Signals: receiving Check when schedule the process to run OS Kernel
blocked
pending 1
OS procedure
OS Kernel

14. Receiving a signal An example: problem 3 Default action
The process terminates [and dumps core]
The process stops until restarted by a SIGCONT signal (ctrl-z)
The process ignore the signal
Can modify (additional action)
“Handle the signal” -- install signal handler
void sigint_handler(int sig);
signal(SIGINT, sigint_handler);
An example: problem 3

15. Problem 3 void handler(int sig){ static int beeps = 0; printf("YO\n");
if (++beeps < 2)
alarm(1); /* next SIGALRM will be delivered in 1s */
else{
printf("MA\n");
kill(getpid(), SIGKILL); }
int main(){
Signal(SIGALRM, handler);
while (1) ;
printf(" is Great!\n");
return 0;
1. Output: YO MA
2. The program will terminate

16. Signals not queued Output: sent SIGUSR2 to parent counter = 1
int counter = 0;
void handler(int sig) {
counter++;
sleep(1);
return; }
int main()
int i;
signal(SIGUSER2, handler);
if (fork() == 0){
for (i = 0; i < 5; i++){
kill(getppid(), SIGUSR2);
printf(“sent SIGUSR2 to parent\n”);
exit(0);
wait(NULL);
printf(“counter = %d\n”, counter);
Output:
sent SIGUSR2 to parent
counter = 1

17. Race hazard
A data structure is shared by two pieces of code that can run concurrently
Different behaviors of program depending upon how the schedule interleaves the execution of code.

18. An example of race hazard
sigchld_handler() {
pid = waitpid(…);
deletejob(pid); }
eval() {
pid = fork();
if(pid == 0)
{ /* child */
execve(…);
/* parent */
/* signal handler may run BEFORE addjob()*/
addjob(…);

19. An okay schedule time Shell Signal Handler Child fork() addjob()
execve()
exit()
sigchld_handler()
deletejobs()

20. A problematic schedule
time
Shell
Signal Handler
Child
fork()
execve()
exit()
sigchld_handler()
deletejobs()
addjob()
Job added to job list after the signal handler tried to delete it!

21. Solution: blocking signals
sigchld_handler() {
pid = waitpid(…);
deletejob(pid); }
eval() {
sigprocmask(SIG_BLOCK, …)
pid = fork();
if(pid == 0)
{ /* child */
sigprocmask(SIG_UNBLOCK, …)
execve(…);
/* parent */
/* signal handler might run BEFORE addjob() */
addjob(…);
More details (page 633)

22. Non-local jump int setjmp(jmp_buf env)
Must called before longjmp
Stores current register context, stack pointer, and PC in the jump buffer env
First called, return 0 if no error
void longjmp(jmp_buf env, int i)
Restores register context from jump buffer env
Jumps back to where previous setjmp is called, behaves like setjmp just completes. But this time it returns i, not 0
Can only jump to an active context
A function that has been called but not yet completed

23. Non-local jump (cont) Let’s see an example: problem 4
int sigsetjmp(jmp_buf env)
Also saves blocked signals
void siglongjmp(jmp_buf env, int i)
Restores blocked signals besides others
Let’s see an example: problem 4

24. Problem 4 jmp_buf stuff; jmp_buf more_stuff; int bar(){
if (setjmp(more_stuff) == 0)
return 3;
else
return 6; }
int foo(){
char c = getc(stdin);
if (c == 'x')
longjmp(stuff, 42);
else if (c == 'y')
longjmp(more_stuff, 17);
return bar();
return -1;
int main() {
int n;
n = setjmp(stuff);
while ((n+=foo())<0)
sleep(1);
printf("%d\n", n); }
Answer:
1. 3
2. 45
3. 45
4. ?
5. ?

25. Summary Process Reaping child processes Signals Non-local jumps
fork(), waitpid(), execl() (and variant)
Reaping child processes
Signals
signal(), install handler, signals not queued
Non-local jumps
Check man page to understand the system calls better
man waitpid (fork, signal, …)
Read test book!