void * conversion problems

[Lovecraft]

I really need some help here. I'm writing a program in c++ on a unix system, using the pthreads package. This here line:

pthread_create(&cars[i],NULL,Car, (void *) i);

is giving me the error:

invalid conversion from `void (*)(int)' to `void*(*)(void*)'

Now, I've done the same thing in another program, and it works just fine, but this won't work. I'm using g++ to compile. Can someone help me out?

[joe_bruin]

well, since you didn't post declaration of Car() and "i", i will speculate. you are trying to pass a function similar to this:

void Car(int i);

as the third argument of pthread_create. this is incorrect, don't do that.

[Valmont]

Originally posted by joe_bruin
well, since you didn't post declaration of Car() and "i", i will speculate. you are trying to pass a function similar to this:

void Car(int i);

as the third argument of pthread_create. this is incorrect, don't do that.

What he does is perfectly legal as long as there is a "car function" taking a void * parameter and returns a void* exit status.

But if I remember correctly:
(void *) i
should be replaced by
&i
Assuming "i" is just an integer.

[Lovecraft]

void *Car(void *arg)

is the car function. I tried &i, but that gave the same error. If seeing more code helps, lemme know, I'll post whatever you need to see. I really am at my wits end.

[joe_bruin]

the error message means the following:
"cannot convert a function pointer for a function that takes in an int and returns a void to a function pointer that takes in a void* and returns a void*".

this code compiles fine.

#include <pthread.h>

void *Car(void *arg)
{
  return NULL;
}

int main()
{
  pthread_t cars[5];
  int i = 0;

  // your call
  pthread_create(&cars[i],NULL,Car, (void *) i);

  return 0;
}

this code generates your error:

#include <pthread.h>

void Car(int arg)
{
}

int main()
{
  pthread_t cars[5];
  int i = 0;

  // your call
  pthread_create(&cars[i],NULL,Car, (void *) i);

  return 0;
}

p.c: In function `int main()':
p.c:13: invalid conversion from `void (*)(int)' to `void*(*)(void*)'

have a look at Car again, something is strange about it (maybe you have a prototype in a header file that doesn't match the body).

btw, there is nothing wrong with "(void *)i" if you intend to have a pointer who's value is i (i'm guessing your Car function will take this value and convert it back into an int).

[Lovecraft]

Gah, I found the problem.

I had a function prototype like this:

void Car(int cid);

but the function was this:

void *Car(void *car);

I forgot to change the prototype when I started to use pthreads.

It compiles fine now. Thanks for the suggestion to check my prototype, I never would have thought to check it. I appreciate the help, thanks so much.

[Valmont]

...as long as there is a "car function" taking a void * parameter and returns a void* exit status...

Lol thanks Joe. I have no time actually