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 PHP
thr_min_stack
thr_min_stack(3C) Standard C Library Functions thr_min_stack(3C)
NAME
thr_min_stack - return the minimum-allowable size for a thread's stack
SYNOPSIS
cc -mt [ flag... ] file...[ library... ]
#include <thread.h>
size_t thr_min_stack(void);
DESCRIPTION
When a thread is created with a user-supplied stack, the user must reserve enough space to run this thread. In a dynamically linked execu-
tion environment, it is very hard to know what the minimum stack requirments are for a thread. The function thr_min_stack() returns the
amount of space needed to execute a null thread. This is a thread that was created to execute a null procedure. A thread that does some-
thing useful should have a stack size that is thr_min_stack() + <some increment>.
Most users should not be creating threads with user-supplied stacks. This functionality was provided to support applications that wanted
complete control over their execution environment.
Typically, users should let the threads library manage stack allocation. The threads library provides default stacks which should meet the
requirements of any created thread.
thr_min_stack() will return the unsigned int THR_MIN_STACK, which is the minimum-allowable size for a thread's stack.
In this implementation the default size for a user-thread's stack is one mega-byte. If the second argument to thr_create(3C) is NULL, then
the default stack size for the newly-created thread will be used. Otherwise, you may specify a stack-size that is at least THR_MIN_STACK,
yet less than the size of your machine's virtual memory.
It is recommended that the default stack size be used.
To determine the smallest-allowable size for a thread's stack, execute the following:
/* cc thisfile.c -lthread */
#define _REENTRANT
#include <thread.h>
#include <stdio.h>
main() {
printf("thr_min_stack() returns %u
",thr_min_stack());
}
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|MT-Level |MT-Safe |
+-----------------------------+-----------------------------+
SEE ALSO
attributes(5), standards(5)
SunOS 5.10 12 May 1998 thr_min_stack(3C)