IS_ERR ERR_PTR PTR_ERR

/*
 * Kernel pointers have redundant information, so we can use a
 * scheme where we can return either an error code or a dentry
 * pointer with the same return value.
 *
 * This should be a per-architecture thing, to allow different
 * error and pointer decisions.
 */
static inline void *ERR_PTR(long error)
{
        return (void *) error;
}

static inline long PTR_ERR(const void *ptr)
{
        return (long) ptr;
}

static inline long IS_ERR(const void *ptr)
{
        return unlikely((unsigned long)ptr > (unsigned long)-1000L);
}

int err;
void *perr;

err = -ENOMEM;
perr = ERR_PTR(err);

printk("err.%d perr.%p \n", err, perr);

if (IS_ERR(perr))
{
  printk("PTR_ERR changed ==> err.%d", (int)PTR_ERR(perr));
}