Getting Started with QNX Neutrino - QNX Software Systems

Threads and processes
© 2009, QNX Software Systems GmbH & Co. KG.
If you want to adjust the priority of a program before you start it, you could use the
nice command, just like in UNIX:
$ nice program3
This instructs the shell to start program3 at a reduced priority.
Or does it?
If you look at what really happens, we told the shell to run a program called nice at
the regular priority. The nice command adjusted its own priority to be lower (this is
where the name “nice” comes from), and then it ran program3 at that lower priority.
Starting a process from within a program
You don’t usually care about the fact that the shell creates processes — this is a basic
assumption about the shell. In some application designs, you’ll certainly be relying on
shell scripts (batches of commands in a file) to do the work for you, but in other cases
you’ll want to create the processes yourself.
For example, in a large multi-process system, you may want to have one master
program start all the other processes for your application based on some kind of
configuration file. Another example would include starting up processes when certain
operating conditions (events) have been detected.
Let’s take a look at the functions that Neutrino provides for starting up other processes
(or transforming into a different program):
• system()
• exec() family of functions
• spawn() family of functions
• fork()
• vfork()
Which function you use depends on two requirements: portability and functionality.
As usual, there’s a trade-off between the two.
The common thing that happens in all the calls that create a new process is the
following. A thread in the original process calls one of the above functions.
Eventually, the function gets the process manager to create an address space for a new
process. Then, the kernel starts a thread in the new process. This thread executes a few
instructions, and calls main(). (In the case of fork() and vfork(), of course, the new
thread begins execution in the new process by returning from the fork() or vfork();
we’ll see how to deal with this shortly.)
Starting a process with the system() call
The system() function is the simplest; it takes a command line, the same as you’d type
it at a shell prompt, and executes it.
28 Chapter 1 • Processes and Threads April 30, 2009