Typecasting void pointer

saurabhchokshi · 04-13-2010, 08:49 PM

Hello Geeks,

I have 32-bit app,

short *app;
int *int_app;
void *p;

// Some code

app = (short *)p;
int_app = (int *)p;

This works fine on 32-bit platform. However it fails on 64-bit platform. It says " cast to pointer from integer of different size"

But, if I use long * pointer, then there won't be any error? I know that void * pointer on 64-bit is 64 bits size.

Any idea about this error?

Thanks,
Saurabh

Sergei Steshenko · 04-13-2010, 09:10 PM

Quote:

Originally Posted by saurabhchokshi

Hello Geeks,

I have 32-bit app,

short *app;
int *int_app;
void *p;

// Some code

app = (short *)p;
int_app = (int *)p;

This works fine on 32-bit platform. However it fails on 64-bit platform. It says " cast to pointer from integer of different size"

But, if I use long * pointer, then there won't be any error? I know that void * pointer on 64-bit is 64 bits size.

Any idea about this error?

Thanks,
Saurabh

Don't use 'int', 'long', etc., just use 'size_t' - look it up using your favorite WEB search engine.

Solution with 'size_t' is portable (at least, I believe so based on what I read about it).

ntubski · 04-14-2010, 10:45 AM

Is there a typo in your post? Seems to work fine here:

Code:

~/tmp$ uname -sm
Linux x86_64
~/tmp$ cat ptr.c
void func(void)
{
    short *app;
    int *int_app;
    void *p;

    app = (short *) p;
    int_app = (int *)p;
}

~/tmp$ make ptr.o
gcc -Wall -Wextra -g -ansi -pedantic   -c -o ptr.o ptr.c
~/tmp$

Quote:

just use 'size_t'

I think intptr_t is the type intended for this use.

Sergei Steshenko · 04-14-2010, 10:56 AM

Quote:

Originally Posted by ntubski

...
I think intptr_t is the type intended for this use.

Probably.

However, because of 'size_t' definition it's equivalent. I.e. should be big enough to accommodate any array index, and array my consist of bytes (the smallest addressable unit in "C") and be as big as the whole address space (minus "epsilon" needed for stack, program itself, etc).

So, due to

Code:

a[i] == *(a + i)

identity/property of "C" 'size_t' is big enough for any pointer.

saurabhchokshi · 04-14-2010, 12:41 PM

That means I can't use char * as a String in C to typecast any void pointer data. As we do in pthread_create().

Any idea how should I use char * for pthread_create thread?

Sergei Steshenko · 04-14-2010, 01:00 PM

Quote:

Originally Posted by saurabhchokshi

That means I can't use char * as a String in C to typecast any void pointer data. As we do in pthread_create().

Any idea how should I use char * for pthread_create thread?

This is the signature:

Code:

int pthread_create
  (
  pthread_t *thread,
  const pthread_attr_t *attr,
  void *(*start_routine) (void *),
  void *arg
  );

.

You probably mean the 'void *arg' part. In "C" 'void *' pointer is compatible with any other pointer, i.e. pointer of any other type can be used where 'void *' pointer is expected.