Data Type used in kernel .

[greg@kroah.com (Greg KH)]

On Fri, Dec 23, 2016 at 11:52:33AM +0000, Nicholas Mc Guire wrote:
> On Fri, Dec 23, 2016 at 06:33:28AM -0500, Pasquier, Thomas wrote:
> > Hello Wasim,
> > 
> > On Fri, Dec 23, 2016 at 6:20 AM, Wasim Akram <wasim7702843288@gmail.com>
> > wrote:
> > 
> > > Hi ,
> > >
> > > I have seen "bool" data type  is used in linux kernel code . Can we
> > > get documentation on data type used in linux kernel development ?
> > >
> > 
> > I think this should be a good start:
> > http://lxr.free-electrons.com/source/include/linux/types.h
> >
> one of the problems though is that there are a number of
> "historic" type systems "left over" in the kernel it seems so
> there actually is a bit of a mess and its not simple to 
> figure out what things really are. For x86_64 I have a list
> of type matches I use for code reading (still not complete)
> which might help.
>  
> x86 64bit:
> ==========
> 
> Type              : equivalent types
> bool              : _Bool,
> char              : signed char, __signed__ char, __s8, s8, int8_t
> unsigned char     : unsigned char, u_char, __u8, u8, unchar, u_int8_t, uint8_t
>                     __ticket_t,
>                     insn_byte_t,
>                     kprobe_opcode_t
> short             : signed short, __signed__ short, s16, int16_t
> unsigned short    : unsigned short, __u16, u16, u_short, ushort, u_int16_t, uint16_t,
>                     __sum16,
>                     umode_t,
>                     __ticketpair_t,
>                     apm_eventinfo_t,
>                     apm_event_t,
>                     Elf32_Sword,
>                     Elf32_Half,
>                     Elf64_Half,
>                     Elf64_SHalf,
>                     __kernel_sa_family_t,
>                     __kernel_gid16_t, gid16_t,
>                     __kernel_uid16_t, uid16_t,
>                     __kernel_old_uid_t, old_uid_t,
>                     __kernel_old_gid_t, old_gid_t,
>                     arch cond: __le16/__be16
> int               : signed int, __signed__ int, __s32, s32, int32_t,  s_int32_t      
>                     Elf64_Sword,
>                     Elf64_Sxword,
>                     key_serial_t,
>                     insn_value_t,
>                     __kernel_daddr_t, daddr_t,
>                     __kernel_ipc_pid_t, pid_t,
>                     __kernel_pid_t,
>                     __kernel_mqd_t, mqd_t,
>                     __kernel_key_t, key_t,
>                     __kernel_timer_t timer_t,
> unsigned int      : unsigned, unsigned int, __u32, u32, u_int32_t, uInt, u_int, uint, uint32_t,
>                     __wsum,
>                     nlink_t,
>                     gfp_t,
>                     fmode_t,
>                     oom_flags_t,
>                     isolate_mode_t,
>                     Elf32_Addr,
>                     Elf32_Off,
>                     Elf32_Word,
>                     Elf64_Word,
>                     key_perm_t,
>                     insn_attr_t,
>                     sk_buff_data_t,
>                     sctp_assoc_t.
>                     __kernel_dev_t, dev_t, 
>                     __kernel_uid32_t, uid_t, projid_t, qid_t
>                     __kernel_gid32_t, gid_t,
>                     __kernel_uid_t, 
>                     __kernel_gid_t,
>                     __kernel_mode_t, mode_t,
>                     arch cond: __le32/__be32
> long              : signed long, __kernel_long_t, 
>                     __kernel_suseconds_t, suseconds_t,
>                     __kernel_ssize_t, ssize_t,
>                     __kernel_ptrdiff_t,
>                     __kernel_off_t, off_t,
>                     __kernel_time_t, time_t,
>                     __kernel_clock_t, clock_t,
> unsigned long     : __kernel_ulong_t, u_long, ulong
>                     uintptr_t,
>                     sector_t,
>                     blkcnt_t,
>                     irq_hw_number_t,
>                     pteval_t,
>                     pmdval_t,
>                     pudval_t,
>                     pgdval_t,
>                     pgprotval_t,
>                     vm_flags_t,
>                     elf_greg_t,
>                     cputime_t,
>                     old_sigset_t,
>                     aio_context_t,
>                     flow_compare_t,
>                     __kernel_ino_t, ino_t 
>                     __kernel_old_dev_t
>                     __kernel_size_t. size_t,
>                     arch cond: __le64/__be64
> long long         : signed long long, __signed__ long long, __s64, s64, int64_t
>                     time64_t,
>                     qsize_t,
>                     __kernel_loff_t, loff_t,
> unsigned long long: u64, __u64, uint64_t, u_int64_t
>                     dma_addr_t,
>                     cycles_t,
>                     phys_addr_t, resource_size_t
>                     cycle_t,
>                     Elf64_Addr,
>                     Elf64_Off,
>                     Elf64_Xword,
>                     cputime64_t,
>                     netdev_features_t

That's crazy.  Just use the data types that the LDD book says to use for
in-kernel code:
	u8, u16, u32, u64, s8, s16, s32, s64, int, bool
and if the data crosses the user/kernel boundry, use the correct ones
for that (__ in front, no int).

You can use others, but you better know what you are doing if you do :)

Hope this helps,

greg k-h