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 ------------------------------------------------------------
17 unsigned char %hhu or %hhx
19 unsigned short int %hu or %hx
23 unsigned long %lu or %lx
24 long long %lld or %llx
25 unsigned long long %llu or %llx
38 If <type> is dependent on a config option for its size (e.g., sector_t,
39 blkcnt_t) or is architecture-dependent for its size (e.g., tcflag_t), use a
40 format specifier of its largest possible type and explicitly cast to it.
44 printk("test: sector number/total blocks: %llu/%llu\n",
45 (unsigned long long)sector, (unsigned long long)blockcount);
47 Reminder: sizeof() returns type size_t.
49 The kernel's printf does not support %n. Floating point formats (%e, %f,
50 %g, %a) are also not recognized, for obvious reasons. Use of any
51 unsupported specifier or length qualifier results in a WARN and early
52 return from vsnprintf().
57 A raw pointer value may be printed with %p which will hash the address
58 before printing. The kernel also supports extended specifiers for printing
59 pointers of different types.
66 %p abcdef12 or 00000000abcdef12
68 Pointers printed without a specifier extension (i.e unadorned %p) are
69 hashed to prevent leaking information about the kernel memory layout. This
70 has the added benefit of providing a unique identifier. On 64-bit machines
71 the first 32 bits are zeroed. The kernel will print ``(ptrval)`` until it
72 gathers enough entropy. If you *really* want the address see %px below.
74 Symbols/Function Pointers
75 -------------------------
79 %pS versatile_init+0x0/0x110
81 %pF versatile_init+0x0/0x110
83 %pSR versatile_init+0x9/0x110
84 (with __builtin_extract_return_addr() translation)
85 %pB prev_fn_of_versatile_init+0x88/0x88
88 The ``S`` and ``s`` specifiers are used for printing a pointer in symbolic
89 format. They result in the symbol name with (S) or without (s)
90 offsets. If KALLSYMS are disabled then the symbol address is printed instead.
92 Note, that the ``F`` and ``f`` specifiers are identical to ``S`` (``s``)
93 and thus deprecated. We have ``F`` and ``f`` because on ia64, ppc64 and
94 parisc64 function pointers are indirect and, in fact, are function
95 descriptors, which require additional dereferencing before we can lookup
96 the symbol. As of now, ``S`` and ``s`` perform dereferencing on those
97 platforms (when needed), so ``F`` and ``f`` exist for compatibility
100 The ``B`` specifier results in the symbol name with offsets and should be
101 used when printing stack backtraces. The specifier takes into
102 consideration the effect of compiler optimisations which may occur
103 when tail-calls are used and marked with the noreturn GCC attribute.
110 %pK 01234567 or 0123456789abcdef
112 For printing kernel pointers which should be hidden from unprivileged
113 users. The behaviour of %pK depends on the kptr_restrict sysctl - see
114 Documentation/sysctl/kernel.txt for more details.
121 %px 01234567 or 0123456789abcdef
123 For printing pointers when you *really* want to print the address. Please
124 consider whether or not you are leaking sensitive information about the
125 kernel memory layout before printing pointers with %px. %px is functionally
126 equivalent to %lx (or %lu). %px is preferred because it is more uniquely
127 grep'able. If in the future we need to modify the way the kernel handles
128 printing pointers we will be better equipped to find the call sites.
135 %pr [mem 0x60000000-0x6fffffff flags 0x2200] or
136 [mem 0x0000000060000000-0x000000006fffffff flags 0x2200]
137 %pR [mem 0x60000000-0x6fffffff pref] or
138 [mem 0x0000000060000000-0x000000006fffffff pref]
140 For printing struct resources. The ``R`` and ``r`` specifiers result in a
141 printed resource with (R) or without (r) a decoded flags member.
145 Physical address types phys_addr_t
146 ----------------------------------
150 %pa[p] 0x01234567 or 0x0123456789abcdef
152 For printing a phys_addr_t type (and its derivatives, such as
153 resource_size_t) which can vary based on build options, regardless of the
154 width of the CPU data path.
158 DMA address types dma_addr_t
159 ----------------------------
163 %pad 0x01234567 or 0x0123456789abcdef
165 For printing a dma_addr_t type which can vary based on build options,
166 regardless of the width of the CPU data path.
170 Raw buffer as an escaped string
171 -------------------------------
177 For printing raw buffer as an escaped string. For the following buffer::
179 1b 62 20 5c 43 07 22 90 0d 5d
181 A few examples show how the conversion would be done (excluding surrounding
184 %*pE "\eb \C\a"\220\r]"
185 %*pEhp "\x1bb \C\x07"\x90\x0d]"
186 %*pEa "\e\142\040\\\103\a\042\220\r\135"
188 The conversion rules are applied according to an optional combination
189 of flags (see :c:func:`string_escape_mem` kernel documentation for the
200 By default ESCAPE_ANY_NP is used.
202 ESCAPE_ANY_NP is the sane choice for many cases, in particularly for
205 If field width is omitted then 1 byte only will be escaped.
207 Raw buffer as a hex string
208 --------------------------
213 %*phC 00:01:02: ... :3f
214 %*phD 00-01-02- ... -3f
217 For printing small buffers (up to 64 bytes long) as a hex string with a
218 certain separator. For larger buffers consider using
219 :c:func:`print_hex_dump`.
226 %pM 00:01:02:03:04:05
227 %pMR 05:04:03:02:01:00
228 %pMF 00-01-02-03-04-05
232 For printing 6-byte MAC/FDDI addresses in hex notation. The ``M`` and ``m``
233 specifiers result in a printed address with (M) or without (m) byte
234 separators. The default byte separator is the colon (:).
236 Where FDDI addresses are concerned the ``F`` specifier can be used after
237 the ``M`` specifier to use dash (-) separators instead of the default
240 For Bluetooth addresses the ``R`` specifier shall be used after the ``M``
241 specifier to use reversed byte order suitable for visual interpretation
242 of Bluetooth addresses which are in the little endian order.
255 For printing IPv4 dot-separated decimal addresses. The ``I4`` and ``i4``
256 specifiers result in a printed address with (i4) or without (I4) leading
259 The additional ``h``, ``n``, ``b``, and ``l`` specifiers are used to specify
260 host, network, big or little endian order addresses respectively. Where
261 no specifier is provided the default network/big endian order is used.
270 %pI6 0001:0002:0003:0004:0005:0006:0007:0008
271 %pi6 00010002000300040005000600070008
272 %pI6c 1:2:3:4:5:6:7:8
274 For printing IPv6 network-order 16-bit hex addresses. The ``I6`` and ``i6``
275 specifiers result in a printed address with (I6) or without (i6)
276 colon-separators. Leading zeros are always used.
278 The additional ``c`` specifier can be used with the ``I`` specifier to
279 print a compressed IPv6 address as described by
280 http://tools.ietf.org/html/rfc5952
284 IPv4/IPv6 addresses (generic, with port, flowinfo, scope)
285 ---------------------------------------------------------
289 %pIS 1.2.3.4 or 0001:0002:0003:0004:0005:0006:0007:0008
290 %piS 001.002.003.004 or 00010002000300040005000600070008
291 %pISc 1.2.3.4 or 1:2:3:4:5:6:7:8
292 %pISpc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345
295 For printing an IP address without the need to distinguish whether it's of
296 type AF_INET or AF_INET6. A pointer to a valid struct sockaddr,
297 specified through ``IS`` or ``iS``, can be passed to this format specifier.
299 The additional ``p``, ``f``, and ``s`` specifiers are used to specify port
300 (IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ``:`` prefix,
301 flowinfo a ``/`` and scope a ``%``, each followed by the actual value.
303 In case of an IPv6 address the compressed IPv6 address as described by
304 http://tools.ietf.org/html/rfc5952 is being used if the additional
305 specifier ``c`` is given. The IPv6 address is surrounded by ``[``, ``]`` in
306 case of additional specifiers ``p``, ``f`` or ``s`` as suggested by
307 https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07
309 In case of IPv4 addresses, the additional ``h``, ``n``, ``b``, and ``l``
310 specifiers can be used as well and are ignored in case of an IPv6
317 %pISfc 1.2.3.4 or [1:2:3:4:5:6:7:8]/123456789
318 %pISsc 1.2.3.4 or [1:2:3:4:5:6:7:8]%1234567890
319 %pISpfc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345/123456789
326 %pUb 00010203-0405-0607-0809-0a0b0c0d0e0f
327 %pUB 00010203-0405-0607-0809-0A0B0C0D0E0F
328 %pUl 03020100-0504-0706-0809-0a0b0c0e0e0f
329 %pUL 03020100-0504-0706-0809-0A0B0C0E0E0F
331 For printing 16-byte UUID/GUIDs addresses. The additional ``l``, ``L``,
332 ``b`` and ``B`` specifiers are used to specify a little endian order in
333 lower (l) or upper case (L) hex notation - and big endian order in lower (b)
334 or upper case (B) hex notation.
336 Where no additional specifiers are used the default big endian
337 order with lower case hex notation will be printed.
349 For printing dentry name; if we race with :c:func:`d_move`, the name might
350 be a mix of old and new ones, but it won't oops. %pd dentry is a safer
351 equivalent of %s dentry->d_name.name we used to use, %pd<n> prints ``n``
352 last components. %pD does the same thing for struct file.
361 %pg sda, sda1 or loop0p1
363 For printing name of block_device pointers.
372 For printing struct va_format structures. These contain a format string
373 and va_list as follows::
380 Implements a "recursive vsnprintf".
382 Do not use this feature without some mechanism to verify the
383 correctness of the format string and va_list arguments.
395 For printing device tree node structures. Default behaviour is
398 - f - device node full_name
399 - n - device node name
400 - p - device node phandle
401 - P - device node path spec (name + @unit)
402 - F - device node flags
403 - c - major compatible string
404 - C - full compatible string
406 The separator when using multiple arguments is ':'
410 %pOF /foo/bar@0 - Node full name
411 %pOFf /foo/bar@0 - Same as above
412 %pOFfp /foo/bar@0:10 - Node full name + phandle
413 %pOFfcF /foo/bar@0:foo,device:--P- - Node full name +
414 major compatible string +
423 Time and date (struct rtc_time)
424 -------------------------------
428 %ptR YYYY-mm-ddTHH:MM:SS
433 For printing date and time as represented by struct rtc_time structure in
434 human readable format.
436 By default year will be incremented by 1900 and month by 1. Use %ptRr (raw)
437 to suppress this behaviour.
449 For printing struct clk structures. %pC and %pCn print the name of the clock
450 (Common Clock Framework) or a unique 32-bit ID (legacy clock framework).
454 bitmap and its derivatives such as cpumask and nodemask
455 -------------------------------------------------------
462 For printing bitmap and its derivatives such as cpumask and nodemask,
463 %*pb outputs the bitmap with field width as the number of bits and %*pbl
464 output the bitmap as range list with field width as the number of bits.
468 Flags bitfields such as page flags, gfp_flags
469 ---------------------------------------------
473 %pGp referenced|uptodate|lru|active|private
474 %pGg GFP_USER|GFP_DMA32|GFP_NOWARN
475 %pGv read|exec|mayread|maywrite|mayexec|denywrite
477 For printing flags bitfields as a collection of symbolic constants that
478 would construct the value. The type of flags is given by the third
479 character. Currently supported are [p]age flags, [v]ma_flags (both
480 expect ``unsigned long *``) and [g]fp_flags (expects ``gfp_t *``). The flag
481 names and print order depends on the particular type.
483 Note that this format should not be used directly in the
484 :c:func:`TP_printk()` part of a tracepoint. Instead, use the show_*_flags()
485 functions from <trace/events/mmflags.h>.
489 Network device features
490 -----------------------
494 %pNF 0x000000000000c000
496 For printing netdev_features_t.
503 If you add other %p extensions, please extend <lib/test_printf.c> with
504 one or more test cases, if at all feasible.
506 Thank you for your cooperation and attention.