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>
13 If variable is of Type, use printk format specifier:
14 ------------------------------------------------------------
18 unsigned long %lu or %lx
19 long long %lld or %llx
20 unsigned long long %llu or %llx
28 If <type> is dependent on a config option for its size (e.g., ``sector_t``,
29 ``blkcnt_t``) or is architecture-dependent for its size (e.g., ``tcflag_t``),
30 use a format specifier of its largest possible type and explicitly cast to it.
34 printk("test: sector number/total blocks: %llu/%llu\n",
35 (unsigned long long)sector, (unsigned long long)blockcount);
37 Reminder: ``sizeof()`` result is of type ``size_t``.
39 The kernel's printf does not support ``%n``. For obvious reasons, floating
40 point formats (``%e, %f, %g, %a``) are also not recognized. Use of any
41 unsupported specifier or length qualifier results in a WARN and early
42 return from vsnprintf.
44 Raw pointer value SHOULD be printed with %p. The kernel supports
45 the following extended format specifiers for pointer types:
50 Pointers printed without a specifier extension (i.e unadorned %p) are
51 hashed to give a unique identifier without leaking kernel addresses to user
52 space. On 64 bit machines the first 32 bits are zeroed.
56 %p abcdef12 or 00000000abcdef12
58 Symbols/Function Pointers
59 =========================
63 %pF versatile_init+0x0/0x110
65 %pS versatile_init+0x0/0x110
66 %pSR versatile_init+0x9/0x110
67 (with __builtin_extract_return_addr() translation)
69 %pB prev_fn_of_versatile_init+0x88/0x88
71 The ``F`` and ``f`` specifiers are for printing function pointers,
72 for example, f->func, &gettimeofday. They have the same result as
73 ``S`` and ``s`` specifiers. But they do an extra conversion on
74 ia64, ppc64 and parisc64 architectures where the function pointers
75 are actually function descriptors.
77 The ``S`` and ``s`` specifiers can be used for printing symbols
78 from direct addresses, for example, __builtin_return_address(0),
79 (void *)regs->ip. They result in the symbol name with (``S``) or
80 without (``s``) offsets. If KALLSYMS are disabled then the symbol
81 address is printed instead.
83 The ``B`` specifier results in the symbol name with offsets and should be
84 used when printing stack backtraces. The specifier takes into
85 consideration the effect of compiler optimisations which may occur
86 when tail-call``s are used and marked with the noreturn GCC attribute.
90 printk("Going to call: %pF\n", gettimeofday);
91 printk("Going to call: %pF\n", p->func);
92 printk("%s: called from %pS\n", __func__, (void *)_RET_IP_);
93 printk("%s: called from %pS\n", __func__,
94 (void *)__builtin_return_address(0));
95 printk("Faulted at %pS\n", (void *)regs->ip);
96 printk(" %s%pB\n", (reliable ? "" : "? "), (void *)*stack);
103 %pK 01234567 or 0123456789abcdef
105 For printing kernel pointers which should be hidden from unprivileged
106 users. The behaviour of ``%pK`` depends on the ``kptr_restrict sysctl`` - see
107 Documentation/sysctl/kernel.txt for more details.
114 %pr [mem 0x60000000-0x6fffffff flags 0x2200] or
115 [mem 0x0000000060000000-0x000000006fffffff flags 0x2200]
116 %pR [mem 0x60000000-0x6fffffff pref] or
117 [mem 0x0000000060000000-0x000000006fffffff pref]
119 For printing struct resources. The ``R`` and ``r`` specifiers result in a
120 printed resource with (``R``) or without (``r``) a decoded flags member.
123 Physical addresses types ``phys_addr_t``
124 ========================================
128 %pa[p] 0x01234567 or 0x0123456789abcdef
130 For printing a ``phys_addr_t`` type (and its derivatives, such as
131 ``resource_size_t``) which can vary based on build options, regardless of
132 the width of the CPU data path. Passed by reference.
134 DMA addresses types ``dma_addr_t``
135 ==================================
139 %pad 0x01234567 or 0x0123456789abcdef
141 For printing a ``dma_addr_t`` type which can vary based on build options,
142 regardless of the width of the CPU data path. Passed by reference.
144 Raw buffer as an escaped string
145 ===============================
151 For printing raw buffer as an escaped string. For the following buffer::
153 1b 62 20 5c 43 07 22 90 0d 5d
155 few examples show how the conversion would be done (the result string
156 without surrounding quotes)::
158 %*pE "\eb \C\a"\220\r]"
159 %*pEhp "\x1bb \C\x07"\x90\x0d]"
160 %*pEa "\e\142\040\\\103\a\042\220\r\135"
162 The conversion rules are applied according to an optional combination
163 of flags (see :c:func:`string_escape_mem` kernel documentation for the
167 - ``c`` - ESCAPE_SPECIAL
169 - ``n`` - ESCAPE_NULL
170 - ``o`` - ESCAPE_OCTAL
172 - ``s`` - ESCAPE_SPACE
174 By default ESCAPE_ANY_NP is used.
176 ESCAPE_ANY_NP is the sane choice for many cases, in particularly for
179 If field width is omitted the 1 byte only will be escaped.
181 Raw buffer as a hex string
182 ==========================
187 %*phC 00:01:02: ... :3f
188 %*phD 00-01-02- ... -3f
191 For printing a small buffers (up to 64 bytes long) as a hex string with
192 certain separator. For the larger buffers consider to use
193 :c:func:`print_hex_dump`.
200 %pM 00:01:02:03:04:05
201 %pMR 05:04:03:02:01:00
202 %pMF 00-01-02-03-04-05
206 For printing 6-byte MAC/FDDI addresses in hex notation. The ``M`` and ``m``
207 specifiers result in a printed address with (``M``) or without (``m``) byte
208 separators. The default byte separator is the colon (``:``).
210 Where FDDI addresses are concerned the ``F`` specifier can be used after
211 the ``M`` specifier to use dash (``-``) separators instead of the default
214 For Bluetooth addresses the ``R`` specifier shall be used after the ``M``
215 specifier to use reversed byte order suitable for visual interpretation
216 of Bluetooth addresses which are in the little endian order.
229 For printing IPv4 dot-separated decimal addresses. The ``I4`` and ``i4``
230 specifiers result in a printed address with (``i4``) or without (``I4``)
233 The additional ``h``, ``n``, ``b``, and ``l`` specifiers are used to specify
234 host, network, big or little endian order addresses respectively. Where
235 no specifier is provided the default network/big endian order is used.
244 %pI6 0001:0002:0003:0004:0005:0006:0007:0008
245 %pi6 00010002000300040005000600070008
246 %pI6c 1:2:3:4:5:6:7:8
248 For printing IPv6 network-order 16-bit hex addresses. The ``I6`` and ``i6``
249 specifiers result in a printed address with (``I6``) or without (``i6``)
250 colon-separators. Leading zeros are always used.
252 The additional ``c`` specifier can be used with the ``I`` specifier to
253 print a compressed IPv6 address as described by
254 http://tools.ietf.org/html/rfc5952
258 IPv4/IPv6 addresses (generic, with port, flowinfo, scope)
259 =========================================================
263 %pIS 1.2.3.4 or 0001:0002:0003:0004:0005:0006:0007:0008
264 %piS 001.002.003.004 or 00010002000300040005000600070008
265 %pISc 1.2.3.4 or 1:2:3:4:5:6:7:8
266 %pISpc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345
269 For printing an IP address without the need to distinguish whether it``s
270 of type AF_INET or AF_INET6, a pointer to a valid ``struct sockaddr``,
271 specified through ``IS`` or ``iS``, can be passed to this format specifier.
273 The additional ``p``, ``f``, and ``s`` specifiers are used to specify port
274 (IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ``:`` prefix,
275 flowinfo a ``/`` and scope a ``%``, each followed by the actual value.
277 In case of an IPv6 address the compressed IPv6 address as described by
278 http://tools.ietf.org/html/rfc5952 is being used if the additional
279 specifier ``c`` is given. The IPv6 address is surrounded by ``[``, ``]`` in
280 case of additional specifiers ``p``, ``f`` or ``s`` as suggested by
281 https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07
283 In case of IPv4 addresses, the additional ``h``, ``n``, ``b``, and ``l``
284 specifiers can be used as well and are ignored in case of an IPv6
291 %pISfc 1.2.3.4 or [1:2:3:4:5:6:7:8]/123456789
292 %pISsc 1.2.3.4 or [1:2:3:4:5:6:7:8]%1234567890
293 %pISpfc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345/123456789
300 %pUb 00010203-0405-0607-0809-0a0b0c0d0e0f
301 %pUB 00010203-0405-0607-0809-0A0B0C0D0E0F
302 %pUl 03020100-0504-0706-0809-0a0b0c0e0e0f
303 %pUL 03020100-0504-0706-0809-0A0B0C0E0E0F
305 For printing 16-byte UUID/GUIDs addresses. The additional 'l', 'L',
306 'b' and 'B' specifiers are used to specify a little endian order in
307 lower ('l') or upper case ('L') hex characters - and big endian order
308 in lower ('b') or upper case ('B') hex characters.
310 Where no additional specifiers are used the default big endian
311 order with lower case hex characters will be printed.
323 For printing dentry name; if we race with :c:func:`d_move`, the name might be
324 a mix of old and new ones, but it won't oops. ``%pd`` dentry is a safer
325 equivalent of ``%s`` ``dentry->d_name.name`` we used to use, ``%pd<n>`` prints
326 ``n`` last components. ``%pD`` does the same thing for struct file.
335 %pg sda, sda1 or loop0p1
337 For printing name of block_device pointers.
346 For printing struct va_format structures. These contain a format string
347 and va_list as follows::
354 Implements a "recursive vsnprintf".
356 Do not use this feature without some mechanism to verify the
357 correctness of the format string and va_list arguments.
368 Base specifier for kobject based structs. Must be followed with
369 character for specific type of kobject as listed below:
375 For printing device tree nodes. The optional arguments are:
376 f device node full_name
378 p device node phandle
379 P device node path spec (name + @unit)
381 c major compatible string
382 C full compatible string
383 Without any arguments prints full_name (same as %pOFf)
384 The separator when using multiple arguments is ':'
388 %pOF /foo/bar@0 - Node full name
389 %pOFf /foo/bar@0 - Same as above
390 %pOFfp /foo/bar@0:10 - Node full name + phandle
391 %pOFfcF /foo/bar@0:foo,device:--P- - Node full name +
392 major compatible string +
411 For printing struct clk structures. ``%pC`` and ``%pCn`` print the name
412 (Common Clock Framework) or address (legacy clock framework) of the
413 structure; ``%pCr`` prints the current clock rate.
417 bitmap and its derivatives such as cpumask and nodemask
418 =======================================================
425 For printing bitmap and its derivatives such as cpumask and nodemask,
426 ``%*pb`` output the bitmap with field width as the number of bits and ``%*pbl``
427 output the bitmap as range list with field width as the number of bits.
431 Flags bitfields such as page flags, gfp_flags
432 =============================================
436 %pGp referenced|uptodate|lru|active|private
437 %pGg GFP_USER|GFP_DMA32|GFP_NOWARN
438 %pGv read|exec|mayread|maywrite|mayexec|denywrite
440 For printing flags bitfields as a collection of symbolic constants that
441 would construct the value. The type of flags is given by the third
442 character. Currently supported are [p]age flags, [v]ma_flags (both
443 expect ``unsigned long *``) and [g]fp_flags (expects ``gfp_t *``). The flag
444 names and print order depends on the particular type.
446 Note that this format should not be used directly in :c:func:`TP_printk()` part
447 of a tracepoint. Instead, use the ``show_*_flags()`` functions from
448 <trace/events/mmflags.h>.
452 Network device features
453 =======================
457 %pNF 0x000000000000c000
459 For printing netdev_features_t.
463 If you add other ``%p`` extensions, please extend lib/test_printf.c with
464 one or more test cases, if at all feasible.
467 Thank you for your cooperation and attention.