Threads and processes
When building an application (realtime, embedded, graphical, or otherwise), the developer may want several algorithms within the application to execute concurrently. This concurrency is achieved by using the POSIX thread model, which defines a process as containing one or more threads of execution.
A thread can be thought of as the minimum unit of
execution,
the unit of scheduling and execution in
the microkernel. A process, on the other hand, can be
thought of as a container
for threads,
defining the address space
within which
threads will execute. A process will always contain at least
one thread.
Depending on the nature of the application, threads might execute independently with no need to communicate between the algorithms (unlikely), or they might need to be tightly coupled, with high-bandwidth communications and tight synchronization. To assist in this, the QNX OS provides many IPC and synchronization services.
The following pthread_* (POSIX Threads) library calls don't involve any microkernel thread calls:
- pthread_attr_destroy()
- pthread_attr_getdetachstate()
- pthread_attr_getinheritsched()
- pthread_attr_getschedparam()
- pthread_attr_getschedpolicy()
- pthread_attr_getscope()
- pthread_attr_getstackaddr()
- pthread_attr_getstacksize()
- pthread_attr_init()
- pthread_attr_setdetachstate()
- pthread_attr_setinheritsched()
- pthread_attr_setschedparam()
- pthread_attr_setschedpolicy()
- pthread_attr_setscope()
- pthread_attr_setstackaddr()
- pthread_attr_setstacksize()
- pthread_cleanup_pop()
- pthread_cleanup_push()
- pthread_equal()
- pthread_getspecific()
- pthread_setspecific()
- pthread_key_create()
- pthread_key_delete()
- pthread_self()
| POSIX call | Microkernel call | Description |
|---|---|---|
| pthread_create() | ThreadCreate() | Create a new thread of execution |
| pthread_exit() | ThreadDestroy() | Destroy a thread |
| pthread_detach() | ThreadDetach() | Detach a thread so it doesn't need to be joined |
| pthread_join() | ThreadJoin() | Join a thread waiting for its exit status |
| pthread_cancel() | ThreadCancel() | Cancel a thread at the next cancellation point |
| N/A | ThreadCtl() | Change a thread's QNX OS-specific thread characteristics |
| pthread_mutex_init() | SyncTypeCreate() | Create a mutex |
| pthread_mutex_destroy() | SyncTypeDestroy() | Destroy a mutex |
| pthread_mutex_lock() | SyncMutexLock() | Lock a mutex |
| pthread_mutex_trylock() | SyncMutexLock() | Conditionally lock a mutex |
| pthread_mutex_unlock() | SyncMutexUnlock() | Unlock a mutex |
| pthread_cond_init() | SyncTypeCreate() | Create a condition variable |
| pthread_cond_destroy() | SyncTypeDestroy() | Destroy a condition variable |
| pthread_cond_wait() | SyncCondvarWait() | Wait on a condition variable |
| pthread_cond_signal() | SyncCondvarSignal() | Signal a condition variable |
| pthread_cond_broadcast() | SyncCondvarSignal() | Broadcast a condition variable |
| pthread_getschedparam() | SchedGet() | Get the scheduling parameters and policy of a thread |
| pthread_setschedparam(), pthread_setschedprio() | SchedSet() | Set the scheduling parameters and policy of a thread |
| pthread_sigmask() | SignalProcmask() | Examine or set a thread's signal mask |
| pthread_kill() | SignalKill() | Send a signal to a specific thread |
The OS can be configured to provide a mix of threads and processes (as defined by POSIX). Each process is MMU-protected from other processes, and each process may contain one or more threads that share the process's address space.
The environment you choose affects not only the concurrency capabilities of the application, but also the IPC and synchronization services the application might make use of.
IPCrefers to communicating processes, we use it here to describe the communication between threads, whether they're within the same process or separate processes.
For information about processes and threads from the programming point of view, see the Processes and Threads chapter of Getting Started with the QNX OS, and the Programming Overview and Processes chapters of the QNX OS Programmer's Guide.