C++ pthreads - using code I used in C gives me a conversion error

Question

I am running the same exact code that I ran in plain C:

pthread_create(&threads[i], &attr, SomeMethod, ptrChar);

And I get the errors:

error: invalid conversion from 'void*(*)(char'*)' to 'void*(*)(void*)'

error: initializing argument 3 of 'int pthread_create(__pthread_t**, __pthread_attr_t* conts*, void*(*)(void*), void*)'

SomeMethod is:

void *SomeMethod(char *direction)

Do I need to do something different in C++? I thought you could just run any C code in C++ and it would work fine?

I am using Cygwin at the moment.

Answer 1

Like it says, they are two different function signatures. You should do:

void *SomeMethod(void* direction) // note, void
{
    char* dir = static_cast<char*>(direction); // and get the value
}

C was casting from one function pointer to the other, apparently. Casting one function pointer to another is undefined behavior in C++.

That said, I'm pretty sure POSIX requires that casts between function pointers be well-defined, so you may be able to do this instead:

pthread_create(&threads[i], &attr, // cast the function pointer
                reinterpret_cast<void* (*)(void*)>(SomeMethod), ptrChar);

Answer 2

Your thread function needs to be:

void *SomeMethod(void *direction);

And you can cast from void* to char* inside the function.