Thread Linux

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.

Thread Creation and Termination:

Example: pthread1.c

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

void *print_message_function( void *ptr );

main()
{
     pthread_t thread1, thread2;
     const char *message1 = "Thread 1";
     const char *message2 = "Thread 2";
     int  iret1, iret2;

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

     iret1 = pthread_create( &thread1, NULL, print_message_function, (void*) message1);
     if(iret1)
     {
         fprintf(stderr,"Error - pthread_create() return code: %d\n",iret1);
         exit(EXIT_FAILURE);
     }

     iret2 = pthread_create( &thread2, NULL, print_message_function, (void*) message2);
     if(iret2)
     {
         fprintf(stderr,"Error - pthread_create() return code: %d\n",iret2);
         exit(EXIT_FAILURE);
     }

     printf("pthread_create() for thread 1 returns: %d\n",iret1);
     printf("pthread_create() for thread 2 returns: %d\n",iret2);

     /* Wait till threads are complete before main continues. Unless we  */
     /* wait we run the risk of executing an exit which will terminate   */
     /* the process and all threads before the threads have completed.   */

     pthread_join( thread1, NULL);
     pthread_join( thread2, NULL); 

     exit(EXIT_SUCCESS);
}

void *print_message_function( void *ptr )
{
     char *message;
     message = (char *) ptr;
     printf("%s \n", message);
}

Compile:

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

The GNU compiler now has the command line option "-pthread" while older versions of the compiler specify the pthread library explicitly with "-lpthread".

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.
• Function call: pthread_join - wait for termination of another thread

      int pthread_join(pthread_t th, void **thread_return);
  

  Arguments:
  • th - thread suspended until the thread identified by th terminates, either by calling pthread_exit() or by being cancelled.
  • thread_return - If thread_return is not NULL, the return value of th is stored in the location pointed to by thread_return.
• Function call: pthread_exit - terminate the calling thread

      void pthread_exit(void *retval);
  

  Arguments:
  • retval - Return value of pthread_exit().
  This routine kills the thread. The pthread_exit() function never returns. If the thread is not detached, the thread id and return value may be examined from another thread by using pthread_join(). 
  Note: the return pointer *retval, must not be of local scope otherwise it would cease to exist once the thread terminates.
• [C++ pitfalls]: The above sample program will compile with the GNU C and C++ compiler g++. The following function pointer representation below will work for C but not C++. Note the subtle differences and avoid the pitfall below:

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

Thread Synchronization:

The threads library provides three synchronization mechanisms:

• mutexes - Mutual exclusion lock: Block access to variables by other threads. This enforces exclusive access by a thread to a variable or set of variables.
• joins - Make a thread wait till others are complete (terminated).
• condition variables - data type pthread_cond_t

sumber : http://www.yolinux.com/TUTORIALS/LinuxTutorialPosixThreads.html