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.
Thread Creation and Termination:
Example: pthread1.c

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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