Linux Tutorial: POSIX Threads

Linux Tutorial: POSIX Threads

Thread Basics:

• Thread operations include thread creation, termination, synchronization (joins,blocking), scheduling, data management and process interaction.
• A thread does not maintain a list of created threads, nor does it know the thread that created it.
• All threads within a process share the same address space.
• Threads in the same process share:
  • Process instructions
  • Most data
  • open files (descriptors)
  • signals and signal handlers
  • current working directory
  • User and group id
• Each thread has a unique:
  • Thread ID
  • set of registers, stack pointer
  • stack for local variables, return addresses
  • signal mask
  • priority
  • Return value: errno
• pthread functions return "0" if OK.

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Thread Creation and Termination:

Example: pthread1.c

view source

print?

01	#include <stdio.h>

02	#include <stdlib.h>

03	#include <pthread.h>

04

05	void *print_message_function( void *ptr );

06

07

main()

08

{

09	pthread_t thread1, thread2;

10	char *message1 = "Thread 1";

11	char *message2 = "Thread 2";

12	int  iret1, iret2;

13

14	/* Create independent threads each of which will execute function */

15

16	iret1 = pthread_create( &thread1, NULL, print_message_function, (void*) message1);

17	iret2 = pthread_create( &thread2, NULL, print_message_function, (void*) message2);

18

19	/* Wait till threads are complete before main continues. Unless we  */

20	/* wait we run the risk of executing an exit which will terminate   */

21	/* the process and all threads before the threads have completed.   */

22

23	pthread_join( thread1, NULL);

24	pthread_join( thread2, NULL);

25

26	printf("Thread 1 returns: %d\n",iret1);

27	printf("Thread 2 returns: %d\n",iret2);

28

exit(0);

29

}

30

31	void *print_message_function( void *ptr )

32

{

33	char *message;

34	message = (char *) ptr;

35	printf("%s \n", message);

36

}

Compile:

• C compiler: cc -lpthread pthread1.c
  or
• C++ compiler: g++ -lpthread pthread1.c

Run: ./a.out
Results:

Thread 1
Thread 2
Thread 1 returns: 0
Thread 2 returns: 0

Details:

• In this example the same function is used in each thread. The arguments are different. The functions need not be the same.
• Threads terminate by explicitly calling pthread_exit, by letting the function return, or by a call to the function exit which will terminate the process including any threads.
• Function call: pthread_create - create a new thread

      int pthread_create(pthread_t * thread, 
                         const pthread_attr_t * attr,
                         void * (*start_routine)(void *), 
                         void *arg);
  

  Arguments:
  • thread - returns the thread id. (unsigned long int defined in bits/pthreadtypes.h)
  • attr - Set to NULL if default thread attributes are used. (else define members of the struct pthread_attr_t defined in bits/pthreadtypes.h) Attributes include:
    • detached state (joinable? Default: PTHREAD_CREATE_JOINABLE. Other option: PTHREAD_CREATE_DETACHED)
    • scheduling policy (real-time? PTHREAD_INHERIT_SCHED,PTHREAD_EXPLICIT_SCHED,SCHED_OTHER)
    • scheduling parameter
    • inheritsched attribute (Default: PTHREAD_EXPLICIT_SCHED Inherit from parent thread: PTHREAD_INHERIT_SCHED)
    • scope (Kernel threads: PTHREAD_SCOPE_SYSTEM User threads: PTHREAD_SCOPE_PROCESS Pick one or the other not both.)
    • guard size
    • stack address (See unistd.h and bits/posix_opt.h _POSIX_THREAD_ATTR_STACKADDR)
    • stack size (default minimum PTHREAD_STACK_SIZE set in pthread.h),
  • void * (*start_routine) - pointer to the function to be threaded. Function has a single argument: pointer to void.
  • *arg - pointer to argument of function. To pass multiple arguments, send a pointer to a structure