CODEWITHSUNDEEP
c++linuxg++pthreads
Aquarius_Girl
Aquarius_Girl

Reputation: 22906

error: invalid conversion from ‘void*’ to ‘void* (*)(void*)’ - pthreads

anisha@linux-y3pi:~> g++ conditionVarTEST.cpp -Wall

conditionVarTEST.cpp: In function ‘int main()’:
conditionVarTEST.cpp:33:53: error: invalid conversion from ‘void*’ to ‘void* (*)(void*)’
conditionVarTEST.cpp:33:53: error:   initializing argument 3 of ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* (*)(void*), void*)’
conditionVarTEST.cpp:34:53: error: invalid conversion from ‘void*’ to ‘void* (*)(void*)’
conditionVarTEST.cpp:34:53: error:   initializing argument 3 of ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* (*)(void*), void*)’

Line number 33 is this: 

pthread_create (&A, NULL, (void *) &functionA, NULL);

Declaration of functionA is this:

void functionA (void*);

Its definition is:

void functionA (void* argA)
{
    while (1)
    {
        pthread_mutex_lock (&mutexA);

        if (count < 0)
        {
            pthread_cond_wait (&conditionVariableA, &mutexA);
        }
        else
        {
            // Do something.
            std :: cout << "\nTime to enjoy!";
        }
        pthread_mutex_unlock (&mutexA);
    }
}

Upvotes: 12

Views: 34374

Answers (4)

Torsten Robitzki
Torsten Robitzki

Reputation: 2555

And as you are using a C++ compiler, you should use a function with C binding, as pthread_create expects a C function:

extern "C" void* functionA (void*);

C++ and C may have the same calling conventions on your current platform, but there is no guaranty, that this will be the case on other platforms or will be so in the future.

Upvotes: 3

mathematician1975
mathematician1975

Reputation: 21351

Use 

pthread_create (&A, NULL, functionA, NULL);

instead of casting.

Also the function you use to pass to pthread_create should return a void* so to avoid any problems later, consider changing the function signature to accomodate this.

Upvotes: 4

Some programmer dude
Some programmer dude

Reputation: 409166

If you look at the manual page you will see that the function argument is

void *(*start_routine) (void *)

That is, a pointer to a function which takes one void * argument and returns void *.

To get rid of your errors, change your function to return void *, and pass it without type-casting it. The return from the thread function can be a simple return NULL if you don't care about the value.

Upvotes: 18

Zeta
Zeta

Reputation: 105886

(void *) &functionA will cast your function pointer functionA which is of type void (*)(void*) to a simple void*. The later can't be converted to the first again, so the compiler reports an error. This is one of the reasons why you shouldn't use C-style casts.

Use pthread_create (&A, NULL, functionA, NULL); instead.

Also, the return type of a thread function should be void*, not void. So change void functionA(void*) to void* functionA(void*).

Upvotes: 4

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