In our initial discussion on Thread Creation earlier in this chapter, we saw the pthread_create(3) API; the second parameter is a pointer to the thread attribute structure: const pthread_attr_t *attr. We mentioned there that passing NULL here, in effect, has the library create a thread with default attributes. While that is indeed the case, the problem is that, for truly portable applications, this is not good enough. Why? Because the default thread attributes actually differ quite widely from implementation to implementation. The right way-specify the thread attributes explicitly at thread creation time.
Firstly, of course, we need to learn what attributes a pthread has. The following table enumerates this:
|
Attribute |
Meaning |
APIs: pthread_attr_[...](3) |
Values Possible |
Linux Default |
|
Detach state |
Create threads as joinable or detached |
pthread_attr_ |
PTHREAD_CREATE_JOINABLE |
PTHREAD_CREATE_JOINABLE |
|
Scheduling/contention scope |
Set of threads against which we compete for resources (CPU) |
pthread_attr_ [get|set]scope |
PTHREAD_SCOPE_SYSTEM |
PTHREAD_SCOPE_SYSTEM |
|
Scheduling/inheritance |
Determines whether scheduling attributes are inherited implicitly from calling a thread or explicitly from the attr structure |
pthread_attr_ [get|set]inheritsched |
PTHREAD_INHERIT_SCHED |
PTHREAD_INHERIT_SCHED |
|
Scheduling/policy |
Determines the scheduling policy of the thread being created |
pthread_attr_ [get|set]schedpolicy |
SCHED_FIFO |
SCHED_OTHER |
|
Scheduling/priority |
Determines the scheduling priority of the thread being created |
pthread_attr_ [get|set]schedparam |
struct sched_param holds |
0 (non real-time) |
|
Stack/guard region |
A guard region for the thread's stack |
pthread_attr_ [get|set]guardsize |
Stack guard region size in bytes |
1 page |
|
Stack/location, size |
Query or set the thread's stack location and size |
pthread_attr_ pthread_attr_ pthread_attr_ |
Stack address and/or stack size, in bytes |
Thread Stack Location: left to the OS Thread Stack Size: 8 MB |
As you can see, clearly understanding what exactly many of these attributes signify requires further information. Please be patient as we proceed through this chapter (and, in fact, this book), as several of these attributes and their meanings will become abundantly clear ( details on scheduling will be shown in Chapter 17, CPU Scheduling on Linux).