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If variable is of Type, use printk format specifier:
int %d or %x
unsigned int %u or %x
long %ld or %lx
unsigned long %lu or %lx
long long %lld or %llx
unsigned long long %llu or %llx
size_t %zu or %zx
ssize_t %zd or %zx
Raw pointer value SHOULD be printed with %p. The kernel supports
the following extended format specifiers for pointer types:
Symbols/Function Pointers:
%pF versatile_init+0x0/0x110
%pf versatile_init
%pS versatile_init+0x0/0x110
%pSR versatile_init+0x9/0x110
(with __builtin_extract_return_addr() translation)
%ps versatile_init
%pB prev_fn_of_versatile_init+0x88/0x88
For printing symbols and function pointers. The 'S' and 's' specifiers
result in the symbol name with ('S') or without ('s') offsets. Where
this is used on a kernel without KALLSYMS - the symbol address is
printed instead.
The 'B' specifier results in the symbol name with offsets and should be
used when printing stack backtraces. The specifier takes into
consideration the effect of compiler optimisations which may occur
when tail-call's are used and marked with the noreturn GCC attribute.
On ia64, ppc64 and parisc64 architectures function pointers are
actually function descriptors which must first be resolved. The 'F' and
'f' specifiers perform this resolution and then provide the same
functionality as the 'S' and 's' specifiers.
Kernel Pointers:
%pK 0x01234567 or 0x0123456789abcdef
For printing kernel pointers which should be hidden from unprivileged
users. The behaviour of %pK depends on the kptr_restrict sysctl - see
Documentation/sysctl/kernel.txt for more details.
Struct Resources:
%pr [mem 0x60000000-0x6fffffff flags 0x2200] or
[mem 0x0000000060000000-0x000000006fffffff flags 0x2200]
%pR [mem 0x60000000-0x6fffffff pref] or
[mem 0x0000000060000000-0x000000006fffffff pref]
For printing struct resources. The 'R' and 'r' specifiers result in a
printed resource with ('R') or without ('r') a decoded flags member.
Physical addresses:
%pa 0x01234567 or 0x0123456789abcdef
For printing a phys_addr_t type (and its derivatives, such as
resource_size_t) which can vary based on build options, regardless of
the width of the CPU data path. Passed by reference.
Raw buffer as a hex string:
%*ph 00 01 02 ... 3f
%*phC 00:01:02: ... :3f
%*phD 00-01-02- ... -3f
%*phN 000102 ... 3f
For printing a small buffers (up to 64 bytes long) as a hex string with
certain separator. For the larger buffers consider to use
MAC/FDDI addresses:
%pM 00:01:02:03:04:05
%pMR 05:04:03:02:01:00
%pMF 00-01-02-03-04-05
%pm 000102030405
%pmR 050403020100
For printing 6-byte MAC/FDDI addresses in hex notation. The 'M' and 'm'
specifiers result in a printed address with ('M') or without ('m') byte
separators. The default byte separator is the colon (':').
Where FDDI addresses are concerned the 'F' specifier can be used after
the 'M' specifier to use dash ('-') separators instead of the default
For Bluetooth addresses the 'R' specifier shall be used after the 'M'
specifier to use reversed byte order suitable for visual interpretation
of Bluetooth addresses which are in the little endian order.
IPv4 addresses:
For printing IPv4 dot-separated decimal addresses. The 'I4' and 'i4'
specifiers result in a printed address with ('i4') or without ('I4')
leading zeros.
The additional 'h', 'n', 'b', and 'l' specifiers are used to specify
host, network, big or little endian order addresses respectively. Where
no specifier is provided the default network/big endian order is used.
IPv6 addresses:
%pI6 0001:0002:0003:0004:0005:0006:0007:0008
%pi6 00010002000300040005000600070008
%pI6c 1:2:3:4:5:6:7:8
For printing IPv6 network-order 16-bit hex addresses. The 'I6' and 'i6'
specifiers result in a printed address with ('I6') or without ('i6')
colon-separators. Leading zeros are always used.
The additional 'c' specifier can be used with the 'I' specifier to
print a compressed IPv6 address as described by
IPv4/IPv6 addresses (generic, with port, flowinfo, scope):
%pIS or 0001:0002:0003:0004:0005:0006:0007:0008
%piS or 00010002000300040005000600070008
%pISc or 1:2:3:4:5:6:7:8
%pISpc or [1:2:3:4:5:6:7:8]:12345
For printing an IP address without the need to distinguish whether it's
of type AF_INET or AF_INET6, a pointer to a valid 'struct sockaddr',
specified through 'IS' or 'iS', can be passed to this format specifier.
The additional 'p', 'f', and 's' specifiers are used to specify port
(IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ':' prefix,
flowinfo a '/' and scope a '%', each followed by the actual value.
In case of an IPv6 address the compressed IPv6 address as described by is being used if the additional
specifier 'c' is given. The IPv6 address is surrounded by '[', ']' in
case of additional specifiers 'p', 'f' or 's' as suggested by
In case of IPv4 addresses, the additional 'h', 'n', 'b', and 'l'
specifiers can be used as well and are ignored in case of an IPv6
Further examples:
%pISfc or [1:2:3:4:5:6:7:8]/123456789
%pISsc or [1:2:3:4:5:6:7:8]%1234567890
%pISpfc or [1:2:3:4:5:6:7:8]:12345/123456789
UUID/GUID addresses:
%pUb 00010203-0405-0607-0809-0a0b0c0d0e0f
%pUB 00010203-0405-0607-0809-0A0B0C0D0E0F
%pUl 03020100-0504-0706-0809-0a0b0c0e0e0f
%pUL 03020100-0504-0706-0809-0A0B0C0E0E0F
For printing 16-byte UUID/GUIDs addresses. The additional 'l', 'L',
'b' and 'B' specifiers are used to specify a little endian order in
lower ('l') or upper case ('L') hex characters - and big endian order
in lower ('b') or upper case ('B') hex characters.
Where no additional specifiers are used the default little endian
order with lower case hex characters will be printed.
dentry names:
For printing dentry name; if we race with d_move(), the name might be
a mix of old and new ones, but it won't oops. %pd dentry is a safer
equivalent of %s dentry-> we used to use, %pd<n> prints
n last components. %pD does the same thing for struct file.
struct va_format:
For printing struct va_format structures. These contain a format string
and va_list as follows:
struct va_format {
const char *fmt;
va_list *va;
Do not use this feature without some mechanism to verify the
correctness of the format string and va_list arguments.
u64 SHOULD be printed with %llu/%llx, (unsigned long long):
printk("%llu", u64_var);
s64 SHOULD be printed with %lld/%llx, (long long):
printk("%lld", s64_var);
If <type> is dependent on a config option for its size (e.g., sector_t,
blkcnt_t) or is architecture-dependent for its size (e.g., tcflag_t), use a
format specifier of its largest possible type and explicitly cast to it.
printk("test: sector number/total blocks: %llu/%llu\n",
(unsigned long long)sector, (unsigned long long)blockcount);
Reminder: sizeof() result is of type size_t.
Thank you for your cooperation and attention.
By Randy Dunlap <> and
Andrew Murray <>