1 =========================================
2 How to get printk format specifiers right
3 =========================================
5 :Author: Randy Dunlap <rdunlap@infradead.org>
6 :Author: Andrew Murray <amurray@mpc-data.co.uk>
14 If variable is of Type, use printk format specifier:
15 ------------------------------------------------------------
19 unsigned long %lu or %lx
20 long long %lld or %llx
21 unsigned long long %llu or %llx
29 If <type> is dependent on a config option for its size (e.g., ``sector_t``,
30 ``blkcnt_t``) or is architecture-dependent for its size (e.g., ``tcflag_t``),
31 use a format specifier of its largest possible type and explicitly cast to it.
35 printk("test: sector number/total blocks: %llu/%llu\n",
36 (unsigned long long)sector, (unsigned long long)blockcount);
38 Reminder: ``sizeof()`` result is of type ``size_t``.
40 The kernel's printf does not support ``%n``. For obvious reasons, floating
41 point formats (``%e, %f, %g, %a``) are also not recognized. Use of any
42 unsupported specifier or length qualifier results in a WARN and early
43 return from vsnprintf.
45 Raw pointer value SHOULD be printed with %p. The kernel supports
46 the following extended format specifiers for pointer types:
48 Symbols/Function Pointers
49 =========================
53 %pF versatile_init+0x0/0x110
55 %pS versatile_init+0x0/0x110
56 %pSR versatile_init+0x9/0x110
57 (with __builtin_extract_return_addr() translation)
59 %pB prev_fn_of_versatile_init+0x88/0x88
61 The ``F`` and ``f`` specifiers are for printing function pointers,
62 for example, f->func, &gettimeofday. They have the same result as
63 ``S`` and ``s`` specifiers. But they do an extra conversion on
64 ia64, ppc64 and parisc64 architectures where the function pointers
65 are actually function descriptors.
67 The ``S`` and ``s`` specifiers can be used for printing symbols
68 from direct addresses, for example, __builtin_return_address(0),
69 (void *)regs->ip. They result in the symbol name with (``S``) or
70 without (``s``) offsets. If KALLSYMS are disabled then the symbol
71 address is printed instead.
73 The ``B`` specifier results in the symbol name with offsets and should be
74 used when printing stack backtraces. The specifier takes into
75 consideration the effect of compiler optimisations which may occur
76 when tail-call``s are used and marked with the noreturn GCC attribute.
84 %pK 0x01234567 or 0x0123456789abcdef
86 For printing kernel pointers which should be hidden from unprivileged
87 users. The behaviour of ``%pK`` depends on the ``kptr_restrict sysctl`` - see
88 Documentation/sysctl/kernel.txt for more details.
95 %pr [mem 0x60000000-0x6fffffff flags 0x2200] or
96 [mem 0x0000000060000000-0x000000006fffffff flags 0x2200]
97 %pR [mem 0x60000000-0x6fffffff pref] or
98 [mem 0x0000000060000000-0x000000006fffffff pref]
100 For printing struct resources. The ``R`` and ``r`` specifiers result in a
101 printed resource with (``R``) or without (``r``) a decoded flags member.
104 Physical addresses types ``phys_addr_t``
105 ========================================
109 %pa[p] 0x01234567 or 0x0123456789abcdef
111 For printing a ``phys_addr_t`` type (and its derivatives, such as
112 ``resource_size_t``) which can vary based on build options, regardless of
113 the width of the CPU data path. Passed by reference.
115 DMA addresses types ``dma_addr_t``
116 ==================================
120 %pad 0x01234567 or 0x0123456789abcdef
122 For printing a ``dma_addr_t`` type which can vary based on build options,
123 regardless of the width of the CPU data path. Passed by reference.
125 Raw buffer as an escaped string
126 ===============================
132 For printing raw buffer as an escaped string. For the following buffer::
134 1b 62 20 5c 43 07 22 90 0d 5d
136 few examples show how the conversion would be done (the result string
137 without surrounding quotes)::
139 %*pE "\eb \C\a"\220\r]"
140 %*pEhp "\x1bb \C\x07"\x90\x0d]"
141 %*pEa "\e\142\040\\\103\a\042\220\r\135"
143 The conversion rules are applied according to an optional combination
144 of flags (see :c:func:`string_escape_mem` kernel documentation for the
148 - ``c`` - ESCAPE_SPECIAL
150 - ``n`` - ESCAPE_NULL
151 - ``o`` - ESCAPE_OCTAL
153 - ``s`` - ESCAPE_SPACE
155 By default ESCAPE_ANY_NP is used.
157 ESCAPE_ANY_NP is the sane choice for many cases, in particularly for
160 If field width is omitted the 1 byte only will be escaped.
162 Raw buffer as a hex string
163 ==========================
168 %*phC 00:01:02: ... :3f
169 %*phD 00-01-02- ... -3f
172 For printing a small buffers (up to 64 bytes long) as a hex string with
173 certain separator. For the larger buffers consider to use
174 :c:func:`print_hex_dump`.
181 %pM 00:01:02:03:04:05
182 %pMR 05:04:03:02:01:00
183 %pMF 00-01-02-03-04-05
187 For printing 6-byte MAC/FDDI addresses in hex notation. The ``M`` and ``m``
188 specifiers result in a printed address with (``M``) or without (``m``) byte
189 separators. The default byte separator is the colon (``:``).
191 Where FDDI addresses are concerned the ``F`` specifier can be used after
192 the ``M`` specifier to use dash (``-``) separators instead of the default
195 For Bluetooth addresses the ``R`` specifier shall be used after the ``M``
196 specifier to use reversed byte order suitable for visual interpretation
197 of Bluetooth addresses which are in the little endian order.
210 For printing IPv4 dot-separated decimal addresses. The ``I4`` and ``i4``
211 specifiers result in a printed address with (``i4``) or without (``I4``)
214 The additional ``h``, ``n``, ``b``, and ``l`` specifiers are used to specify
215 host, network, big or little endian order addresses respectively. Where
216 no specifier is provided the default network/big endian order is used.
225 %pI6 0001:0002:0003:0004:0005:0006:0007:0008
226 %pi6 00010002000300040005000600070008
227 %pI6c 1:2:3:4:5:6:7:8
229 For printing IPv6 network-order 16-bit hex addresses. The ``I6`` and ``i6``
230 specifiers result in a printed address with (``I6``) or without (``i6``)
231 colon-separators. Leading zeros are always used.
233 The additional ``c`` specifier can be used with the ``I`` specifier to
234 print a compressed IPv6 address as described by
235 http://tools.ietf.org/html/rfc5952
239 IPv4/IPv6 addresses (generic, with port, flowinfo, scope)
240 =========================================================
244 %pIS 1.2.3.4 or 0001:0002:0003:0004:0005:0006:0007:0008
245 %piS 001.002.003.004 or 00010002000300040005000600070008
246 %pISc 1.2.3.4 or 1:2:3:4:5:6:7:8
247 %pISpc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345
250 For printing an IP address without the need to distinguish whether it``s
251 of type AF_INET or AF_INET6, a pointer to a valid ``struct sockaddr``,
252 specified through ``IS`` or ``iS``, can be passed to this format specifier.
254 The additional ``p``, ``f``, and ``s`` specifiers are used to specify port
255 (IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ``:`` prefix,
256 flowinfo a ``/`` and scope a ``%``, each followed by the actual value.
258 In case of an IPv6 address the compressed IPv6 address as described by
259 http://tools.ietf.org/html/rfc5952 is being used if the additional
260 specifier ``c`` is given. The IPv6 address is surrounded by ``[``, ``]`` in
261 case of additional specifiers ``p``, ``f`` or ``s`` as suggested by
262 https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07
264 In case of IPv4 addresses, the additional ``h``, ``n``, ``b``, and ``l``
265 specifiers can be used as well and are ignored in case of an IPv6
272 %pISfc 1.2.3.4 or [1:2:3:4:5:6:7:8]/123456789
273 %pISsc 1.2.3.4 or [1:2:3:4:5:6:7:8]%1234567890
274 %pISpfc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345/123456789
281 %pUb 00010203-0405-0607-0809-0a0b0c0d0e0f
282 %pUB 00010203-0405-0607-0809-0A0B0C0D0E0F
283 %pUl 03020100-0504-0706-0809-0a0b0c0e0e0f
284 %pUL 03020100-0504-0706-0809-0A0B0C0E0E0F
286 For printing 16-byte UUID/GUIDs addresses. The additional 'l', 'L',
287 'b' and 'B' specifiers are used to specify a little endian order in
288 lower ('l') or upper case ('L') hex characters - and big endian order
289 in lower ('b') or upper case ('B') hex characters.
291 Where no additional specifiers are used the default big endian
292 order with lower case hex characters will be printed.
304 For printing dentry name; if we race with :c:func:`d_move`, the name might be
305 a mix of old and new ones, but it won't oops. ``%pd`` dentry is a safer
306 equivalent of ``%s`` ``dentry->d_name.name`` we used to use, ``%pd<n>`` prints
307 ``n`` last components. ``%pD`` does the same thing for struct file.
316 %pg sda, sda1 or loop0p1
318 For printing name of block_device pointers.
327 For printing struct va_format structures. These contain a format string
328 and va_list as follows::
335 Implements a "recursive vsnprintf".
337 Do not use this feature without some mechanism to verify the
338 correctness of the format string and va_list arguments.
349 Base specifier for kobject based structs. Must be followed with
350 character for specific type of kobject as listed below:
356 For printing device tree nodes. The optional arguments are:
357 f device node full_name
359 p device node phandle
360 P device node path spec (name + @unit)
362 c major compatible string
363 C full compatible string
364 Without any arguments prints full_name (same as %pOFf)
365 The separator when using multiple arguments is ':'
369 %pOF /foo/bar@0 - Node full name
370 %pOFf /foo/bar@0 - Same as above
371 %pOFfp /foo/bar@0:10 - Node full name + phandle
372 %pOFfcF /foo/bar@0:foo,device:--P- - Node full name +
373 major compatible string +
392 For printing struct clk structures. ``%pC`` and ``%pCn`` print the name
393 (Common Clock Framework) or address (legacy clock framework) of the
394 structure; ``%pCr`` prints the current clock rate.
398 bitmap and its derivatives such as cpumask and nodemask
399 =======================================================
406 For printing bitmap and its derivatives such as cpumask and nodemask,
407 ``%*pb`` output the bitmap with field width as the number of bits and ``%*pbl``
408 output the bitmap as range list with field width as the number of bits.
412 Flags bitfields such as page flags, gfp_flags
413 =============================================
417 %pGp referenced|uptodate|lru|active|private
418 %pGg GFP_USER|GFP_DMA32|GFP_NOWARN
419 %pGv read|exec|mayread|maywrite|mayexec|denywrite
421 For printing flags bitfields as a collection of symbolic constants that
422 would construct the value. The type of flags is given by the third
423 character. Currently supported are [p]age flags, [v]ma_flags (both
424 expect ``unsigned long *``) and [g]fp_flags (expects ``gfp_t *``). The flag
425 names and print order depends on the particular type.
427 Note that this format should not be used directly in :c:func:`TP_printk()` part
428 of a tracepoint. Instead, use the ``show_*_flags()`` functions from
429 <trace/events/mmflags.h>.
433 Network device features
434 =======================
438 %pNF 0x000000000000c000
440 For printing netdev_features_t.
444 If you add other ``%p`` extensions, please extend lib/test_printf.c with
445 one or more test cases, if at all feasible.
448 Thank you for your cooperation and attention.