12.2. Thread Limits

We discussed the sysconf function in Section 2.5.4. The Single UNIX Specification defines several limits associated with the operation of threads, which we didn't show in Figure 2.10. As with other system limits, the thread limits can be queried using sysconf. Figure 12.1 summarizes these limits.

Figure 12.1. Thread limits and name arguments to sysconf

Name of limit	Description	name argument
PTHREAD_DESTRUCTOR_ITERATIONS	maximum number of times an implementation will try to destroy the thread-specific data when a thread exits (Section 12.6)	_SC_THREAD_DESTRUCTOR_ITERATIONS
PTHREAD_KEYS_MAX	maximum number of keys that can be created by a process (Section 12.6)	_SC_THREAD_KEYS_MAX
PTHREAD_STACK_MIN	minimum number of bytes that can be used for a thread's stack (Section 12.3)	_SC_THREAD_STACK_MIN
PTHREAD_THREADS_MAX	maximum number of threads that can be created in a process (Section 12.3)	_SC_THREAD_THREADS_MAX

As with the other limits reported by sysconf, use of these limits is intended to promote application portability among different operating system implementations. For example, if your application requires that you create four threads for every file you manage, you might have to limit the number of files you can manage concurrently if the system won't let you create enough threads.

Figure 12.2 shows the values of the thread limits for the four implementations described in this book. When the implementation doesn't define the corresponding sysconf symbol (starting with _SC_), "no symbol" is listed. If the implementation's limit is indeterminate, "no limit" is listed. This doesn't mean that the value is unlimited, however. An "unsupported" entry means that the implementation defines the corresponding sysconf limit symbol, but the sysconf function doesn't recognize it.

Note that although an implementation may not provide access to these limits, that doesn't mean that the limits don't exist. It just means that the implementation doesn't provide us with a way to get at them using sysconf.

Figure 12.2. Examples of thread configuration limits

Limit	FreeBSD 5.2.1	Linux 2.4.22	Mac OS X 10.3	Solaris 9
PTHREAD_DESTRUCTOR_ITERATIONS	no symbol	unsupported	no symbol	no limit
PTHREAD_KEYS_MAX	no symbol	unsupported	no symbol	no limit
PTHREAD_STACK_MIN	no symbol	unsupported	no symbol	4,096
PTHREAD_THREADS_MAX	no symbol	unsupported	no symbol	no limit