invalid conversion from ‘void* (*)()’ to ‘void* (*)(void*)’ when using pthread in C++

Hi,
I am using pthread in my main.cpp file and got error in the following code:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

pthread_mutex_t count_mutex     = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t condition_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t  condition_cond  = PTHREAD_COND_INITIALIZER;

void *functionCount1();
void *functionCount2();
int  count = 0;
#define COUNT_DONE  10
#define COUNT_HALT1  3
#define COUNT_HALT2  6

int main()
{
   pthread_t thread1, thread2;

   pthread_create( &thread1, NULL, &functionCount1, NULL);
   pthread_create( &thread2, NULL, &functionCount2, NULL);
   pthread_join( thread1, NULL);
   pthread_join( thread2, NULL);

   return 0;
}

void *functionCount1()
{
   for(;;)
   {
      pthread_mutex_lock( &condition_mutex );
      while( count >= COUNT_HALT1 && count <= COUNT_HALT2 )
      {
         pthread_cond_wait( &condition_cond, &condition_mutex );
      }
      pthread_mutex_unlock( &condition_mutex );

      pthread_mutex_lock( &count_mutex );
      count++;
      printf("Counter value functionCount1: %d\n",count);
      pthread_mutex_unlock( &count_mutex );

      if(count >= COUNT_DONE) return(NULL);
    }
}

void *functionCount2()
{
    for(;;)
    {
       pthread_mutex_lock( &condition_mutex );
       if( count < COUNT_HALT1 || count > COUNT_HALT2 )
       {
          pthread_cond_signal( &condition_cond );
       }
       pthread_mutex_unlock( &condition_mutex );

       pthread_mutex_lock( &count_mutex );
       count++;
       printf("Counter value functionCount2: %d\n",count);
       pthread_mutex_unlock( &count_mutex );

       if(count >= COUNT_DONE) return(NULL);
    }

}

g++ gives error:

/home/C/test/main.cpp|96|error: invalid conversion from ‘void* (*)()’ to ‘void* (*)(void*)’|
/home/C/test/main.cpp|96|error: initializing argument 3 of ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* (*)(void*), void*)’|
/home/C/test/main.cpp|97|error: invalid conversion from ‘void* (*)()’ to ‘void* (*)(void*)’|
/home/C/test/main.cpp|97|error: initializing argument 3 of ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* (*)(void*), void*)’|

Could you explain a little bit about these mistakes and how to fix it? Thanks in advance!

Your thread functions should take a void * parameter.

--
Mats

Originally Posted by matsp

Your thread functions should take a void * parameter.

--
Mats

And even if you don't need that parameter you still need to declare it. If the compiler whines about "unused parameter" just don't give it a name

Originally Posted by brewbuck

just don't give it a name

could you do it in C?

Originally Posted by vart

could you do it in C?

I don't think so.

--
Mats

Yes, I have tried to add a parameter to the two functions. It's odd that both

Code:

void *functionCount1(void * p)
void *functionCount2(void * p)

Code:

void *functionCount1(int * p)
void *functionCount2(int * p)

still result in the same error

/home/C/test/main.cpp|96|error: invalid conversion from ‘void* (*)()’ to ‘void* (*)(void*)’|
/home/C/test/main.cpp|96|error: initializing argument 3 of ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* (*)(void*), void*)’|
/home/C/test/main.cpp|97|error: invalid conversion from ‘void* (*)()’ to ‘void* (*)(void*)’|
/home/C/test/main.cpp|97|error: initializing argument 3 of ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* (*)(void*), void*)’|

Seems like the two functions are always recognized as void* (*)(), instead of taking a parameter.

By the way, I copied this example from Linux Tutorial: POSIX Threads as my first multi-threading program, where the code is C. However I am going to apply it to my C++ code.

Did you change your prototypes too?

--
Mats

Originally Posted by brewbuck

And even if you don't need that parameter you still need to declare it. If the compiler whines about "unused parameter" just don't give it a name

I believe in C, you can use...
(void)unused_param;
...instead.

Originally Posted by Elysia

I believe in C, you can use...
(void)unused_param;
...instead.

Yes - that also works in C++ of course.

--
Mats

Did you change your prototypes too?

My bad. Now it works! Thanks!

In case my function takes multiple parameters possibly with different types, how to pass them into pthread_create in the last parameter "void * arg"?

Make a struct or class that you pass along.
Btw, why is this in the C forum when it is C++?

Thanks Elysia!
Sorry to spam here with C++ stuff. I would be careful next time.

and one more programmer has been doomed to eternal suffering... Hopefully he knows what is waiting for him. Synchronization problems, deadlocks, sudden crashes due to wild pointers hitting from other threads, harder debugging, starvations, hard to spot performance issues, pain of asm while writing atomic compare and swap...