1========================================= 2How to get printk format specifiers right 3========================================= 4 5:Author: Randy Dunlap <rdunlap@infradead.org> 6:Author: Andrew Murray <amurray@mpc-data.co.uk> 7 8Integer types 9============= 10 11:: 12 13 If variable is of Type, use printk format specifier: 14 ------------------------------------------------------------ 15 int %d or %x 16 unsigned int %u or %x 17 long %ld or %lx 18 unsigned long %lu or %lx 19 long long %lld or %llx 20 unsigned long long %llu or %llx 21 size_t %zu or %zx 22 ssize_t %zd or %zx 23 s32 %d or %x 24 u32 %u or %x 25 s64 %lld or %llx 26 u64 %llu or %llx 27 28If <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``), 30use a format specifier of its largest possible type and explicitly cast to it. 31 32Example:: 33 34 printk("test: sector number/total blocks: %llu/%llu\n", 35 (unsigned long long)sector, (unsigned long long)blockcount); 36 37Reminder: ``sizeof()`` result is of type ``size_t``. 38 39The kernel's printf does not support ``%n``. For obvious reasons, floating 40point formats (``%e, %f, %g, %a``) are also not recognized. Use of any 41unsupported specifier or length qualifier results in a WARN and early 42return from vsnprintf. 43 44Raw pointer value SHOULD be printed with %p. The kernel supports 45the following extended format specifiers for pointer types: 46 47Pointer Types 48============= 49 50Pointers printed without a specifier extension (i.e unadorned %p) are 51hashed to give a unique identifier without leaking kernel addresses to user 52space. On 64 bit machines the first 32 bits are zeroed. If you _really_ 53want the address see %px below. 54 55:: 56 57 %p abcdef12 or 00000000abcdef12 58 59Symbols/Function Pointers 60========================= 61 62:: 63 64 %pF versatile_init+0x0/0x110 65 %pf versatile_init 66 %pS versatile_init+0x0/0x110 67 %pSR versatile_init+0x9/0x110 68 (with __builtin_extract_return_addr() translation) 69 %ps versatile_init 70 %pB prev_fn_of_versatile_init+0x88/0x88 71 72The ``F`` and ``f`` specifiers are for printing function pointers, 73for example, f->func, &gettimeofday. They have the same result as 74``S`` and ``s`` specifiers. But they do an extra conversion on 75ia64, ppc64 and parisc64 architectures where the function pointers 76are actually function descriptors. 77 78The ``S`` and ``s`` specifiers can be used for printing symbols 79from direct addresses, for example, __builtin_return_address(0), 80(void *)regs->ip. They result in the symbol name with (``S``) or 81without (``s``) offsets. If KALLSYMS are disabled then the symbol 82address is printed instead. 83 84The ``B`` specifier results in the symbol name with offsets and should be 85used when printing stack backtraces. The specifier takes into 86consideration the effect of compiler optimisations which may occur 87when tail-call``s are used and marked with the noreturn GCC attribute. 88 89Examples:: 90 91 printk("Going to call: %pF\n", gettimeofday); 92 printk("Going to call: %pF\n", p->func); 93 printk("%s: called from %pS\n", __func__, (void *)_RET_IP_); 94 printk("%s: called from %pS\n", __func__, 95 (void *)__builtin_return_address(0)); 96 printk("Faulted at %pS\n", (void *)regs->ip); 97 printk(" %s%pB\n", (reliable ? "" : "? "), (void *)*stack); 98 99Kernel Pointers 100=============== 101 102:: 103 104 %pK 01234567 or 0123456789abcdef 105 106For printing kernel pointers which should be hidden from unprivileged 107users. The behaviour of ``%pK`` depends on the ``kptr_restrict sysctl`` - see 108Documentation/sysctl/kernel.txt for more details. 109 110Unmodified Addresses 111==================== 112 113:: 114 115 %px 01234567 or 0123456789abcdef 116 117For printing pointers when you _really_ want to print the address. Please 118consider whether or not you are leaking sensitive information about the 119Kernel layout in memory before printing pointers with %px. %px is 120functionally equivalent to %lx. %px is preferred to %lx because it is more 121uniquely grep'able. If, in the future, we need to modify the way the Kernel 122handles printing pointers it will be nice to be able to find the call 123sites. 124 125Struct Resources 126================ 127 128:: 129 130 %pr [mem 0x60000000-0x6fffffff flags 0x2200] or 131 [mem 0x0000000060000000-0x000000006fffffff flags 0x2200] 132 %pR [mem 0x60000000-0x6fffffff pref] or 133 [mem 0x0000000060000000-0x000000006fffffff pref] 134 135For printing struct resources. The ``R`` and ``r`` specifiers result in a 136printed resource with (``R``) or without (``r``) a decoded flags member. 137Passed by reference. 138 139Physical addresses types ``phys_addr_t`` 140======================================== 141 142:: 143 144 %pa[p] 0x01234567 or 0x0123456789abcdef 145 146For printing a ``phys_addr_t`` type (and its derivatives, such as 147``resource_size_t``) which can vary based on build options, regardless of 148the width of the CPU data path. Passed by reference. 149 150DMA addresses types ``dma_addr_t`` 151================================== 152 153:: 154 155 %pad 0x01234567 or 0x0123456789abcdef 156 157For printing a ``dma_addr_t`` type which can vary based on build options, 158regardless of the width of the CPU data path. Passed by reference. 159 160Raw buffer as an escaped string 161=============================== 162 163:: 164 165 %*pE[achnops] 166 167For printing raw buffer as an escaped string. For the following buffer:: 168 169 1b 62 20 5c 43 07 22 90 0d 5d 170 171few examples show how the conversion would be done (the result string 172without surrounding quotes):: 173 174 %*pE "\eb \C\a"\220\r]" 175 %*pEhp "\x1bb \C\x07"\x90\x0d]" 176 %*pEa "\e\142\040\\\103\a\042\220\r\135" 177 178The conversion rules are applied according to an optional combination 179of flags (see :c:func:`string_escape_mem` kernel documentation for the 180details): 181 182 - ``a`` - ESCAPE_ANY 183 - ``c`` - ESCAPE_SPECIAL 184 - ``h`` - ESCAPE_HEX 185 - ``n`` - ESCAPE_NULL 186 - ``o`` - ESCAPE_OCTAL 187 - ``p`` - ESCAPE_NP 188 - ``s`` - ESCAPE_SPACE 189 190By default ESCAPE_ANY_NP is used. 191 192ESCAPE_ANY_NP is the sane choice for many cases, in particularly for 193printing SSIDs. 194 195If field width is omitted the 1 byte only will be escaped. 196 197Raw buffer as a hex string 198========================== 199 200:: 201 202 %*ph 00 01 02 ... 3f 203 %*phC 00:01:02: ... :3f 204 %*phD 00-01-02- ... -3f 205 %*phN 000102 ... 3f 206 207For printing a small buffers (up to 64 bytes long) as a hex string with 208certain separator. For the larger buffers consider to use 209:c:func:`print_hex_dump`. 210 211MAC/FDDI addresses 212================== 213 214:: 215 216 %pM 00:01:02:03:04:05 217 %pMR 05:04:03:02:01:00 218 %pMF 00-01-02-03-04-05 219 %pm 000102030405 220 %pmR 050403020100 221 222For printing 6-byte MAC/FDDI addresses in hex notation. The ``M`` and ``m`` 223specifiers result in a printed address with (``M``) or without (``m``) byte 224separators. The default byte separator is the colon (``:``). 225 226Where FDDI addresses are concerned the ``F`` specifier can be used after 227the ``M`` specifier to use dash (``-``) separators instead of the default 228separator. 229 230For Bluetooth addresses the ``R`` specifier shall be used after the ``M`` 231specifier to use reversed byte order suitable for visual interpretation 232of Bluetooth addresses which are in the little endian order. 233 234Passed by reference. 235 236IPv4 addresses 237============== 238 239:: 240 241 %pI4 1.2.3.4 242 %pi4 001.002.003.004 243 %p[Ii]4[hnbl] 244 245For printing IPv4 dot-separated decimal addresses. The ``I4`` and ``i4`` 246specifiers result in a printed address with (``i4``) or without (``I4``) 247leading zeros. 248 249The additional ``h``, ``n``, ``b``, and ``l`` specifiers are used to specify 250host, network, big or little endian order addresses respectively. Where 251no specifier is provided the default network/big endian order is used. 252 253Passed by reference. 254 255IPv6 addresses 256============== 257 258:: 259 260 %pI6 0001:0002:0003:0004:0005:0006:0007:0008 261 %pi6 00010002000300040005000600070008 262 %pI6c 1:2:3:4:5:6:7:8 263 264For printing IPv6 network-order 16-bit hex addresses. The ``I6`` and ``i6`` 265specifiers result in a printed address with (``I6``) or without (``i6``) 266colon-separators. Leading zeros are always used. 267 268The additional ``c`` specifier can be used with the ``I`` specifier to 269print a compressed IPv6 address as described by 270http://tools.ietf.org/html/rfc5952 271 272Passed by reference. 273 274IPv4/IPv6 addresses (generic, with port, flowinfo, scope) 275========================================================= 276 277:: 278 279 %pIS 1.2.3.4 or 0001:0002:0003:0004:0005:0006:0007:0008 280 %piS 001.002.003.004 or 00010002000300040005000600070008 281 %pISc 1.2.3.4 or 1:2:3:4:5:6:7:8 282 %pISpc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345 283 %p[Ii]S[pfschnbl] 284 285For printing an IP address without the need to distinguish whether it``s 286of type AF_INET or AF_INET6, a pointer to a valid ``struct sockaddr``, 287specified through ``IS`` or ``iS``, can be passed to this format specifier. 288 289The additional ``p``, ``f``, and ``s`` specifiers are used to specify port 290(IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ``:`` prefix, 291flowinfo a ``/`` and scope a ``%``, each followed by the actual value. 292 293In case of an IPv6 address the compressed IPv6 address as described by 294http://tools.ietf.org/html/rfc5952 is being used if the additional 295specifier ``c`` is given. The IPv6 address is surrounded by ``[``, ``]`` in 296case of additional specifiers ``p``, ``f`` or ``s`` as suggested by 297https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07 298 299In case of IPv4 addresses, the additional ``h``, ``n``, ``b``, and ``l`` 300specifiers can be used as well and are ignored in case of an IPv6 301address. 302 303Passed by reference. 304 305Further examples:: 306 307 %pISfc 1.2.3.4 or [1:2:3:4:5:6:7:8]/123456789 308 %pISsc 1.2.3.4 or [1:2:3:4:5:6:7:8]%1234567890 309 %pISpfc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345/123456789 310 311UUID/GUID addresses 312=================== 313 314:: 315 316 %pUb 00010203-0405-0607-0809-0a0b0c0d0e0f 317 %pUB 00010203-0405-0607-0809-0A0B0C0D0E0F 318 %pUl 03020100-0504-0706-0809-0a0b0c0e0e0f 319 %pUL 03020100-0504-0706-0809-0A0B0C0E0E0F 320 321For printing 16-byte UUID/GUIDs addresses. The additional 'l', 'L', 322'b' and 'B' specifiers are used to specify a little endian order in 323lower ('l') or upper case ('L') hex characters - and big endian order 324in lower ('b') or upper case ('B') hex characters. 325 326Where no additional specifiers are used the default big endian 327order with lower case hex characters will be printed. 328 329Passed by reference. 330 331dentry names 332============ 333 334:: 335 336 %pd{,2,3,4} 337 %pD{,2,3,4} 338 339For printing dentry name; if we race with :c:func:`d_move`, the name might be 340a mix of old and new ones, but it won't oops. ``%pd`` dentry is a safer 341equivalent of ``%s`` ``dentry->d_name.name`` we used to use, ``%pd<n>`` prints 342``n`` last components. ``%pD`` does the same thing for struct file. 343 344Passed by reference. 345 346block_device names 347================== 348 349:: 350 351 %pg sda, sda1 or loop0p1 352 353For printing name of block_device pointers. 354 355struct va_format 356================ 357 358:: 359 360 %pV 361 362For printing struct va_format structures. These contain a format string 363and va_list as follows:: 364 365 struct va_format { 366 const char *fmt; 367 va_list *va; 368 }; 369 370Implements a "recursive vsnprintf". 371 372Do not use this feature without some mechanism to verify the 373correctness of the format string and va_list arguments. 374 375Passed by reference. 376 377kobjects 378======== 379 380:: 381 382 %pO 383 384 Base specifier for kobject based structs. Must be followed with 385 character for specific type of kobject as listed below: 386 387 Device tree nodes: 388 389 %pOF[fnpPcCF] 390 391 For printing device tree nodes. The optional arguments are: 392 f device node full_name 393 n device node name 394 p device node phandle 395 P device node path spec (name + @unit) 396 F device node flags 397 c major compatible string 398 C full compatible string 399 Without any arguments prints full_name (same as %pOFf) 400 The separator when using multiple arguments is ':' 401 402 Examples: 403 404 %pOF /foo/bar@0 - Node full name 405 %pOFf /foo/bar@0 - Same as above 406 %pOFfp /foo/bar@0:10 - Node full name + phandle 407 %pOFfcF /foo/bar@0:foo,device:--P- - Node full name + 408 major compatible string + 409 node flags 410 D - dynamic 411 d - detached 412 P - Populated 413 B - Populated bus 414 415 Passed by reference. 416 417 418struct clk 419========== 420 421:: 422 423 %pC pll1 424 %pCn pll1 425 426For printing struct clk structures. ``%pC`` and ``%pCn`` print the name 427(Common Clock Framework) or address (legacy clock framework) of the 428structure. 429 430Passed by reference. 431 432bitmap and its derivatives such as cpumask and nodemask 433======================================================= 434 435:: 436 437 %*pb 0779 438 %*pbl 0,3-6,8-10 439 440For printing bitmap and its derivatives such as cpumask and nodemask, 441``%*pb`` output the bitmap with field width as the number of bits and ``%*pbl`` 442output the bitmap as range list with field width as the number of bits. 443 444Passed by reference. 445 446Flags bitfields such as page flags, gfp_flags 447============================================= 448 449:: 450 451 %pGp referenced|uptodate|lru|active|private 452 %pGg GFP_USER|GFP_DMA32|GFP_NOWARN 453 %pGv read|exec|mayread|maywrite|mayexec|denywrite 454 455For printing flags bitfields as a collection of symbolic constants that 456would construct the value. The type of flags is given by the third 457character. Currently supported are [p]age flags, [v]ma_flags (both 458expect ``unsigned long *``) and [g]fp_flags (expects ``gfp_t *``). The flag 459names and print order depends on the particular type. 460 461Note that this format should not be used directly in :c:func:`TP_printk()` part 462of a tracepoint. Instead, use the ``show_*_flags()`` functions from 463<trace/events/mmflags.h>. 464 465Passed by reference. 466 467Network device features 468======================= 469 470:: 471 472 %pNF 0x000000000000c000 473 474For printing netdev_features_t. 475 476Passed by reference. 477 478If you add other ``%p`` extensions, please extend lib/test_printf.c with 479one or more test cases, if at all feasible. 480 481 482Thank you for your cooperation and attention. 483