1. Thread Programming

2. Topics Thread Thread NPTL thread library commands Thread Safety
Thread libraries
Threading issues
Linux Threads

3. 4 Reasons for threads Express logically concurrent tasks
Run work in the background
Exploit multiple processors
Manage I/O devices

5. Process (fork) vs Thread creation
Fork() is relatively expensive to launch
Difficult to share information between processes
Faster creation (10x or better..
Sharing information is easy.. Just copy information into a shared heap.
-> That requires synchronization between threads

6. Thread Advantages Sharing data between threads is easier
Thread Creation is faster
Used extensively in Operating Systems

7. Thread Disadvantages
First let’s look at threads in memory space

8. Stack and Stack Frames square (int x) STACK doCalc(int val)
{ return x*x }
doCalc(int val)
{ printf (“square is %d\n”, square(val) }
main (int x, int y)
{ key = 9999
doCalc (key) }
STACK
Frames for C run-time start up functions
Frame for doCalc
Frame for square
Stack for main

10. 4 Threads executing in process in linux
argv, environ
Stack for main thread
Stack for thread 3
Stack for thread 2
Stack for thread 1
Shared libraries, shared memory
Code is re-entrant
Heap
Uninitialized Data (BSS)
Initialized Data
Thread 3 executing here
Text (program code)
Main thread executing here
Thread 1 executing here
Thread 2 executing here

11. Threads share Process ID and parent process ID Controlling terminal
Credentials
Open files and locks
Resource limits
CPU times consumed.

12. Attributes distinct for Threads
Thread ID
Thread-specific data
Real-time scheduling policy
CPU affinity
Stack (local variables and function call linkage)
Processor registers in switc

13. Thread resource Shared State Per-Thread State Per-Thread State
Thread Control Block
Heap
Thread Control Block
Stack Information
Stack Information
Saved Registers
Saved Registers
Global Variables
Thread Metadata
Thread Metadata
Code
Thread Stack
Thread Stack

14. Thread Lifecycle

15. Thread Safety
A function is said to be thread-safe if it can be safely invoked by multiple threads at the same time.
Conversely, if a function is not thread-safe, then we can’t call it from one thread while it is being executed in another thread.

16. Two threads call yield

17. Thread Disadvantages Must ensure thread safety
Bug in one thread can exhibit itself in ALL threads
Threads are competing for finite virtual address space.

18. Compiling Thread Programs
On Linux compile with the -pthread option.
The effects include
_REENTRANT preprocessor macro is defined.
Links with libpthread lib

19. Creating Threads Thread points to buffer for unique thread ID
#include <pthread.h>
int pthread_create( pthread_t *thread.
const pthread_attr_t *attr,
void *(*start_routine)(void *),
void *arg));
Thread points to buffer for unique thread ID
If attr is NULL, the default attributes are used.
Upon its creation, the thread executes start_routine, with arg as its sole argument.

20. Thread ID
#include <pthread.h>
pthread_t pthread_self(void));
Each thread within a process is uniquely identified by the Thread Id
This is returned on pthread_create()

21. Thread Termination
#include <pthread.h>
void pthread_exit(void *retval));
The thread’s start function returns with return value
Thread calls pthread_exit()
Thread is cancelled by external program
Main thread performs a return (causes all threads to terminate immediately)

22. Thread Cancellation
#include <pthread.h>
void pthread_cancel(pthread_t thread));
Threads can protect themselves from cancellation by setting its flag.
When cancellation is enabled, it is done only when the thread reaches its next cancellation point.

23. Joining with a Terminated Thread
#include <pthread.h>
int pthread_join(pthread_ t thread, void **retval);
Returns 0 on success or a positive error number on error
Waits for the thread to terminate.

24. Detaching a Thread When we don’t care to wait for a thread to finish.
#include <pthread.h>
int pthread_detach (pthread_ t thread);
When we don’t care to wait for a thread to finish.

25. Create Threads example

26. To see thread information ps -m

27. ps m –o pid,tid,command