SIGNALS IN POSIX

An important POSIX® concept is the distinction between "threads" and "processes."

Threads are schedulable entities that run within a process. (Visit here if you missed our introduction to processes.) Each process will have one main thread, and it may also have several more threads which share the address space of the process invoked via pthread_create(). A POSIX system call will refer to a process ID or to a thread ID—these IDs are not interchangeable.

Note that some operating systems simply refer to "tasks" instead of distinguishing between "threads" and "processes." Such an operating system probably does not support the POSIX standard, and would not be able to run calls which refer to a process (such as signals).

How POSIX threads work

• A thread is a flow-of-control that runs within a process context.
• Threads in the same process share the same virtual address space. They can communicate and share data using globals.
• Threads are the running entities of LynxOS®.
• Threads do not have a parent-child relationship with each other.
• Any thread created within a process can terminate itself or can read the exit status of any other thread.
• Exception: If any thread calls exit(), the process terminates. All threads in the process are then terminated.
• Each process has its own protected address space. Communications between processes require kernel services.

LynuxWorks™ also supports kernel threads and has been awarded a patent for its unique approach to handling hardware events in a real-time environment. Visit here to read how LynxOS incorporates this technology.

POSIX thread-exit functions

Function	Main Thread	Other Threads
return(), } or pthread_exit()	All die	This thread dies
exit() or _exit()	All die	All die

 

"But is it POSIX-conformant?"

POSIX for embedded developers What is POSIX? POSIX.1, .1b and .1c POSIX Function Calls Reference Processes and Virtual Memory Parent-Child Processes Threads in POSIX Signals in POSIX LynxOS: POSIX-conformant RTOS POSIX PDF BROCHURE (170 kB) POSIX TRAINING: Real-time system course O'REILLY PROGRAMMING GUIDES for LynuxWorks operating systems POSIX Programmers Guide Pthreads Programming POSIX.4 Programmers Guide A savvy engineer just needs to ask a few quick questions about an operating system in order to determine if it might fully support the POSIX standards. "Does each process in the operating system reside in a different name space and have its own symbols table? "Does the operating system support the fork() call?" "Can the operating system distinguish between threads and processes?" "Does the operating system support signals?"

SIGNALS IN POSIX