Thread: Passing Parameters To SDL_CreateThread

Hello, I need to pass a parameter (string/filename) into SDL_CreateThread but for some reason I am not able to figure this out.

Code:

 
int processFile(string sFilePath)
{ 
    // process file here 
    return 0; 
}  

int main() 
{     
    string sFilePath =  "/home/myuser/test.txt";     
    thread = SDL_CreateThread(processFile, "processFile", &sFilePath);  
}

The error I get is

Code:

 
main.cpp|124|error: invalid conversion from ‘int (*)(std::string) {aka int (*)(std::basic_string)}’ to ‘int (*)(void*)’ [-fpermissive]|

How can I pass the filename into the thread?

Thanks in advance!

The problem lies with your callback: the callback is expected to be a function that has a void* parameter and returns an int, but you passed a pointer to a function that has a std::string parameter and returns an int.

thanks for the reply.

So it needs to change to this?

Code:

int processFile(string sFilePath)
{ 
    // process file here 
    return 0; 
}  

int main() 
{     
    int threadReturnValue;
    string sFilePath =  "/home/myuser/test.txt";     
    thread = SDL_CreateThread(processFile, sFilePath, threadReturnValue);  
}

I do appologize for the noob question but it still gives me this

Code:

main.cpp|124|error: invalid conversion from ‘int (*)(std::string) {aka int (*)(std::basic_string<char>)}’ to ‘int (*)(void*)’ [-fpermissive]|

I thought that I passed parameters through data but I am unsure how to do so. I also am not sure what 'name' is used for in that fuction either.

Code:

SDL_Thread* SDL_CreateThread(SDL_ThreadFunction fn,
                             const char*        name,
                             void*              data)

SDL_CreateThread - SDL Wiki'

The first parameter needs to be of type "SDL_ThreadFunction". That means you need a signature of "int fn(void* data)". See remarks.

The third parameter is a void*. Any pointer can safely be stored as a void*, and then restored back to its original pointer type. However, I would not use a std::string object itself. Just pass in a "const char*" version of the string, then the thread function can cast the void* back to const char*. It is up to you to ensure the lifetime of the memory that the void* points to is valid when the thread function accesses it. In your current code, sFilePath will destruct once main returns (which is probably before the thread would run).

Also notice in the manual that you are required to call either SDL_WaitThread() or SDL_DetachThread() after a successful CreateThread().
SDL_WaitThread - SDL Wiki'

gg

No, more like:

Code:

int processFile(void* data)
{
    string& sFilePath = *static_cast<std::string*>(data);
    // process file here
    return 0;
}

int main()
{
    string sFilePath =  "/home/myuser/test.txt";
    thread = SDL_CreateThread(processFile, "processFile", &sFilePath);
}

Thanks! I think now there is some kind of string problem because it doesnt like the string being passed into data

Code:

main.cpp|126|error: invalid conversion from ‘const void*’ to ‘void*’ [-fpermissive]|

I tried

Code:

    const string sFilePath =  "/home/myuser/test.txt"; 
    thread = SDL_CreateThread(processFile, "processFile", &sFilePath);  //this is the line it errors on

but it did not work

Remove the const from the declaration of sFilePath. That said, Codeplug made a point about object lifetime that you should consider.

>> The error happens on the 2nd line.
You are not making it easy for us to help you. Post what you are compiling and the errors your received.

gg

Sorry, its the same error over and over again

Error:

Code:

main.cpp|126|error: invalid conversion from ‘const void*’ to ‘void*’ [-fpermissive]|

Code:

Code:

int processFile(void* data)
{
    string& sFilePath = *static_cast<std::string*>(data);
}

int main()
{
    string sFilePath =  "/home/myuser/test.txt";
    thread = SDL_CreateThread(processFile, "processFile", &sFilePath);
}

I don't see line 126.

gg

Sample code from the manual - replaced printf with cout - added std::string parameter:

Code:

#include "SDL_thread.h"
#include "SDL_timer.h"

#include <iostream>
#include <string>
using namespace std;

// Very simple thread - counts 0 to 9 delaying 50ms between increments
static int TestThread(void *ptr)
{
    const string& threadMsg = *static_cast<string*>(ptr);
    int cnt;
    
    cout << threadMsg << endl;

    for (cnt = 0; cnt < 10; ++cnt) {
        cout << "Thread counter: " << cnt << endl;
        SDL_Delay(50);
    }

    // Return the final value to the SDL_WaitThread function above
    return cnt;
}

int main(int argc, char *argv[])
{
    SDL_Thread *thread;
    int         threadReturnValue;

    cout << "Simple SDL_CreateThread test:" << endl;

    // Simply create a thread
    string threadMsg = "Hello World";
    thread = SDL_CreateThread(TestThread, "TestThread", &threadMsg);

    if (NULL == thread) {
        cout << "SDL_CreateThread failed: " << SDL_GetError() << endl;
    } else {
        // Wait for the thread to complete. The thread functions return code will
        //       be placed in the "threadReturnValue" variable when it completes.
        //
        SDL_WaitThread(thread, &threadReturnValue);
        cout << "Thread returned value: " << threadReturnValue << endl;
    }

    return 0;
}

Code:

C:\MinGW-Projects\gg>g++ sdl_main.cpp -I c:\MinGW\include\SDL2 -lSDL2main -lSDL2_mixer -lSDL2 -lwinmm -luser32 -lgdi32 -lole32 -limm32 -loleaut32 -lversion -Wall

C:\MinGW-Projects\gg>a
Simple SDL_CreateThread test:
Hello World
Thread counter: 0
Thread counter: 1
Thread counter: 2
Thread counter: 3
Thread counter: 4
Thread counter: 5
Thread counter: 6
Thread counter: 7
Thread counter: 8
Thread counter: 9
Thread returned value: 10

"Works for me." TM

>> This is 100% of the code.
Very good. It seems strange that it compiles without including <string>.

gg