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Re: [HACKERS] Threads vs Processes

From: "Keith Bottner" <kbottner(at)comcast(dot)net>
To: "'Merlin Moncure'" <merlin(dot)moncure(at)rcsonline(dot)com>,"'Tom Lane'" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: <pgsql-hackers(at)postgresql(dot)org>,<pgsql-hackers-win32(at)postgresql(dot)org>,"'Bruce Momjian'" <pgman(at)candle(dot)pha(dot)pa(dot)us>,"'Shridhar Daithankar'" <shridhar_daithankar(at)persistent(dot)co(dot)in>,"'Claudio Natoli'" <claudio(dot)natoli(at)memetrics(dot)com>
Subject: Re: [HACKERS] Threads vs Processes
Date: 2003-09-25 20:03:57
Message-ID: 004901c383a0$27fd8d00$7d00a8c0@juxtapose (view raw, whole thread or download thread mbox)
Lists: pgsql-hackerspgsql-hackers-win32
Actually you can use a DLL with LoadLibrary as long as you do the following.

When a process uses load-time linking with this DLL, the entry-point
function is sufficient to manage the thread local storage. Problems can
occur with a process that uses run-time linking because the entry-point
function is not called for threads that exist before the LoadLibrary
function is called, so TLS memory is not allocated for these threads. The
following example solves this problem by checking the value returned by the
TlsGetValue function and allocating memory if the value indicates that the
TLS slot for this thread is not set.

LPVOID lpvData; 
// Retrieve a data pointer for the current thread.
lpvData = TlsGetValue(dwTlsIndex); 
// If NULL, allocate memory for this thread.
if (lpvData == NULL) 
    lpvData = (LPVOID) LocalAlloc(LPTR, 256); 
    if (lpvData != NULL) 
        TlsSetValue(dwTlsIndex, lpvData); 

Unless gcc has extension as Borland and Microsoft do you will have to
utilize the API and not the compiler/linker customizations that make access
variables declared as __declspec(thread) or __thread under Borland.


-----Original Message-----
From: pgsql-hackers-owner(at)postgresql(dot)org
[mailto:pgsql-hackers-owner(at)postgresql(dot)org] On Behalf Of Merlin Moncure
Sent: Thursday, September 25, 2003 2:49 PM
To: Tom Lane
Cc: pgsql-hackers(at)postgresql(dot)org; pgsql-hackers-win32(at)postgresql(dot)org; Bruce
Momjian; Shridhar Daithankar; Claudio Natoli
Subject: Re: [HACKERS] Threads vs Processes 

Both Microsoft and windows compilers support thread local storage.  *If* you
guys go with the threading model and *if* it does not introduce any serious
portability issues with gcc (big ifs, and I'm not familiar with gcc), than
IMO TLS is really the way to go.  I don't think any reorganization of
postgres's static variables is necessary.  TLS is implemented in the win32
API, not the C Libs, so by giving up the syntax sugar you can make direct
calls and keep compiler independence in win32.

Microsoft syntax is __desclspec(thread) and Borland syntax is simply
__thread.  All TLS variables *must* be static (or implicitly static through
extern, i.e. no 'auto' variables) and their addresses can not be assumed to
be constant.  

Taking addresses of TLS variables should be considered illegal, as well as
pointers to TLS variables.  Another gotcha is that DLLs that have __thread
variables will GPF if loaded with LoadLibrary (they should be static
linked).  Of course, pg does not use dlls, but it's worth noting.


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