Tried this on two machines:

SunOS hornet 5.10 Generic sun4u sparc SUNW,Ultra-4
    with GCC 4.0.1
Linux numenor 2.6.13 #9 Mon Sep 19 19:03:35 PDT 2005 i686 unknown unknown GNU/Linux
    with GCC 3.3.6

The following code produces spurios warning:
/* Cut here */
int x_read(int h, void *buf, unsigned len);
int x_write(int h, const void *buf, unsigned len);

typedef int (*x_io)(int h, void *buf, unsigned len);
int blockio(int h, long long offset, void *buf, x_io action);

int bug(int h, unsigned where, void *buf)
{
        return blockio(h, (long long)where << 10, buf, x_write);
}
/* Cut here */
sample.c: In function `bug':
sample.c:9: warning: passing arg 4 of `blockio' from incompatible pointer type