Hi Neeraj, the statement (unsigned long)&((struct module *)0L)->persist_start has nothing to do with NULL. The above statement will not get converted in to phase 2 but directly goes to phase 3. NULL doesnot come in to picture. You can test it like this. #undef NULL #define NULL 100 (unsigned long)&((struct module *)0L)->persist_start (unsigned long)&((struct module *)NULL)->persist_start the first statement will give the correct offset of the persist_start, whereas the second one will give the persist_start+100. what i mean is the first staement is independent of the NULL value. sreedhar. -----Original Message----- From: Neeraj [mailto:neerajk@cse.iitk.ac.in] Sent: Friday, April 19, 2002 2:02 AM To: kernelnewbies@nl.linux.org Subject: Re: sys_init_module() => NULL isn't used anywhere in this code. 0 is used. Essentially, you've => subtracted 0 from the original address, which is unnecessary. Hi John, I think you misinterpreted the thing. As far as I remember, The C standard says that if a 0(zero) value is assigned to a pointer or is type-casted to a pointer then it is converted to the NULL pointer, i.e. it is treated as the NULL pointer. The value of the NULL pointer is arch/compiler dependent and cannot be guaranteed that it will be zero. The code in module.c makes use of this point that it presumes that the value of NULL pointer is zero. In my understanding the compiler interprets the code as : 1. (unsigned long)&((struct module *)0L)->persist_start // this parses to 2. 2. (unsigned long)&(NULL)->persist_start) // this parses to 3. 3. (unsigned long)&(value at address (0 + offset of persist_start)) // this parses to 4. 4. (unsigned long)(0 + offset of persist_start) // and this gives the offset of persist_start in the structure So, in this manner NULL comes into play. Do correct me if I am wrong in interpreting the things. neeraj => => On Thu, 18 Apr 2002, Neeraj wrote: => => > Thanks John. => > => > Later I understood the code. But I would like to ask that is this code portable => > ? becoz when you are using (struct module *)0L then it is going to become a NULL => > pointer and it is not necessary NULL will have a value 0 (zero). So would'nt it => > be better if this code is written as : => > => > ((unsigned long)&((struct module *)0L)->persist_start - (unsigned long)(struct => > module *)0L) => > => > I think this is portable! => > => > -neeraj => > => > => 0L refers to the number 0 as a Long integer. => > => => > => Basically, what is happening is, the kernel is acting as if the module => > => struct is located at address 0. Obviously, this structure isn't there, but => > => the effect is that the address returned is the size of the data contained => > => in the structure before the persist_start member. => > => => > => John => > => => > => On Thu, 18 Apr 2002, Neeraj wrote: => > => => > => > Hello friends, => > => > => > => > I have just started reading the kernel code and hence a kernel newbie. Right now => > => > I am reading the code of Linux kernel version 2.4.3. The file module.c conatins => > => > a function sys_init_module. I am not able to the understand the following LOC. => > => > => > => > <snip> => > => > => > => > if (mod_user_size < (unsigned long)&((struct module *)0L)->persist_start => > => > => > => > </snip> => > => > => > => > What does 0L refers to ? Can somebody please explain it to me. => > => > => > => > Thanks => > => > neeraj => > => > => > => > => > => > => > => > -- => > => > Kernelnewbies: Help each other learn about the Linux kernel. => > => > Archive: http://mail.nl.linux.org/kernelnewbies/ => > => > FAQ: http://kernelnewbies.org/faq/ => > => > => > => => > => => > => > -- => > Kernelnewbies: Help each other learn about the Linux kernel. => > Archive: http://mail.nl.linux.org/kernelnewbies/ => > FAQ: http://kernelnewbies.org/faq/ => > => => -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/ -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
