Getting Started with QNX Neutrino - QNX Software Systems

Threads and processes
© 2009, QNX Software Systems GmbH & Co. KG.
Starting a thread
• When fork() does work with multiple threads, you’ll need to register a
pthread_atfork() handler and lock every single mutex before you fork,
complicating the design.
• The child of fork() duplicates all open file descriptors. As we’ll see in the Resource
Manager chapter later, this causes a lot of work — most of which will be
unnecessary if the child then immediately does an exec() and closes all the file
descriptors anyway.
The choice between vfork() and the spawn() family boils down to portability, and what
you want the child and parent to be doing. The vfork() function will pause until the
child calls exec() or exits, whereas the spawn() family of functions can allow both to
run concurrently. The vfork() function, however, is subtly different between operating
systems.
Now that we’ve seen how to start another process, let’s see how to start another thread.
Any thread can create another thread in the same process; there are no restrictions
(short of memory space, of course!). The most common way of doing this is via the
POSIX pthread_create() call:
#include
int
pthread_create (pthread_t *thread,
const pthread_attr_t *attr,
void *(*start_routine) (void *),
void *arg);
The pthread_create() function takes four arguments:
thread
attr
start_routine
arg
a pointer to a pthread_t where the thread ID is stored
an attributes structure
the routine where the thread begins
an argument passed to the thread’s start_routine
Note that the thread pointer and the attributes structure (attr) are optional — you can
pass them as NULL.
The thread parameter can be used to store the thread ID of the newly created thread.
You’ll notice that in the examples below, we’ll pass a NULL, meaning that we don’t
care what the ID is of the newly created thread. If we did care, we could do something
like this:
pthread_t tid;
pthread_create (&tid, ...
printf ("Newly created thread id is %d\n", tid);
36 Chapter 1 • Processes and Threads April 30, 2009