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Full Discussion: Multi-threading questions
Top Forums Programming Multi-threading questions Post 42698 by Perderabo on Sunday 2nd of November 2003 10:34:52 PM
Old 11-02-2003
Re: Multi-threading questions
I'll try to answer your questions, but I have only written a few sample pthread programs. But it's not like I'm a thread expert.

Quote:
Originally posted by DreamWarrior
I know that errno is thread safe when the _REENTRANT macro is defined...what about when it is not?
Then errno is not thread safe. If you insist on invoking a non thread safe library then your expected results are undefined. My guess is that the library will use the one-per-process errno and get the last error of any system call invoked by any thread. I would not expect repeatable results.

Quote:
Originally posted by DreamWarrior
2) I've seen functions re-written to be thread safe by doing their work in a local variable, then copying that local variable out to a user passed in storage area. What I wonder is, what about just using the user defined storage area directly?
When I've seen stuff like that, it was to warn against using a local static variable and then passing the address of that local static variable back to the caller. If you have a already written function that does that, a simple memcopy at the end will render it thread safe. I don't see a real problem in using the caller's area directly. But if you do that, you will be dereferencing a pointer, perhaps repeatedly. That may not be a speed win.

So they were not rewriten to USE a local variable. They were rewritten to not return a pointer to a local variable.

BTW, at very high optimization levels a memcpy will be "inlined", that is, it becomes a sequence of instructions rather than a true call. It may even become a single machine language instruction.

Quote:
Originally posted by DreamWarrior
Therefore, how do I provide both thread safe interfaces and standard interfaces in one library without requring an application to link with the pthread library.
Actually you have it backwards. Thread-safe interfaces are standard. A process is always a collection of threads. Sometimes it simply has one thread. You don't need a thread library unless you are assuming multiple threads, so don't assume that.

I haven't looked at the source code for the standard libraries, but I guess that there are 3 cases.

1. Some stuff is so primative that it always was thread safe.

2. Some stuff was not thread safe, but it could be made thread safe with changing the interface presented to the caller. So this was done.

3. Some stuff could not be made thread safe without visible changes. So seperate versions were made. For the most these are functions that tripped on the issue in your 2nd question.
 

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LEARN ABOUT HPUX

pthread_join

pthread_join(3T)														  pthread_join(3T)

NAME

       pthread_join() - wait for the termination of a specified thread

SYNOPSIS

PARAMETERS

	      thread	Thread whose termination is awaited by the caller.

	      value_ptr Pointer to the location where the exit status of thread is returned.

DESCRIPTION

       The  function  waits  for the termination of the target thread.	If the target thread has already terminated, this function returns immedi-
       ately.  Only threads created with a detachstate attribute value of may be specified in the target thread parameter.  On	successful  return
       from the value_ptr argument, if it is not a null pointer, will contain the value passed to by the terminating thread.

       When  a	call  returns successfully, the caller is guaranteed the target thread has terminated.	If more than one thread calls for the same
       target thread, one thread is guaranteed to return successfully.	Undefined behavior results for other callers specifying the same thread.

       If the thread calling is canceled, the target thread shall not be joined.  The exit status of the target thread will remain  available  for
       another thread to call

       If the target thread was canceled, its exit status is

       It is unspecified whether a thread that has exited, but remains unjoined, counts against the limit.

RETURN VALUE

       Upon successful completion, returns zero. Otherwise, an error number is returned to indicate the error (the variable is not set).

ERRORS

       If any of the following occur, the function returns the corresponding error number:

	      [EINVAL]	     The value specified by thread does not refer to a joinable thread.

	      [ESRCH]	     No thread could be found corresponding to thread.

       For each of the following conditions, if the condition is detected, the function returns the corresponding error number:

	      [EDEADLK]      This operation would result in process deadlock or thread specifies the calling thread.

AUTHOR

       was derived from the IEEE POSIX P1003.1c standard.

SEE ALSO

       pthread_create(3T), wait(2).

STANDARDS CONFORMANCE

								  Pthread Library						  pthread_join(3T)
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