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 %u or %x
19 unsigned short int %u or %x
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.
61 Some of the extended specifiers print the data on the given address instead
62 of printing the address itself. In this case, the following error messages
63 might be printed instead of the unreachable information::
65 (null) data on plain NULL address
66 (efault) data on invalid address
67 (einval) invalid data on a valid address
74 %p abcdef12 or 00000000abcdef12
76 Pointers printed without a specifier extension (i.e unadorned %p) are
77 hashed to prevent leaking information about the kernel memory layout. This
78 has the added benefit of providing a unique identifier. On 64-bit machines
79 the first 32 bits are zeroed. The kernel will print ``(ptrval)`` until it
80 gathers enough entropy. If you *really* want the address see %px below.
89 For printing error pointers (i.e. a pointer for which IS_ERR() is true)
90 as a symbolic error name. Error values for which no symbolic name is
91 known are printed in decimal, while a non-ERR_PTR passed as the
92 argument to %pe gets treated as ordinary %p.
94 Symbols/Function Pointers
95 -------------------------
99 %pS versatile_init+0x0/0x110
101 %pSR versatile_init+0x9/0x110
102 (with __builtin_extract_return_addr() translation)
103 %pB prev_fn_of_versatile_init+0x88/0x88
106 The ``S`` and ``s`` specifiers are used for printing a pointer in symbolic
107 format. They result in the symbol name with (S) or without (s)
108 offsets. If KALLSYMS are disabled then the symbol address is printed instead.
110 The ``B`` specifier results in the symbol name with offsets and should be
111 used when printing stack backtraces. The specifier takes into
112 consideration the effect of compiler optimisations which may occur
113 when tail-calls are used and marked with the noreturn GCC attribute.
115 Probed Pointers from BPF / tracing
116 ----------------------------------
123 The ``k`` and ``u`` specifiers are used for printing prior probed memory from
124 either kernel memory (k) or user memory (u). The subsequent ``s`` specifier
125 results in printing a string. For direct use in regular vsnprintf() the (k)
126 and (u) annotation is ignored, however, when used out of BPF's bpf_trace_printk(),
127 for example, it reads the memory it is pointing to without faulting.
134 %pK 01234567 or 0123456789abcdef
136 For printing kernel pointers which should be hidden from unprivileged
137 users. The behaviour of %pK depends on the kptr_restrict sysctl - see
138 Documentation/admin-guide/sysctl/kernel.rst for more details.
145 %px 01234567 or 0123456789abcdef
147 For printing pointers when you *really* want to print the address. Please
148 consider whether or not you are leaking sensitive information about the
149 kernel memory layout before printing pointers with %px. %px is functionally
150 equivalent to %lx (or %lu). %px is preferred because it is more uniquely
151 grep'able. If in the future we need to modify the way the kernel handles
152 printing pointers we will be better equipped to find the call sites.
162 For printing the pointer differences, use the %t modifier for ptrdiff_t.
166 printk("test: difference between pointers: %td\n", ptr2 - ptr1);
173 %pr [mem 0x60000000-0x6fffffff flags 0x2200] or
174 [mem 0x0000000060000000-0x000000006fffffff flags 0x2200]
175 %pR [mem 0x60000000-0x6fffffff pref] or
176 [mem 0x0000000060000000-0x000000006fffffff pref]
178 For printing struct resources. The ``R`` and ``r`` specifiers result in a
179 printed resource with (R) or without (r) a decoded flags member.
183 Physical address types phys_addr_t
184 ----------------------------------
188 %pa[p] 0x01234567 or 0x0123456789abcdef
190 For printing a phys_addr_t type (and its derivatives, such as
191 resource_size_t) which can vary based on build options, regardless of the
192 width of the CPU data path.
196 DMA address types dma_addr_t
197 ----------------------------
201 %pad 0x01234567 or 0x0123456789abcdef
203 For printing a dma_addr_t type which can vary based on build options,
204 regardless of the width of the CPU data path.
208 Raw buffer as an escaped string
209 -------------------------------
215 For printing raw buffer as an escaped string. For the following buffer::
217 1b 62 20 5c 43 07 22 90 0d 5d
219 A few examples show how the conversion would be done (excluding surrounding
222 %*pE "\eb \C\a"\220\r]"
223 %*pEhp "\x1bb \C\x07"\x90\x0d]"
224 %*pEa "\e\142\040\\\103\a\042\220\r\135"
226 The conversion rules are applied according to an optional combination
227 of flags (see :c:func:`string_escape_mem` kernel documentation for the
238 By default ESCAPE_ANY_NP is used.
240 ESCAPE_ANY_NP is the sane choice for many cases, in particularly for
243 If field width is omitted then 1 byte only will be escaped.
245 Raw buffer as a hex string
246 --------------------------
251 %*phC 00:01:02: ... :3f
252 %*phD 00-01-02- ... -3f
255 For printing small buffers (up to 64 bytes long) as a hex string with a
256 certain separator. For larger buffers consider using
257 :c:func:`print_hex_dump`.
264 %pM 00:01:02:03:04:05
265 %pMR 05:04:03:02:01:00
266 %pMF 00-01-02-03-04-05
270 For printing 6-byte MAC/FDDI addresses in hex notation. The ``M`` and ``m``
271 specifiers result in a printed address with (M) or without (m) byte
272 separators. The default byte separator is the colon (:).
274 Where FDDI addresses are concerned the ``F`` specifier can be used after
275 the ``M`` specifier to use dash (-) separators instead of the default
278 For Bluetooth addresses the ``R`` specifier shall be used after the ``M``
279 specifier to use reversed byte order suitable for visual interpretation
280 of Bluetooth addresses which are in the little endian order.
293 For printing IPv4 dot-separated decimal addresses. The ``I4`` and ``i4``
294 specifiers result in a printed address with (i4) or without (I4) leading
297 The additional ``h``, ``n``, ``b``, and ``l`` specifiers are used to specify
298 host, network, big or little endian order addresses respectively. Where
299 no specifier is provided the default network/big endian order is used.
308 %pI6 0001:0002:0003:0004:0005:0006:0007:0008
309 %pi6 00010002000300040005000600070008
310 %pI6c 1:2:3:4:5:6:7:8
312 For printing IPv6 network-order 16-bit hex addresses. The ``I6`` and ``i6``
313 specifiers result in a printed address with (I6) or without (i6)
314 colon-separators. Leading zeros are always used.
316 The additional ``c`` specifier can be used with the ``I`` specifier to
317 print a compressed IPv6 address as described by
318 http://tools.ietf.org/html/rfc5952
322 IPv4/IPv6 addresses (generic, with port, flowinfo, scope)
323 ---------------------------------------------------------
327 %pIS 1.2.3.4 or 0001:0002:0003:0004:0005:0006:0007:0008
328 %piS 001.002.003.004 or 00010002000300040005000600070008
329 %pISc 1.2.3.4 or 1:2:3:4:5:6:7:8
330 %pISpc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345
333 For printing an IP address without the need to distinguish whether it's of
334 type AF_INET or AF_INET6. A pointer to a valid struct sockaddr,
335 specified through ``IS`` or ``iS``, can be passed to this format specifier.
337 The additional ``p``, ``f``, and ``s`` specifiers are used to specify port
338 (IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ``:`` prefix,
339 flowinfo a ``/`` and scope a ``%``, each followed by the actual value.
341 In case of an IPv6 address the compressed IPv6 address as described by
342 http://tools.ietf.org/html/rfc5952 is being used if the additional
343 specifier ``c`` is given. The IPv6 address is surrounded by ``[``, ``]`` in
344 case of additional specifiers ``p``, ``f`` or ``s`` as suggested by
345 https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07
347 In case of IPv4 addresses, the additional ``h``, ``n``, ``b``, and ``l``
348 specifiers can be used as well and are ignored in case of an IPv6
355 %pISfc 1.2.3.4 or [1:2:3:4:5:6:7:8]/123456789
356 %pISsc 1.2.3.4 or [1:2:3:4:5:6:7:8]%1234567890
357 %pISpfc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345/123456789
364 %pUb 00010203-0405-0607-0809-0a0b0c0d0e0f
365 %pUB 00010203-0405-0607-0809-0A0B0C0D0E0F
366 %pUl 03020100-0504-0706-0809-0a0b0c0e0e0f
367 %pUL 03020100-0504-0706-0809-0A0B0C0E0E0F
369 For printing 16-byte UUID/GUIDs addresses. The additional ``l``, ``L``,
370 ``b`` and ``B`` specifiers are used to specify a little endian order in
371 lower (l) or upper case (L) hex notation - and big endian order in lower (b)
372 or upper case (B) hex notation.
374 Where no additional specifiers are used the default big endian
375 order with lower case hex notation will be printed.
387 For printing dentry name; if we race with :c:func:`d_move`, the name might
388 be a mix of old and new ones, but it won't oops. %pd dentry is a safer
389 equivalent of %s dentry->d_name.name we used to use, %pd<n> prints ``n``
390 last components. %pD does the same thing for struct file.
399 %pg sda, sda1 or loop0p1
401 For printing name of block_device pointers.
410 For printing struct va_format structures. These contain a format string
411 and va_list as follows::
418 Implements a "recursive vsnprintf".
420 Do not use this feature without some mechanism to verify the
421 correctness of the format string and va_list arguments.
433 For printing device tree node structures. Default behaviour is
436 - f - device node full_name
437 - n - device node name
438 - p - device node phandle
439 - P - device node path spec (name + @unit)
440 - F - device node flags
441 - c - major compatible string
442 - C - full compatible string
444 The separator when using multiple arguments is ':'
448 %pOF /foo/bar@0 - Node full name
449 %pOFf /foo/bar@0 - Same as above
450 %pOFfp /foo/bar@0:10 - Node full name + phandle
451 %pOFfcF /foo/bar@0:foo,device:--P- - Node full name +
452 major compatible string +
468 For printing information on fwnode handles. The default is to print the full
469 node name, including the path. The modifiers are functionally equivalent to
472 - f - full name of the node, including the path
473 - P - the name of the node including an address (if there is one)
477 %pfwf \_SB.PCI0.CIO2.port@1.endpoint@0 - Full node name
478 %pfwP endpoint@0 - Node name
482 %pfwf /ocp@68000000/i2c@48072000/camera@10/port/endpoint - Full name
483 %pfwP endpoint - Node name
485 Time and date (struct rtc_time)
486 -------------------------------
490 %ptR YYYY-mm-ddTHH:MM:SS
495 For printing date and time as represented by struct rtc_time structure in
496 human readable format.
498 By default year will be incremented by 1900 and month by 1. Use %ptRr (raw)
499 to suppress this behaviour.
511 For printing struct clk structures. %pC and %pCn print the name of the clock
512 (Common Clock Framework) or a unique 32-bit ID (legacy clock framework).
516 bitmap and its derivatives such as cpumask and nodemask
517 -------------------------------------------------------
524 For printing bitmap and its derivatives such as cpumask and nodemask,
525 %*pb outputs the bitmap with field width as the number of bits and %*pbl
526 output the bitmap as range list with field width as the number of bits.
530 Flags bitfields such as page flags, gfp_flags
531 ---------------------------------------------
535 %pGp referenced|uptodate|lru|active|private
536 %pGg GFP_USER|GFP_DMA32|GFP_NOWARN
537 %pGv read|exec|mayread|maywrite|mayexec|denywrite
539 For printing flags bitfields as a collection of symbolic constants that
540 would construct the value. The type of flags is given by the third
541 character. Currently supported are [p]age flags, [v]ma_flags (both
542 expect ``unsigned long *``) and [g]fp_flags (expects ``gfp_t *``). The flag
543 names and print order depends on the particular type.
545 Note that this format should not be used directly in the
546 :c:func:`TP_printk()` part of a tracepoint. Instead, use the show_*_flags()
547 functions from <trace/events/mmflags.h>.
551 Network device features
552 -----------------------
556 %pNF 0x000000000000c000
558 For printing netdev_features_t.
565 If you add other %p extensions, please extend <lib/test_printf.c> with
566 one or more test cases, if at all feasible.
568 Thank you for your cooperation and attention.