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
30 If <type> is dependent on a config option for its size (e.g., sector_t,
31 blkcnt_t) or is architecture-dependent for its size (e.g., tcflag_t), use a
32 format specifier of its largest possible type and explicitly cast to it.
36 printk("test: sector number/total blocks: %llu/%llu\n",
37 (unsigned long long)sector, (unsigned long long)blockcount);
39 Reminder: sizeof() returns type size_t.
41 The kernel's printf does not support %n. Floating point formats (%e, %f,
42 %g, %a) are also not recognized, for obvious reasons. Use of any
43 unsupported specifier or length qualifier results in a WARN and early
44 return from vsnprintf().
49 A raw pointer value may be printed with %p which will hash the address
50 before printing. The kernel also supports extended specifiers for printing
51 pointers of different types.
58 %p abcdef12 or 00000000abcdef12
60 Pointers printed without a specifier extension (i.e unadorned %p) are
61 hashed to prevent leaking information about the kernel memory layout. This
62 has the added benefit of providing a unique identifier. On 64-bit machines
63 the first 32 bits are zeroed. If you *really* want the address see %px
66 Symbols/Function Pointers
67 -------------------------
71 %pF versatile_init+0x0/0x110
73 %pS versatile_init+0x0/0x110
74 %pSR versatile_init+0x9/0x110
75 (with __builtin_extract_return_addr() translation)
77 %pB prev_fn_of_versatile_init+0x88/0x88
80 The ``F`` and ``f`` specifiers are for printing function pointers,
81 for example, f->func, &gettimeofday. They have the same result as
82 ``S`` and ``s`` specifiers. But they do an extra conversion on
83 ia64, ppc64 and parisc64 architectures where the function pointers
84 are actually function descriptors.
86 The ``S`` and ``s`` specifiers can be used for printing symbols
87 from direct addresses, for example, __builtin_return_address(0),
88 (void *)regs->ip. They result in the symbol name with (S) or
89 without (s) offsets. If KALLSYMS are disabled then the symbol
90 address is printed instead.
92 The ``B`` specifier results in the symbol name with offsets and should be
93 used when printing stack backtraces. The specifier takes into
94 consideration the effect of compiler optimisations which may occur
95 when tail-calls are used and marked with the noreturn GCC attribute.
99 printk("Going to call: %pF\n", gettimeofday);
100 printk("Going to call: %pF\n", p->func);
101 printk("%s: called from %pS\n", __func__, (void *)_RET_IP_);
102 printk("%s: called from %pS\n", __func__,
103 (void *)__builtin_return_address(0));
104 printk("Faulted at %pS\n", (void *)regs->ip);
105 printk(" %s%pB\n", (reliable ? "" : "? "), (void *)*stack);
112 %pK 01234567 or 0123456789abcdef
114 For printing kernel pointers which should be hidden from unprivileged
115 users. The behaviour of %pK depends on the kptr_restrict sysctl - see
116 Documentation/sysctl/kernel.txt for more details.
123 %px 01234567 or 0123456789abcdef
125 For printing pointers when you *really* want to print the address. Please
126 consider whether or not you are leaking sensitive information about the
127 kernel memory layout before printing pointers with %px. %px is functionally
128 equivalent to %lx (or %lu). %px is preferred because it is more uniquely
129 grep'able. If in the future we need to modify the way the kernel handles
130 printing pointers we will be better equipped to find the call sites.
137 %pr [mem 0x60000000-0x6fffffff flags 0x2200] or
138 [mem 0x0000000060000000-0x000000006fffffff flags 0x2200]
139 %pR [mem 0x60000000-0x6fffffff pref] or
140 [mem 0x0000000060000000-0x000000006fffffff pref]
142 For printing struct resources. The ``R`` and ``r`` specifiers result in a
143 printed resource with (R) or without (r) a decoded flags member.
147 Physical address types phys_addr_t
148 ----------------------------------
152 %pa[p] 0x01234567 or 0x0123456789abcdef
154 For printing a phys_addr_t type (and its derivatives, such as
155 resource_size_t) which can vary based on build options, regardless of the
156 width of the CPU data path.
160 DMA address types dma_addr_t
161 ----------------------------
165 %pad 0x01234567 or 0x0123456789abcdef
167 For printing a dma_addr_t type which can vary based on build options,
168 regardless of the width of the CPU data path.
172 Raw buffer as an escaped string
173 -------------------------------
179 For printing raw buffer as an escaped string. For the following buffer::
181 1b 62 20 5c 43 07 22 90 0d 5d
183 A few examples show how the conversion would be done (excluding surrounding
186 %*pE "\eb \C\a"\220\r]"
187 %*pEhp "\x1bb \C\x07"\x90\x0d]"
188 %*pEa "\e\142\040\\\103\a\042\220\r\135"
190 The conversion rules are applied according to an optional combination
191 of flags (see :c:func:`string_escape_mem` kernel documentation for the
202 By default ESCAPE_ANY_NP is used.
204 ESCAPE_ANY_NP is the sane choice for many cases, in particularly for
207 If field width is omitted then 1 byte only will be escaped.
209 Raw buffer as a hex string
210 --------------------------
215 %*phC 00:01:02: ... :3f
216 %*phD 00-01-02- ... -3f
219 For printing small buffers (up to 64 bytes long) as a hex string with a
220 certain separator. For larger buffers consider using
221 :c:func:`print_hex_dump`.
228 %pM 00:01:02:03:04:05
229 %pMR 05:04:03:02:01:00
230 %pMF 00-01-02-03-04-05
234 For printing 6-byte MAC/FDDI addresses in hex notation. The ``M`` and ``m``
235 specifiers result in a printed address with (M) or without (m) byte
236 separators. The default byte separator is the colon (:).
238 Where FDDI addresses are concerned the ``F`` specifier can be used after
239 the ``M`` specifier to use dash (-) separators instead of the default
242 For Bluetooth addresses the ``R`` specifier shall be used after the ``M``
243 specifier to use reversed byte order suitable for visual interpretation
244 of Bluetooth addresses which are in the little endian order.
257 For printing IPv4 dot-separated decimal addresses. The ``I4`` and ``i4``
258 specifiers result in a printed address with (i4) or without (I4) leading
261 The additional ``h``, ``n``, ``b``, and ``l`` specifiers are used to specify
262 host, network, big or little endian order addresses respectively. Where
263 no specifier is provided the default network/big endian order is used.
272 %pI6 0001:0002:0003:0004:0005:0006:0007:0008
273 %pi6 00010002000300040005000600070008
274 %pI6c 1:2:3:4:5:6:7:8
276 For printing IPv6 network-order 16-bit hex addresses. The ``I6`` and ``i6``
277 specifiers result in a printed address with (I6) or without (i6)
278 colon-separators. Leading zeros are always used.
280 The additional ``c`` specifier can be used with the ``I`` specifier to
281 print a compressed IPv6 address as described by
282 http://tools.ietf.org/html/rfc5952
286 IPv4/IPv6 addresses (generic, with port, flowinfo, scope)
287 ---------------------------------------------------------
291 %pIS 1.2.3.4 or 0001:0002:0003:0004:0005:0006:0007:0008
292 %piS 001.002.003.004 or 00010002000300040005000600070008
293 %pISc 1.2.3.4 or 1:2:3:4:5:6:7:8
294 %pISpc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345
297 For printing an IP address without the need to distinguish whether it's of
298 type AF_INET or AF_INET6. A pointer to a valid struct sockaddr,
299 specified through ``IS`` or ``iS``, can be passed to this format specifier.
301 The additional ``p``, ``f``, and ``s`` specifiers are used to specify port
302 (IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ``:`` prefix,
303 flowinfo a ``/`` and scope a ``%``, each followed by the actual value.
305 In case of an IPv6 address the compressed IPv6 address as described by
306 http://tools.ietf.org/html/rfc5952 is being used if the additional
307 specifier ``c`` is given. The IPv6 address is surrounded by ``[``, ``]`` in
308 case of additional specifiers ``p``, ``f`` or ``s`` as suggested by
309 https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07
311 In case of IPv4 addresses, the additional ``h``, ``n``, ``b``, and ``l``
312 specifiers can be used as well and are ignored in case of an IPv6
319 %pISfc 1.2.3.4 or [1:2:3:4:5:6:7:8]/123456789
320 %pISsc 1.2.3.4 or [1:2:3:4:5:6:7:8]%1234567890
321 %pISpfc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345/123456789
328 %pUb 00010203-0405-0607-0809-0a0b0c0d0e0f
329 %pUB 00010203-0405-0607-0809-0A0B0C0D0E0F
330 %pUl 03020100-0504-0706-0809-0a0b0c0e0e0f
331 %pUL 03020100-0504-0706-0809-0A0B0C0E0E0F
333 For printing 16-byte UUID/GUIDs addresses. The additional ``l``, ``L``,
334 ``b`` and ``B`` specifiers are used to specify a little endian order in
335 lower (l) or upper case (L) hex notation - and big endian order in lower (b)
336 or upper case (B) hex notation.
338 Where no additional specifiers are used the default big endian
339 order with lower case hex notation will be printed.
351 For printing dentry name; if we race with :c:func:`d_move`, the name might
352 be a mix of old and new ones, but it won't oops. %pd dentry is a safer
353 equivalent of %s dentry->d_name.name we used to use, %pd<n> prints ``n``
354 last components. %pD does the same thing for struct file.
363 %pg sda, sda1 or loop0p1
365 For printing name of block_device pointers.
374 For printing struct va_format structures. These contain a format string
375 and va_list as follows::
382 Implements a "recursive vsnprintf".
384 Do not use this feature without some mechanism to verify the
385 correctness of the format string and va_list arguments.
397 For printing kobject based structs (device nodes). Default behaviour is
400 - f - device node full_name
401 - n - device node name
402 - p - device node phandle
403 - P - device node path spec (name + @unit)
404 - F - device node flags
405 - c - major compatible string
406 - C - full compatible string
408 The separator when using multiple arguments is ':'
412 %pOF /foo/bar@0 - Node full name
413 %pOFf /foo/bar@0 - Same as above
414 %pOFfp /foo/bar@0:10 - Node full name + phandle
415 %pOFfcF /foo/bar@0:foo,device:--P- - Node full name +
416 major compatible string +
434 For printing struct clk structures. %pC and %pCn print the name
435 (Common Clock Framework) or address (legacy clock framework) of the
436 structure; %pCr prints the current clock rate.
440 bitmap and its derivatives such as cpumask and nodemask
441 -------------------------------------------------------
448 For printing bitmap and its derivatives such as cpumask and nodemask,
449 %*pb outputs the bitmap with field width as the number of bits and %*pbl
450 output the bitmap as range list with field width as the number of bits.
454 Flags bitfields such as page flags, gfp_flags
455 ---------------------------------------------
459 %pGp referenced|uptodate|lru|active|private
460 %pGg GFP_USER|GFP_DMA32|GFP_NOWARN
461 %pGv read|exec|mayread|maywrite|mayexec|denywrite
463 For printing flags bitfields as a collection of symbolic constants that
464 would construct the value. The type of flags is given by the third
465 character. Currently supported are [p]age flags, [v]ma_flags (both
466 expect ``unsigned long *``) and [g]fp_flags (expects ``gfp_t *``). The flag
467 names and print order depends on the particular type.
469 Note that this format should not be used directly in the
470 :c:func:`TP_printk()` part of a tracepoint. Instead, use the show_*_flags()
471 functions from <trace/events/mmflags.h>.
475 Network device features
476 -----------------------
480 %pNF 0x000000000000c000
482 For printing netdev_features_t.
489 If you add other %p extensions, please extend <lib/test_printf.c> with
490 one or more test cases, if at all feasible.
492 Thank you for your cooperation and attention.