Multithreading..Output

[venkatacplpl]

#include <iostream>
#include <cstdlib>
#include <pthread.h>
using namespace std;
#define NUM_THREADS 5
void *PrintHello(void *threadid)
{
long tid;
tid = (long)threadid;
cout << "Hello World! Thread ID, " << tid << endl;
pthread_exit(NULL);
}
int main ()
{
pthread_t threads[NUM_THREADS];
int rc;
int i;
for( i=0; i < NUM_THREADS; i++ ){
cout << "main() : creating thread, " << i << endl;
rc = pthread_create(&threads[i], NULL,
PrintHello, (void *)i);
if (rc){
cout << "Error:unable to create thread," << rc << endl;
exit(-1);
}
}
pthread_exit(NULL);
}



Hi All,
I am new to Multithreading and came across above program and output of the program is:
main() : creating thread, 0
main() : creating thread, 1
main() : creating thread, 2
main() : creating thread, 3
main() : creating thread, 4
Hello World! Thread ID, 0
Hello World! Thread ID, 1
Hello World! Thread ID, 2
Hello World! Thread ID, 3
Hello World! Thread ID, 4

I guess output of the program should be as below
main() : creating thread, 0
Hello World! Thread ID, 0

main() : creating thread, 1
Hello World! Thread ID, 1

main() : creating thread, 2
Hello World! Thread ID, 2

main() : creating thread, 3
Hello World! Thread ID, 3

main() : creating thread, 4
Hello World! Thread ID, 4

for loop to go next itearion it should have executed thread in present iteration.


why all the treads are waiting without executing themselves till for loop is
complete?

I am clueless.Hope I am making sense and please throw some light on Joining and detaching threads also..when do we Join and detach threads?

[ne555]

> why all the treads are waiting without executing themselves till for loop is complete?
¿why should the program that is creating the threads block itself so the newly created could use the processor?

[venkatacplpl]

@ne555

Thanks for your answer....but my doubt is cant many processes/threads run simeltaneously on single processer?


Thanks in advance

[JLBorges]

> but my doubt is cant many processes/threads run simeltaneously on single processer?

On a single processor, multithreading is generally implemented by time-division multiplexing (as in multitasking): the processor switches between different threads. This context switching generally happens frequently enough that the user perceives the threads or tasks as running at the same time. On a multiprocessor or multi-core system, threads can be truly concurrent, with every processor or core executing a separate thread simultaneously.
http://en.wikipedia.org/wiki/Thread_(computing)

Try this:

#include <iostream>
#include <thread>
#include <chrono>
#include <atomic>

void print( const char* cstr )
{
    static std::atomic<int> cnt(0) ;

    std::cout << cstr ;
    if( ++cnt%10 == 9 ) std::cout << std::endl ;
    else std::cout << std::flush ;
}

void foo( const char* cstr )
{
    for( int i = 0; i < 20 ; ++i )
    {
        print(cstr) ;
        std::this_thread::sleep_for( std::chrono::milliseconds(1) ) ;
    }
}

int main()
{
    const char* const strings[] = { "foo ", "bar ", "baz ", "FOO ", "BAR ", "BAZ " } ;

    for( auto cstr : strings )
    {
        std::thread t( foo, cstr ) ;
        t.detach() ;

        print( "main " ) ;

        std::this_thread::sleep_for( std::chrono::milliseconds(2) ) ;
    }

    print( "sleeping for a long period... " ) ;
    std::this_thread::sleep_for( std::chrono::seconds(1) ) ;
    print( "done!" ) ;
}

http://coliru.stacked-crooked.com/a/72720960e04593be