1. Programming with Threads
Topics
Shared variables
The need for synchronization
Synchronizing with semaphores
Thread safety and reentrancy
Races and deadlocks

2. Shared Variables in Threaded C Programs
Question: Which variables in a threaded C program are shared variables?
The answer is not as simple as “global variables are shared” and “stack variables are private”.
Requires answers to the following questions:
What is the memory model for threads?
How are variables mapped to memory instances?
How many threads reference each of these instances?

3. Threads Memory Model Conceptual model:
Each thread runs in the context of a process.
Each thread has its own separate thread context.
Thread ID, stack, stack pointer, program counter, condition codes, and general purpose registers.
All threads share the remaining process context.
Code, data, heap, and shared library code
Open files and installed handlers
Operationally, this model is not strictly enforced:
While register values are truly separate and protected....
Any thread can read and write the stack of any other thread.

4. Practice Problem
Problem 13.6, p. 871 (handout)

5. Shared Variable Analysis
Which variables are shared?
Variable Referenced by Referenced by Referenced by
instance main thread? peer thread 0? peer thread 1?
ptr yes yes yes
cnt no yes yes
i.m yes no no
msgs.m yes yes yes
myid.p0 no yes no
myid.p1 no no yes
ptr, cnt, and msgs are shared.
i and myid are not shared (i is passed by value)
demo sharing.c (on cs01)

6. Synchronization
An improperly synchronized program
badcnt.c (p. 872)

7. Synchronization An improperly synchronized program badcnt.c (p. 872)
examine count function in badcnt.s

8. Synchronization An improperly synchronized program badcnt.c (p. 872)
examine count function in badcnt.s
cnt++;
movl _cnt,%eax
incl %eax
movl %eax,_cnt

9. Synchronization An improperly synchronized program badcnt.c (p. 872)
examine count function in badcnt.s
compile without the force-mem option that forces memory operands to be copied to registers before performing arithmetic
cnt++;
movl _cnt,%eax
incl %eax
movl %eax,_cnt

10. Synchronization An improperly synchronized program badcnt.c (p. 872)
examine count function in badcnt.s
compile without the force-mem option that forces memory operands to be copied to registers before performing arithmetic
cnt++;
movl _cnt,%eax
incl %eax
movl %eax,_cnt
cnt++;
incl _cnt

11. Assembly Code for Counter Loop
C code for counter loop
Corresponding asm code
(gcc -fforce-mem)
for (i=0; i<NITERS; i++)
cnt++;
.L9:
movl -4(%ebp),%eax
cmpl $ ,%eax
jle .L12
jmp .L10
.L12:
movl _cnt,%eax # Load
incl %eax # Update
movl %eax,_cnt # Store
.L11:
leal 1(%eax),%edx
movl %edx,-4(%ebp)
jmp .L9
.L10:
Head (Hi)
Load cnt (Li)
Update cnt (Ui)
Store cnt (Si)
Tail (Ti)

12. Concurrent Execution
Any sequential interleaving is possible, but some orderings are incorrect
%eaxi is the contents of %eax in thread i’s context
H=Head T=Tail L=Load U=Update S=Store
i (thread)
instri
%eax1
%eax2
cnt 1 H1 - - 1 L1 - 1 U1 1 - 1 S1 1 - 1 2 H2 - - 1 2 L2 - 1 1 2 U2 - 2 1 2 S2 - 2 2 2 T2 - 2 2 1 T1 1 - 2 OK

13. Concurrent Execution
Incorrect ordering: two threads increment the counter, but the result is 1 instead of 2.
i (thread)
instri
%eax1
%eax2
cnt 1 H1 - - 1 L1 - 1 U1 1 - 2 H2 - - 2 L2 - 1 S1 1 - 1 1 T1 1 - 1 2 U2 - 1 1 2 S2 - 1 1 2 T2 - 1 1
Oops!

14. Concurrent Execution How about this ordering? H1 L1 H2 L2 U2 S2 U1 S1
i (thread)
instri
cnt
%eax1
%eax2

15. Concurrent Execution How about this ordering?L1 H2 L2 U2 S2 U1 S1 T1 T2 1 2 -
i (thread)
instri
cnt
%eax1
%eax2
We can clarify our understanding of concurrent
execution with the help of the progress graph

16. Progress Graphs A progress graph depicts the execution state
Thread 2
A progress graph depicts
the execution state
of concurrent threads.
Each axis corresponds to
the sequential order of
instructions in a thread.
Each point corresponds to
a possible execution state.
E.g., (L1, S2) denotes state
where thread 1 has
completed L1 and thread
2 has completed S2. T2
(L1, S2) S2 U2 L2 H2
Thread 1 H1 L1 U1 S1 T1

17. Trajectories in Progress GraphsL1 U1 S1 T1 H2 L2 U2 S2 T2
Thread 1
Thread 2
A trajectory is a sequence
of legal state transitions
that describes one possible
concurrent execution of
the threads.
Example:
H1, L1, U1, H2, L2,
S1, T1, U2, S2, T2

18. Critical Sections and Unsafe Regions
Thread 2
L, U, and S form a
critical section with
respect to the shared
variable cnt.
Instructions in critical
sections should
not be interleaved.
Sets of states where such
interleaving occurs
form unsafe regions. T2 S2
critical section U2
Unsafe region L2 H2
Thread 1 H1 L1 U1 S1 T1
critical section

19. Safe and Unsafe Trajectories
Thread 2
Def: A trajectory is safe
iff it doesn’t cross into
the unsafe region.
Claim: A trajectory is
correct iff it is safe.
Safe trajectory T2 S2
Unsafe region
Unsafe
trajectory
critical section U2 L2 H2
Thread 1 H1 L1 U1 S1 T1
critical section

20. Semaphores Question: How can we guarantee a safe trajectory?
We must synchronize the threads so that they never enter an unsafe state
Classic solution: Dijkstra's P and V operations on semaphores.
semaphore: non-negative integer synchronization variable.
Variable “s” indicates the number of resources available
P(s): [ while (s == 0) wait(); s--; ]
P(s) requests resource “s”
Dutch for "Proberen" (test)
V(s): [ s++; ]
V(s) releases resource “s”
Dutch for "Verhogen" (increment)
OS guarantees that operations between brackets [ ] are atomic operations (executed indivisibly).
Only one P or V operation at a time can modify ‘s’
When while loop in P terminates, only that P can decrement ‘s’

21. Safe Sharing with Semaphores
Here is how we would use P and V operations to synchronize the threads that update cnt.
/* Semaphore s is initially 1 */
/* Thread routine */
void *count(void *arg) {
int i;
for (i=0; i<NITERS; i++) {
P(s);
cnt++;
V(s); }
return NULL;

22. Safe Sharing With Semaphores
Thread 2 1
Provide mutually exclusive access to shared variable by surrounding critical section with P and V operations on semaphore
s (initially set to 1).
Semaphore invariant
creates a forbidden region
that encloses unsafe region and is never
touched by any trajectory. T2 1
V(s)
Forbidden region -1 -1 -1 -1 S2 -1 -1 -1 -1 U2
Unsafe region -1 -1 -1 -1 L2 -1 -1 -1 -1
P(s) 1 H2 1 H1
P(s) L1 U1 S1
V(s) T1
Initially
s = 1
Thread 1

23. Semaphore Wrappers /* Initialize semaphore sem to value */
/* pshared=0 if thread, pshared=1 if process */
void Sem_init(sem_t *sem, int pshared, unsigned int value) {
if (sem_init(sem, pshared, value) < 0)
unix_error("Sem_init"); }
/* P operation on semaphore sem */
void P(sem_t *sem) {
if (sem_wait(sem))
unix_error("P");
/* V operation on semaphore sem */
void V(sem_t *sem) {
if (sem_post(sem))
unix_error("V");

24. Demo: goodcnt.c (demo, cs01, not in text)

25. Signaling With Semaphores
producer
thread
consumer
thread
shared
buffer
Common synchronization pattern:
Producer waits for slot, inserts item in buffer, and “signals” consumer.
Consumer waits for item, removes it from buffer, and “signals” producer.
“signals” in this context have nothing to do with Unix signals
Examples
Multimedia processing:
Producer creates MPEG video frames, consumer renders the frames
Event-driven graphical user interfaces
Producer (person) creates mouse clicks, mouse movements, and keyboard hits and inserts corresponding events in buffer.
Consumer (computer) retrieves events from buffer and paints the display.

26. Examples Demo: Shared FIFO Buffer Demo: Concurrent Echo Server
sbuf.h/sbuf.c - shared FIFO buffer (p )
sbufdriver.c – test driver
Demo: Concurrent Echo Server
echoservert_pre.c (p. 884)
echo_cnt (p. 885)

27. Thread-Safe and Reentrant Functions
Functions called from a thread must be thread-safe.
A function is reentrant iff it
Accesses NO shared variables
Does not require synchronization operations (initialization)
A function can be thread-safe, but not reentrant
A thread-safe application can have shared variables
But uses semaphores to access shared variables
Less efficient than reentrant implementation
Reentrant
functions
All functions
Thread-unsafe
Thread-safe

28. Thread Safety Functions that are thread-unsafe include functions that
fail to protect shared variables
rely on persistent state across invocations
return a pointer to a static variable
call thread-unsafe functions

29. Thread-Unsafe Functions
Failing to protect shared variables.
Fix: Use P and V semaphore operations.
Issue: Synchronization operations will slow down code.
Example: goodcnt.c

30. Thread-Unsafe Functions
Relying on persistent state across multiple function invocations.
Random number generator relies on static state
Fix: Rewrite function so that caller passes in all necessary state.
// rand - return pseudo-random integer on
static unsigned int next = 1;
int rand(void) {
next = (next * ) ;
return (unsigned int)(next/65536) % 32768; }
// srand - set seed for rand()
void srand(unsigned int seed) {
next = seed;
srand(20);
int x = rand();

31. Thread-Unsafe Functions
Relying on persistent state across multiple function invocations.
Random number generator relies on static state
Fix: Rewrite function so that caller passes in the necessary state.
// rand - return pseudo-random integer on
int rand(unsigned int *next) {
*next = (*next * ) ;
return (unsigned int)(*next/65536) % 32768; }
int seed = 20;
int x = rand(&seed);

32. Thread-Unsafe Functions
Returning a ptr to a static variable.
Fixes:
Reentrant version, caller passes pointer to buf.
Issue: Requires changes in caller and callee.
Thread-safe version, Lock-and-copy wrapper
Issue: caller must free memory.
char *asctime(struct tm *CLOCK) {
static char buf[50];
...
return buf; }
// see ‘man asctime’
buf = Malloc(...);
asctime_r(&tm, buf);
char *asctime_t(struct tm *CLOCK) {
char *buf = Malloc(...);
P(&mutex);
char *p = asctime(tm);
strcpy(buf, p);
V(&mutex);
return buf; }

33. Thread-Unsafe Functions
Calling thread-unsafe functions.
Calling one thread-unsafe function makes an entire function thread-unsafe.
Fix: Modify the function so it calls only thread-safe functions

34. Thread-Safe Library Functions
Most functions in the Standard C Library (at the back of your K&R text) are thread-safe.
Examples: malloc, free, printf, scanf
Exceptions: asctime, ctime, gmtime, localtime
These four functions return pointers to static objects that may be overwritten by other calls.
Exceptions: rand
Reentrant versions: asctime_r, rand_r
Most Unix system calls are thread-safe, with a few exceptions:
Thread-unsafe function Reentrant version
gethostbyaddr gethostbyaddr_r
gethostbyname gethostbyname_r
inet_ntoa (none)

35. Races
A race occurs when the correctness of the program depends on one thread reaching point ‘x’ before another thread reaches point ‘y’. (demo, run several times)
// race between creating next thread and
// accessing variable “i”
int main() {
pthread_t tid[N];
int i;
for (i = 0; i < N; i++)
Pthread_create(&tid[i], NULL, thread, &i);
Pthread_join(tid[i], NULL);
exit(0); }
// thread routine
void *thread(void *vargp) {
int myid = *((int *)vargp);
printf("Hello from thread %d\n", myid);
return NULL;

36. Deadlocks Locking introduces the potential for deadlock:
waiting for a condition that will never be true.
Any trajectory that enters
the deadlock region will
eventually reach the
deadlock state, waiting for either s or t to become nonzero.
Other trajectories luck out and skirt the deadlock region.
Thread 2
V(a)
deadlock
state
forbidden
region for a
V(b)
P(a)
deadlock
region
forbidden
region for b
P(b)
P(a)
P(b)
V(a)
V(b)
Thread 1
Initially, a = b= 1

37. Threads Summary
Threads provide another mechanism for writing concurrent programs.
Threads are growing in popularity
Somewhat cheaper than processes
Easy to share data between threads
However, the ease of sharing has a cost:
Easy to introduce subtle synchronization errors
Tread carefully with threads!
For more info:
D. Butenhof, “Programming with Posix Threads”, Addison-Wesley, 1997.