12-24-2002
I don't exactly have an answer, but I may be able to shed some light here.
When you use fopen(), you get a stream. But this is built on open() and open() would give you an fd. The fd is a kernel thing and the stream is a library thing built on top on it.
In the same way, a lwp is a kernel thing and a thread (or a pthread) is a library thing.
What I have just discovered this morning is that the lwp's and the threads are not in a one-to-one correspondence.
Look at "man pthread_attr_init". You will see language like 'This thread is not "bound" to a LWP, and is also called an unbound thread.'
I gotta read up on threads sometimes. Since you are trying to count them, maybe you should do the same. In any event, it looks like counting LWP's is not going to help you counting threads.
Here is my guess as to why 3 lwps:
Consider what would happen if you did an fopen(), but all of the fd's were in use. If you had not yet reached the limit on fd's, when the open() system call occurred, the kernel would allocate another chunk of fd's to the process, not just one.
Or if you need more stack, your stack grows by a page, not just the few bytes that you need.
Or if you write another byte to a disk file, the file may grow by a full block.
The kernel may not be easily able to give you just one lwp, or it may just be attempting to be efficient.
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LEARN ABOUT NETBSD
pthread_barrier
PTHREAD_BARRIER(3) BSD Library Functions Manual PTHREAD_BARRIER(3)
NAME
pthread_barrier -- barrier interface
LIBRARY
POSIX Threads Library (libpthread, -lpthread)
SYNOPSIS
#include <pthread.h>
int
pthread_barrier_init(pthread_barrier_t * restrict barrier, const pthread_barrierattr_t * restrict attr, unsigned int count);
int
pthread_barrier_destroy(pthread_barrier_t *barrier);
int
pthread_barrier_wait(pthread_barrier_t *barrier);
DESCRIPTION
The pthread_barrier_init() function creates a new barrier with attributes attr and count. The count parameter indicates the number of
threads which will participate in the barrier. The pthread_barrierattr_init(3) function may be used to specify the attributes supplied in
attr. If attr is NULL, the default attributes are used. Barriers are most commonly used in the decomposition of parallel loops.
The pthread_barrier_destroy() function causes the resources allocated to barrier to be released. No threads should be blocked on barrier.
The pthread_barrier_wait() function causes the current thread to wait on the barrier specified. Once as many threads as specified by the
count parameter to the corresponding pthread_barrier_init() call have called pthread_barrier_wait(), all threads will wake up, return from
their respective pthread_barrier_wait() calls and continue execution.
RETURN VALUES
If successful, pthread_barrier_init() will return zero and put the new barrier id into barrier, otherwise an error number will be returned to
indicate the error.
If successful, pthread_barrier_destroy() will return zero. Otherwise an error value will be returned.
If successful, pthread_barrier_wait() will return zero for all waiting threads except for one. One thread will receive status
PTHREAD_BARRIER_SERIAL_THREAD, which is intended to indicate that this thread may be used to update shared data. It is the responsibility of
this thread to insure the visibility and atomicity of any updates to shared data with respect to the other threads participating in the bar-
rier. In the case of failure, an error value will be returned.
ERRORS
The pthread_barrier_init() function may fail if:
[EINVAL] The value specified by count is zero or attr is invalid.
The pthread_barrier_destroy() function may fail if:
[EBUSY] The barrier still has active threads associated with it.
[EINVAL] The value specified by barrier is invalid.
The pthread_barrier_wait() function may fail if:
[EINVAL] The value specified by barrier is invalid.
SEE ALSO
pthread_barrierattr(3), pthread_cond(3), pthread_mutex(3)
STANDARDS
These functions conform to IEEE Std 1003.1-2001 (``POSIX.1'').
BSD
July 8, 2010 BSD