Thread: _beginthread with std::string params

  1. #1
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    _beginthread with std::string params

    Im trying to call _beginthread with multiple arguments but I get undefined behavior.
    It either wont be called or just idles forever without printing out anything.

    Code:
    #define WIN32_LEAN_AND_MEAN
    #include <windows.h>
    #include <process.h>
    #include <string>
    #include <iostream>
    using namespace std;
    
    typedef struct
    {
        string data1;
        string data2;
    } t;
    
    t arg;
    
    void Thread(void *param)
    {
        cout << " Thread \n";
        t *args = (t*) param;
        string x = args->data1;
        string y = args->data2;
    
        cout << x << y << " \n";
    
       _endthread();
    }
    
    void foo(string a, string b)
    {
        arg.data1 = a;
        arg.data2 = b;
        HANDLE handle = (HANDLE) _beginthread(Thread, 0, (void*) &arg);
        cout << " foo " << handle << "\n";
    }
    
    int main()
    {
        foo("Hello"," World");
    
        return 0;
    }
    Windows 10 with Visual Studio 14.0.

  2. #2
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    When main ends, the constructor of your global variable is called, leading to undefined behavior. You must block until Thread is finished with its data.
    But why aren't you using std::thread instead? Regardless, you must still block until your thread is complete as when main exits, all your threads die.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  3. #3
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    Thanks, that makes sense.

    I was having problem with std::thread in a Win32 loop. But now I discovered thread.detach(); and I do use std::thread.
    Windows 10 with Visual Studio 14.0.

  4. #4
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    Quote Originally Posted by Elysia View Post
    When main ends, the constructor of your global variable is called, leading to undefined behavior. You must block until Thread is finished with its data.
    But why aren't you using std::thread instead? Regardless, you must still block until your thread is complete as when main exits, all your threads die.
    Typo: constructor should be destructor.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  5. #5
    and the hat of int overfl Salem's Avatar
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    It's even worse than main ending.

    > cout << " foo " << handle << "\n";
    arg is going out of scope at this point, so the &arg you passed to the thread is already off in the weeds somewhere.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  6. #6
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    arg is a global variable, though? How can it go out of scope at the end of foo?
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  7. #7
    and the hat of int overfl Salem's Avatar
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    Mis-read
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  8. #8
    C++まいる!Cをこわせ!
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    Yeah, I thought arg was a local variable the first time I read the code too. Probably because it sounds like a local variable.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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