The Windows DLL is thread safe. You do not have to call my_init() and
my_thread_init() because Windows DLLs receive events when they are attached to
a new process and when they are attached to a new thread in a process. This is
one of the nicer features of Windows shared libraries. Other than that, you
don't have to do anything special. I am a heavy user of libmysql under Win32.
You simply mysql_init() your MYSQL struct, and then mysql_real_connect() and
you're ready to mysql_query().
You should not call my_init() or my_thread_init() as the previous poster
suggested. This could result in memory leaks.
>From libmysql/dll.c
BOOL APIENTRY LibMain(HANDLE hInst,DWORD ul_reason_being_called,LPVOID
lpReserved)
{
switch (ul_reason_being_called) {
case DLL_PROCESS_ATTACH: /* case of libentry call in win 3.x */
if (!inited++)
{
s_hModule=hInst;
libmysql_init();
main_thread=GetCurrentThreadId();
}
break;
case DLL_THREAD_ATTACH:
threads++;
my_thread_init();
break;
case DLL_PROCESS_DETACH: /* case of wep call in win 3.x */
if (!--inited) /* Safety */
{
/* my_thread_init() */ /* This may give extra safety */
my_end(0);
}
break;
case DLL_THREAD_DETACH:
/* Main thread will free by my_end() */
threads--;
if (main_thread != GetCurrentThreadId())
my_thread_end();
break;
default:
break;
} /* switch */
return TRUE;
UNREFERENCED_PARAMETER(lpReserved);
} /* LibMain */
----- Original Message -----
From: "John McCaskey" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>; <[email protected]>
Sent: Friday, October 07, 2005 10:31 AM
Subject: RE: How thread-safe is mysql_real_connect()?
Sean,
First let me thank you for all the great posts and info I've seen you
put on this list for others.
I've been working in C with MySQL in a very multithreaded environment
for several years and think I can explain the thread safety issues
clearly. Rather than try to respond point by point to your question I'm
going to give a summary and if that doesn't help please respond again
and I'll answer specific questions.
First, mysql is in fact pretty much threadsafe when using the _r
library. You definitely do need to use the _r library and not the
normal one as the SIGPIPE discussion applies to both, the non _r library
has additional safety issues surrounding mysql_real_connect() and should
not be used. On windows you don't really need to do anything here I
believe because "the Windows binaries are by default compiled to be
thread-safe." (from
http://dev.mysql.com/doc/mysql/en/threaded-clients.html). To validate
this in your client code you should in the main() function close to
startup use mysql_thread_safe() to verify your linked in version is
thread safe.
The next thing you need to do is initialize mysql globally before
creating any threads that will use it. Simply call my_init(); in your
main thread. After this you can go ahead and create any threads. In
the threads you create you need to call mysql_thread_init(); and when
you end the thread mysql_thread_end(); in between these calls you can
just use mysql as normal and the mysql_real_connect function will be
thread safe, you do not need to perform any locking of your own to make
only one call at a time or anything along those lines.
Here is some pseudo code of what you need to do:
int main(int argc, char **argv) {
if(!mysql_thread_safe()) {
fprintf(stderr, "Not Thread safe!!!");
return 1;
}
my_init();
// your regular init code
// create the threads that will use mysql
CreateThread(....);
}
void *mysql_thread(void *arg) {
mysql_thread_init();
//regular mysql code and whatever else here
//use mysql_real_connect and mysql_real_query
//and whatever without worrying about thread safety
mysql_thread_end();
}
John A. McCaskey
Software Development Engineer
Klir Technologies, Inc.
[EMAIL PROTECTED]
206.902.2027
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
Sent: Friday, October 07, 2005 9:01 AM
To: [EMAIL PROTECTED]; [email protected]
Subject: How thread-safe is mysql_real_connect()?
(please excuse the double post but I wanted to reach the two audiences I
thought could help the best)
This is a question about the interpreting the documentation in the
manual
for the C API.
I searched the list archives (all lists) going back 365 days for the
terms
(unquoted): "mysql_real_connect thread" (I also looked for
alternatives:"mysql_real_connect threaded", "mysql_real_connect multi
threaded", etc.). I searched on Google Groups for: mysql_real_connect
thread and found a few interesting hits. However, I am still not 100%
clear on how to interpret some of the information on this page:
http://dev.mysql.com/doc/mysql/en/threaded-clients.html
I do a lot of MySQL administration and development using mostly the CLI
and a few other tools but I am writing a multithreaded client to
automate
certain background processing and I need a bit of advice. According to
the
page in question the function mysql_real_connect() is not "thread-safe".
Does that simply mean that I cannot call that function from more than
one
thread at a time or does that mean that the connection created by one
call
to the function will be visible to the other threads or what? Just how
not
"thread-safe" is it?
Each thread will have it's own MYSQL structure and I will need to use
two
different connections per thread at the same time (am I going to need a
separate call to mysql_init() for each connection?). I know how to wrap
all of my calls to mysql_real_connect() in a critical section or
protect
them with a mutex if that's all I need to do . If it's not that simple
and
I do need to compile and link against another library (as the page
suggests - sort of) can someone help me to configure my Microsoft Visual
C++ .NET (v7) to do it? I said "sort-of" because the page also says that
the binary distributions (which I am working with ) already contain the
threadsafe library so I wonder if I need to rebuild anything or not.
How
can I tell?
I am an experienced but not well-seasoned C++ developer (not using c#
for
this). I know the language and can write and debug code just fine (I can
make stand-alone apps and DLLs all day); it's just that some of the
complier/linker options and settings that confound me and I am having
trouble translating the advice on the page into specifics I can work
with
for my environment.
I know I probably left out some simple pieces of information, just let
me
know and I will respond ASAP. Please remember to CC: both lists on all
responses.
Shawn Green
Database Administrator
Unimin Corporation - Spruce Pine
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