c++ _beginthread can't pass string as a parameter

Question

I would like to start multiple threads using a sample code like below:

void ThreadFunction(void* param) {
    cout << (string)param << endl;
    _endthread();
    }
int main(int argc, char* argv[]) {
    for (unsigned int i = 1; i <= 10; i++) {
        string String = "This is test nr ";
        String += i;
        _beginthread(ThreadFunction, 0, (void*)&String);
        }
    }

However, I can't get this to work (bad allocation error). What am I doing wrong?

When you say that you "can't get this to work", what do you mean by that? Can you please elaborate? And also please try to create a [Minimal, Complete, and Verifiable Example](http://stackoverflow.com/help/mcve) to show us. — Some programmer dude, Aug 11 '17 at 08:47
what is `_beginthread` ? In what way this code is not working? Are there error message? Does it crash? — 463035818_is_not_an_ai, Aug 11 '17 at 08:47
As soon as you reach the `}` at the end of the for-loop, the string goes away. — Bo Persson, Aug 11 '17 at 08:55
@Someprogrammerdude, all I try is to pass a string to a thread. However, I can't get this to work in any way. I get errors like `bad alloc` and such — TVA van Hesteren, Aug 11 '17 at 08:57
Also, you cast the `param` to `string` when it really is a `string*`. — Bo Persson, Aug 11 '17 at 08:58
@BoPersson, the string is not passed by a global reference. Would this be a problem? Could yo provide me a sample? — TVA van Hesteren, Aug 11 '17 at 08:58
No you don't pass a `string` to the thread function, you pass a *pointer* to a `string`. — Some programmer dude, Aug 11 '17 at 08:58
@Someprogrammerdude, makes more sense. Sorry, I'm quite new to c++ programming. Could you provide me a simple sample I can work on? — TVA van Hesteren, Aug 11 '17 at 08:59
" it crashes by telling me the bad alloc error " this should be part of the question, not just a comment — 463035818_is_not_an_ai, Aug 11 '17 at 08:59
@TVA - If you want to learn C++, and not Windows, you should perhaps look at [`std::thread`](http://en.cppreference.com/w/cpp/thread/thread) which will copy the parameters to the thread function and not just pass a pointer. — Bo Persson, Aug 11 '17 at 09:04
"new to C++ programming" and "multiple threads" is not an ideal combination. Learn single-threaded C++ programming first. [Book list.](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — molbdnilo, Aug 11 '17 at 09:08

score 1 · Accepted Answer · answered Aug 11 '17 at 09:10

You cannot pass a string as you are doing, but you can pass a string pointer. BUT! You have to be careful that the string stays valid at least until the threads starts... This is usually done by creating the strings on the heap, using new, but it can also work using global objects. Here's how your code could work.

#include <unistd.h>
#include <string>
#include <vector>
#include <sstream>
#include <pthread.h>

void ThreadFunction(void* param) 
{
    std::string* s = reinterpret_cast<std::string*>(param);  
    cout << *s << endl;  
}                        // <-- no need to call endthread when exiting gracefully

std::vector<std::string> myStrings;

int main(int argc, char* argv[]) 
{
    // since we will use pointers to strings in the vector myStrings,
    // we need to be sure it is final before using them.
    // you could store some form of smart pointer in the vector to avoid 
    // this issue
    for (unsigned int i = 0; i < 10; i++) 
    {
       std::stringstream ss;
       ss << "This is test nr " << i;
       myStrings.emplace_back(ss.str());
    }

    for (unsigned int i = 0; i < myStrings.size(); i++) 
    {
        _beginthread(FunctionName, 0, &myStrings[i]);
    }

    // we must pause this thread and wait a little while for the threads
    // to run.  _beginthread does not block, and exiting the program too
    // quickly would prevent our background threads from executing...

    sleep(1);

    return 0;
}

Great! I got this to work. I however slightly modified the code in order to remove the sleep. I initialize a boolean `ProcessStarted` on `false` just before the `_beginthread`. Within the thread I set `ProcessStarted=true` just after I safely stored the parameter in a string locally for the tread. Works like a charm! Thanks — TVA van Hesteren, Aug 11 '17 at 09:33
I'm glad I could help. If you are serious about c++, you should really learn how to use the standard library, and start with C++11 or later. The standard library is portable, very practical and is a great time-saver. C++ has seen a lot of changes in the 2011 standard, and it should be the starting point for any new student, since all major compilers support it.. C++ 14 is the current standard. There are lots of sites that document the standard library, but I found that cppreference.com is somewhat easier to read. — Michaël Roy, Aug 11 '17 at 10:00

c++ _beginthread can't pass string as a parameter

1 Answers1