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Hi. Can you clear something up for me?
My collegue is declaring vars inside while loops (and maybe in for loops as well...)
Does this allocate new memory each time for the variable and hence considered wasteful?
Example:
while (m_socket->Read((char *) buffer, bufferLength, false, &from) > 0) {
UINT portNumber = ntohs(from.sin_port);
ProcessMessage(buffer, portNumber);
}
compiling in C++, VisualStudio 2005
Thanks.
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You should really check the generated machine code to be 100% sure, but I bet money it will be exactly the same as if you put the portNumber declaration before the loop. Compilers are pretty smart these days 
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Nemanja Trifunovic wrote: it will be exactly the same as if you put the portNumber declaration before the loop. Compilers are pretty smart these days
Sorry, this is incorrect.  If you put portNumber outside the loop, it's scope will be larger. This has nothing to do with smart compilers, but with the semantics of cpp.
Bram van Kampen
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Bram van Kampen wrote: Sorry, this is incorrect.
Sorry, it is correct
Bram van Kampen wrote: If you put portNumber outside the loop, it's scope will be larger.
True, but that will have an effect on the generated machine code only if it is actually used in that larger scope.
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Not wanting to split hairs, there's more to CPPCode than the eventual Machine code. (Readibility, Intent, Scope, etc). It will probably generate similar machine code, in particular in the quoted example. If the var where a class with a non-trivial constructor, in theory the Constructor / Destructor would be called each for each itteration of the loop, unless if the Optimiser decided that nothing was to be gained by this, and rearranged the machine code to place it outside the loop. As for scope, well, that's clear forsofar as the scope brackets are concerned. There has been confusion in the past about the scope of a var declared in the test statement of a loop (e.g. for(int i=0;;) In older MS compilers, this particular case left the Symbol Table in an inconsistent state. You could not declare a New i,(Already Declared), Nor use the Old one(Undeclared Identifier.
Bram van Kampen
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Actually you are both correct, the language clearly defines scope and how variables should be accessed inside these scope regions. But there is a difference between language semantics and compiler output. Especially when optimization is involved.
Optimizing compilers such as MSVC can unroll loops, change/combine variables or eliminate them completely. In fact some compilers may choose to completely remove some of your function calls. SecureZeroMemory [^]exists for this very reason as you can plainly read in its documentation.
Microsoft has only recently made a modest attempt to adhere to ISO standards. In fact Microsoft only recently added Force Conformance in for Loop Scope[^] I believe in VC2005. Prior compiler releases allowed out of loop scope variable reuse.
Keep in mind that almost all compilers are not fully ISO conformant and they all optimize differently. Here is an excellent article over at Dr. Dobb's[^] illustrating the history of ISO compiler conformance.
Best Wishes.
-David Delaune
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Yes, but you have to start somewhere. ISO is a good starting point. MS might not always comply, thats where you deviate from the starting point.
Bram van Kampen
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No, it does not! After ProcessMessage() returns, the code hits the '}' the scopr with portNumber in it is destroyed. Then the 'while'test is done, and, if TRUE, the code this the'{ starting the scope again, and constructing the var portNumber all over again. This is sort of trivial with UINTS, these have trivial default Constructors and Destructors. In case of classes, you will have the overhead of the Constructor / Destructor Call.
Bram van Kampen
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Hi Friends,
Few days back i have installed visual studio 2008 on my machine.
When i tried to build the samples, i am unable build any solution.
I am getting
Rebuild All: 0 succeeded, 0 failed, 1 skipped. for every solution .
can someone help me in fixing the issue.
Thanks in advance.
Appu..
"Never explain yourself to anyone.
Because the person who likes you does n't need it.
And the person who dislikes you won't believe it."
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hello my friends,
i am no a advanced Visual C++ developer, but i am only a new VC++ / MFC developer (starting from 15 jun 2008)
please:
ho to host a ActiveX Control in my MFC View Class ?
otherwise:
ho to add a other controls to my MFC View ?
(this is a generik View, (no RichEdit, Edit or HTML View)
thanks!
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Hello, I'm pretty new to programming. I have 2 programs: a server and a client. i want the server to accept messages and do any message processing on a different thread while i keep it's main thread available to whatever i want (this is only something i'm doing to get accustomed with threads & sockets). So, i have an AbstractStream class which is inherited by an ServerStreamSocket class and a AbstractServerCommsThread class which is base class for ServerSocketCommsThread. In the main of the program i create a ServerStreamSocket * ss and after initalising the socket i pass it the thread to be used: ServerSocketCommsThread *thr = new ServerSocketCommsThread(ss). The thread should be started by thr->Run(). Run()creates a thread using a friend function ThreadMe to which it passes a pointer to the object so the threading class has acces to the ServerStreamSocket. Now, i tried doing something really stupid in the threaded function, like outputting a character for 400 time and outputting a different character in the main function to see if they are sequential or not, and unfortunately they are sequential. Any idea why this is?
Here the threading class implementation and the main function:
int main()
{
ServerStreamSocket *ss = new ServerStreamSocket();
ss->init(11000);
ServerSocketCommsThread *sockThr = new ServerSocketCommsThread(ss);
sockThr->Run();
for (int i = 0; i < 400; i++)
cout << "o ";
return 0;
}
class ServerSocketCommsThread : public AbstractServerCommsThread
{
private:
AbstractStream *stream;
pthread_t tid;
public:
void Run();
public:
ServerSocketCommsThread( AbstractStream *pAS);
~ServerSocketCommsThread();
//threaded function must be static or global
friend void* ThreadMe(void *arg);
};
ServerSocketCommsThread::ServerSocketCommsThread( AbstractStream *pAS)
{
this->stream = pAS;
}
void ServerSocketCommsThread::Run()
{
pthread_create(&tid, NULL, ThreadMe, (void*)this);
//return;
}
void* ThreadMe( void *arg)
{
ServerSocketCommsThread *hdl = (ServerSocketCommsThread*)arg;
char in[300];
hdl->stream->write("Starting thread", 300);
cout << "thread started!";
/*char recBuff[300];
while(1){
hdl->stream->read(recBuff, 300);
cout << recBuff << endl;
}*/
for (int i = 0; i < 400; i++)
cout << "x ";
}
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tiberiuv wrote: Now, i tried doing something really stupid in the threaded function, like outputting a character for 400 time and outputting a different character in the main function to see if they are sequential or not, and unfortunately they are sequential. Any idea why this is?
If I understand you correctly you believe you have a problem because you get an output that consists of 400 'x' followed by 400 'o', or the other way around....
Right?
Consider that the scheduler lets each thread run for approx. 10ms unless the thread is preempted by another thread with higher priority that is ready to run.
This means that your threads have plenty of time to write 400 chars to the console, or output window, before the next thread is allowed to run.
This is not a problem in your code or the POSIX thread library, this is how the OS works.
Can you pin-point in a few sentences (less than ten and without code initially) what your undesired behaviour is, you'll get more answers. 
The heading of your post suggests that it's about POSIX threads, but I don't see any problem related to the use of them.
(Just for the record: yes, I've used the POSIX thread library port for Win32.)
"It's supposed to be hard, otherwise anybody could do it!" - selfquote
"High speed never compensates for wrong direction!" - unknown
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Thanks for the answer. My desired behavior was to actually see that a separate thread is created and that the output from the thread is intertwined with the output from the main thread. I had the impression i had done this before and it worked (by creating two threads directly from the main function) but it's getting clear to me that this is not the case
And yes, it wasn't a POSIX threads question as much as it was a threading question. But right now it's clear to me what is happening. Thank you.
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tiberiuv wrote: My desired behavior was to actually see that a separate thread is created and that the output from the thread is intertwined with the output from the main thread. I had the impression i had done this before and it worked (by creating two threads directly from the main function) but it's getting clear to me that this is not the case
Well, depends on what you mean by "working".
It does work, in the sense that a new thread will be created with pthread_create().
However, I think you are implementing it in a strange way and mixing Win32, MFC and pthread.
I suggest you read this excellent article[^] to get started with multithreading using the MFC framework and also understand how and when to use multithreading.
The article also helps you avoid common pitfalls. Even though the implementation details may differ from the POSIX thread library, the concept of multithreading is still the same.
The POSIX thread port library adds a lot of limitations to the Win32 API and should, in my opinion, only be used to write code that is supposed to be portable between Win32 and e.g. Linux. One of the worst limitations is that you cannot wait on multiple synchronization objects. Compare with ::WaitForMultipleObjects().
There are also some features of the POSIX thread library that are unsupported in the Win32 port, such as cleanup handlers.
"It's supposed to be hard, otherwise anybody could do it!" - selfquote
"High speed never compensates for wrong direction!" - unknown
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Ah, damn. I asked this in the wrong place. I'm actually doing this under Linux. No MFC/Visual C++ involved here. Sorry.
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I still have one small question, if you do not mind
on the server side i have something like this:
while( pThread->stream->read( recBuff, MAX_LEN) )
{
cout << recBuff;
}
and on the client side i have something like this:
while ( true )
{
cin >> sendBuff;
cs->write( sendBuff, MAX_LEN);
}
So when i write a sentence like: "timmy hates guns" on receiving i get it like i would have entered every word followed by a new line. I think that this is just what the client is doing. It's sending each word at a time in rapid succession. For this reason, some words do not pop out at the other end. I can end up with "timmy guns" and so on. Any idea why the client isn't sending the whole sentence? I'm using ::send(socket, (void*)msg, strlen(msg)+1, 0) to send over the network . Thanks
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Try using cin.getline() instead of the >> operator since the operator break at least on every whitespace and possibly on every char.
"It's supposed to be hard, otherwise anybody could do it!" - selfquote
"High speed never compensates for wrong direction!" - unknown
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you my friend, are a genius 
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tiberiuv wrote: you my friend, are a genius
My most humble thanks.
Glad I could help you.
"It's supposed to be hard, otherwise anybody could do it!" - selfquote
"High speed never compensates for wrong direction!" - unknown
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Hi every one!
Can any one explain me why is this happening
struct SOME
{
char s0[1];
char s1[2];
char s2[1];
};
int main()
{
SOME *sm = new SOME;
delete sm;
return 0;
}
before allocation (with new)
0xcccccccc
{
s0=0xcccccccc <bad ptr="">
s1=0xcccccccd <bad ptr="">
s2=0xcccccccf <bad ptr="">
}
after allocation
0x00332e98
{
s0=0x00332e98 "
πΊ««««««««ξώξώ"
s1=0x00332e99 "πΊ««««««««ξώξώ"
s2=0x00332e9b "Ί««««««««ξώξώ"
}
after deallocation (with delete)
0x00332e98 <----- Is that a problem?
{
s0=0x00332e98 "" <-----
s1=0x00332e99 "3" <-----
s2=0x00332e9b "" <-----
}
thanks!
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I don't see a problem. What specifically are you asking?
"Love people and use things, not love things and use people." - Unknown
"The brick walls are there for a reason...to stop the people who don't want it badly enough." - Randy Pausch
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first:
in that below, why the memory addresses of s0, s1, s2 are different and has these values
0xcccccccc
{
s0=0xcccccccc <bad ptr="">
s1=0xcccccccd <bad ptr="">
s2=0xcccccccf <bad ptr="">
}
second:
after deallocation the memory addresses are still assigned to s0, s1, s2
0x00332e98
{
s0=0x00332e98 ""
s1=0x00332e99 "3" is that a problem
s2=0x00332e9b ""
}
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Dennis L wrote: in that below, why the memory addresses of s0, s1, s2 are different...
As opposed to all being the same?
Dennis L wrote: after deallocation the memory addresses are still assigned to s0, s1, s2
And what would you expect them to be? All delete does is tell the memory manager that address 0x00332e98 is available for use again.
"Love people and use things, not love things and use people." - Unknown
"The brick walls are there for a reason...to stop the people who don't want it badly enough." - Randy Pausch
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first
I have read that all local variables that are not initialized explicitly are being intialiazed to 0xcccccccc with the Microsoft compilers. The first OK but the other two?
second
what happens with this address if the program exits
or
is this
void func1()
{
SOME *sm = new SOME;
delete sm;
}
void func2()
{
func1();
// What happents here?
}
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