Re: multithreaded c++ question

"James Kanze" <>
11 Apr 2007 03:04:44 -0700
On Apr 10, 8:57 pm, Chris Roth <> wrote:

I'm using 7.1 on Windows XP.

I'm more familiar with Posix threads, but I think the basic
principles are similar.

I have a class that hold a container of doubles. One function (foo) for
the class calls a sub-function (bar) for each of the doubles. I'd like
to multithread foo so that the bar sub-functions can run on multiple
threads. I'd like to imlpement this with _beginthreadex as I'm using

What is _beginthreadex, and what is its relationship to

Please provide some working code around the following details:

#include <windows.h> // for HANDLE
#include <process.h> // for _beginthreadex()
#include <vector>

using namespace std;

class A
        vector<double> v;
        double d; // some other variable common to each thread;
        double bar( double x );
        vector<double> foo();

vector<double> A::foo()
        vector r( v.size() );
        for( int i=0; i<int(v.size()); ++i )
                r[i] = bar( v[i] );
        return r;

double A::bar( double x )
        double r = x*d; // some function using x and d
                        // obviosly more complicated in the real code...
        return r;

Now what I'd like is for multiple instances of bar to run on my two
cores. Can you help me please?

The first, and most important question, is: does bar modify d in
your actual code? If so, you'll need a lock around each access
to d, which is likely to make the threaded code much, much
slower. Similar considerations apply if you modify the topology
(size or capacity) of any shared vector.

Secondly, what is the size of the vector, and how often is foo()
called. Creating a thread is expensive; if foo() is called a
lot, you'll want to use a pool of threads, so you don't have to
create new threads each time foo is called, especially if the
vectors aren't that big.

Finally: I'd define a few helper objects (maybe just structs) to
define what each thread should do. Maybe something along the
lines of:

    struct ThreadData
        A* object ;
        vector< double >* result ;
        int first ;
        int last ;
        ThreadIdType id ;

        ThreadData( A* owner, vector<double>*r )
            : object( owner )
            , result( r )
    } ;

with an overloaded foo:

    A::foo( ThreadData const& data )
        for ( int i = data.first ; i < data.last ; ++ i ) {
            (*result)[ i ] = bar( v[ i ] ) ;

If you're starting the thread in foo, you'll need an `extern
"C"' function (or maybe some special Windows linkage) to kick
things off:

    extern "C"
    void* // or whatever Windows requires...
    threadStarter( void* param )
        ThreadData* data( static_cast< ThreadData* >( param ) ) ;
        data->object->foo( *data ) ;
        return NULL ; // or whatever...

Given this, foo becomes:

    vector< double >
        vector< double > r( v.size() ) ;
        static int const threadCount = numberOfCores ;
        size_t perThread = v.size() / threadCount ;
        size_t extras = v.size() % threadCount ;
        size_t currentIndex = 0 ;
        std::vector< ThreadData >
                            threads( threadCount,
                                     ThreadData( this, &r ) ) ;
        for ( size_t i = 0 ; i < threadCount ; ++ i ) {
            threads[ i ].first = currentIndex ;
            currentIndex += perThread ;
            if ( extras != 0 ) {
                ++ currentIndex ;
                -- extras ;
            threads[ i ].last = currentIndex ;
            StartThread( &threads[ i ].id, threadStarter,
&threads[ i ] ) ;
        for ( size_t i = 0 ; i < threadCount ; ++ i ) {
            JoinThread( &threads[ i ].id ) ;

Obviously, you'll have to use whatever the Windows API requires
for StartThread and JoinThread (CreateThread and
WaitForSingleObject, I think). And you really should add some
error handling as well. (Be very careful about leaving the
function once you've started some threads. Those threads are
using a local variable in the function, and will continue using
even if you leave the function. In case of error, you must
somehow cause all already started threads to stop, and then join
with them.)

James Kanze (GABI Software)
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