path: root/Documentation/printk-formats.txt
diff options
authorAndrew Murray <>2011-06-15 12:57:09 -0700
committerLinus Torvalds <>2011-06-15 21:52:50 -0700
commit04c55715cbd5de526046745bca3d3b6ffa6641c6 (patch)
treefe63e426a56f951b8bb864ca850ceca61165d793 /Documentation/printk-formats.txt
parenta1b6ae8ed01736dae19a33e74ea4e8dd88f891b5 (diff)
Documentation: update printk-formats.txt
This patch updates the incomplete documentation concerning the printk extended format specifiers. Signed-off-by: Andrew Murray <> Signed-off-by: Randy Dunlap <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'Documentation/printk-formats.txt')
1 files changed, 117 insertions, 2 deletions
diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt
index 1b5a5ddbc3ef..5df176ed59b8 100644
--- a/Documentation/printk-formats.txt
+++ b/Documentation/printk-formats.txt
@@ -9,7 +9,121 @@ If variable is of Type, use printk format specifier:
size_t %zu or %zx
ssize_t %zd or %zx
-Raw pointer value SHOULD be printed with %p.
+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
+ %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.
+MAC/FDDI addresses:
+ %pM 00:01:02:03:04:05
+ %pMF 00-01-02-03-04-05
+ %pm 000102030405
+ 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
+ separator.
+IPv4 addresses:
+ %pI4
+ %pi4
+ %p[Ii][hnbl]
+ 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
+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.
+struct va_format:
+ %pV
+ 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):
@@ -32,4 +146,5 @@ Reminder: sizeof() result is of type size_t.
Thank you for your cooperation and attention.
-By Randy Dunlap <>
+By Randy Dunlap <> and
+Andrew Murray <>