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 8 9Integer types 10============= 11 12:: 13 14 If variable is of Type, use printk format specifier: 15 ------------------------------------------------------------ 16 char %d or %x 17 unsigned char %u or %x 18 short int %d or %x 19 unsigned short int %u or %x 20 int %d or %x 21 unsigned int %u or %x 22 long %ld or %lx 23 unsigned long %lu or %lx 24 long long %lld or %llx 25 unsigned long long %llu or %llx 26 size_t %zu or %zx 27 ssize_t %zd or %zx 28 s8 %d or %x 29 u8 %u or %x 30 s16 %d or %x 31 u16 %u or %x 32 s32 %d or %x 33 u32 %u or %x 34 s64 %lld or %llx 35 u64 %llu or %llx 36 37 38If <type> is dependent on a config option for its size (e.g., sector_t, 39blkcnt_t) or is architecture-dependent for its size (e.g., tcflag_t), use a 40format specifier of its largest possible type and explicitly cast to it. 41 42Example:: 43 44 printk("test: sector number/total blocks: %llu/%llu\n", 45 (unsigned long long)sector, (unsigned long long)blockcount); 46 47Reminder: sizeof() returns type size_t. 48 49The 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 51unsupported specifier or length qualifier results in a WARN and early 52return from vsnprintf(). 53 54Pointer types 55============= 56 57A raw pointer value may be printed with %p which will hash the address 58before printing. The kernel also supports extended specifiers for printing 59pointers of different types. 60 61Some of the extended specifiers print the data on the given address instead 62of printing the address itself. In this case, the following error messages 63might be printed instead of the unreachable information:: 64 65 (null) data on plain NULL address 66 (efault) data on invalid address 67 (einval) invalid data on a valid address 68 69Plain Pointers 70-------------- 71 72:: 73 74 %p abcdef12 or 00000000abcdef12 75 76Pointers printed without a specifier extension (i.e unadorned %p) are 77hashed to prevent leaking information about the kernel memory layout. This 78has the added benefit of providing a unique identifier. On 64-bit machines 79the first 32 bits are zeroed. The kernel will print ``(ptrval)`` until it 80gathers enough entropy. If you *really* want the address see %px below. 81 82Error Pointers 83-------------- 84 85:: 86 87 %pe -ENOSPC 88 89For printing error pointers (i.e. a pointer for which IS_ERR() is true) 90as a symbolic error name. Error values for which no symbolic name is 91known are printed in decimal, while a non-ERR_PTR passed as the 92argument to %pe gets treated as ordinary %p. 93 94Symbols/Function Pointers 95------------------------- 96 97:: 98 99 %pS versatile_init+0x0/0x110 100 %ps versatile_init 101 %pSR versatile_init+0x9/0x110 102 (with __builtin_extract_return_addr() translation) 103 %pB prev_fn_of_versatile_init+0x88/0x88 104 105 106The ``S`` and ``s`` specifiers are used for printing a pointer in symbolic 107format. They result in the symbol name with (S) or without (s) 108offsets. If KALLSYMS are disabled then the symbol address is printed instead. 109 110The ``B`` specifier results in the symbol name with offsets and should be 111used when printing stack backtraces. The specifier takes into 112consideration the effect of compiler optimisations which may occur 113when tail-calls are used and marked with the noreturn GCC attribute. 114 115Kernel Pointers 116--------------- 117 118:: 119 120 %pK 01234567 or 0123456789abcdef 121 122For printing kernel pointers which should be hidden from unprivileged 123users. The behaviour of %pK depends on the kptr_restrict sysctl - see 124Documentation/admin-guide/sysctl/kernel.rst for more details. 125 126Unmodified Addresses 127-------------------- 128 129:: 130 131 %px 01234567 or 0123456789abcdef 132 133For printing pointers when you *really* want to print the address. Please 134consider whether or not you are leaking sensitive information about the 135kernel memory layout before printing pointers with %px. %px is functionally 136equivalent to %lx (or %lu). %px is preferred because it is more uniquely 137grep'able. If in the future we need to modify the way the kernel handles 138printing pointers we will be better equipped to find the call sites. 139 140Struct Resources 141---------------- 142 143:: 144 145 %pr [mem 0x60000000-0x6fffffff flags 0x2200] or 146 [mem 0x0000000060000000-0x000000006fffffff flags 0x2200] 147 %pR [mem 0x60000000-0x6fffffff pref] or 148 [mem 0x0000000060000000-0x000000006fffffff pref] 149 150For printing struct resources. The ``R`` and ``r`` specifiers result in a 151printed resource with (R) or without (r) a decoded flags member. 152 153Passed by reference. 154 155Physical address types phys_addr_t 156---------------------------------- 157 158:: 159 160 %pa[p] 0x01234567 or 0x0123456789abcdef 161 162For printing a phys_addr_t type (and its derivatives, such as 163resource_size_t) which can vary based on build options, regardless of the 164width of the CPU data path. 165 166Passed by reference. 167 168DMA address types dma_addr_t 169---------------------------- 170 171:: 172 173 %pad 0x01234567 or 0x0123456789abcdef 174 175For printing a dma_addr_t type which can vary based on build options, 176regardless of the width of the CPU data path. 177 178Passed by reference. 179 180Raw buffer as an escaped string 181------------------------------- 182 183:: 184 185 %*pE[achnops] 186 187For printing raw buffer as an escaped string. For the following buffer:: 188 189 1b 62 20 5c 43 07 22 90 0d 5d 190 191A few examples show how the conversion would be done (excluding surrounding 192quotes):: 193 194 %*pE "\eb \C\a"\220\r]" 195 %*pEhp "\x1bb \C\x07"\x90\x0d]" 196 %*pEa "\e\142\040\\\103\a\042\220\r\135" 197 198The conversion rules are applied according to an optional combination 199of flags (see :c:func:`string_escape_mem` kernel documentation for the 200details): 201 202 - a - ESCAPE_ANY 203 - c - ESCAPE_SPECIAL 204 - h - ESCAPE_HEX 205 - n - ESCAPE_NULL 206 - o - ESCAPE_OCTAL 207 - p - ESCAPE_NP 208 - s - ESCAPE_SPACE 209 210By default ESCAPE_ANY_NP is used. 211 212ESCAPE_ANY_NP is the sane choice for many cases, in particularly for 213printing SSIDs. 214 215If field width is omitted then 1 byte only will be escaped. 216 217Raw buffer as a hex string 218-------------------------- 219 220:: 221 222 %*ph 00 01 02 ... 3f 223 %*phC 00:01:02: ... :3f 224 %*phD 00-01-02- ... -3f 225 %*phN 000102 ... 3f 226 227For printing small buffers (up to 64 bytes long) as a hex string with a 228certain separator. For larger buffers consider using 229:c:func:`print_hex_dump`. 230 231MAC/FDDI addresses 232------------------ 233 234:: 235 236 %pM 00:01:02:03:04:05 237 %pMR 05:04:03:02:01:00 238 %pMF 00-01-02-03-04-05 239 %pm 000102030405 240 %pmR 050403020100 241 242For printing 6-byte MAC/FDDI addresses in hex notation. The ``M`` and ``m`` 243specifiers result in a printed address with (M) or without (m) byte 244separators. The default byte separator is the colon (:). 245 246Where FDDI addresses are concerned the ``F`` specifier can be used after 247the ``M`` specifier to use dash (-) separators instead of the default 248separator. 249 250For Bluetooth addresses the ``R`` specifier shall be used after the ``M`` 251specifier to use reversed byte order suitable for visual interpretation 252of Bluetooth addresses which are in the little endian order. 253 254Passed by reference. 255 256IPv4 addresses 257-------------- 258 259:: 260 261 %pI4 1.2.3.4 262 %pi4 001.002.003.004 263 %p[Ii]4[hnbl] 264 265For printing IPv4 dot-separated decimal addresses. The ``I4`` and ``i4`` 266specifiers result in a printed address with (i4) or without (I4) leading 267zeros. 268 269The additional ``h``, ``n``, ``b``, and ``l`` specifiers are used to specify 270host, network, big or little endian order addresses respectively. Where 271no specifier is provided the default network/big endian order is used. 272 273Passed by reference. 274 275IPv6 addresses 276-------------- 277 278:: 279 280 %pI6 0001:0002:0003:0004:0005:0006:0007:0008 281 %pi6 00010002000300040005000600070008 282 %pI6c 1:2:3:4:5:6:7:8 283 284For printing IPv6 network-order 16-bit hex addresses. The ``I6`` and ``i6`` 285specifiers result in a printed address with (I6) or without (i6) 286colon-separators. Leading zeros are always used. 287 288The additional ``c`` specifier can be used with the ``I`` specifier to 289print a compressed IPv6 address as described by 290http://tools.ietf.org/html/rfc5952 291 292Passed by reference. 293 294IPv4/IPv6 addresses (generic, with port, flowinfo, scope) 295--------------------------------------------------------- 296 297:: 298 299 %pIS 1.2.3.4 or 0001:0002:0003:0004:0005:0006:0007:0008 300 %piS 001.002.003.004 or 00010002000300040005000600070008 301 %pISc 1.2.3.4 or 1:2:3:4:5:6:7:8 302 %pISpc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345 303 %p[Ii]S[pfschnbl] 304 305For printing an IP address without the need to distinguish whether it's of 306type AF_INET or AF_INET6. A pointer to a valid struct sockaddr, 307specified through ``IS`` or ``iS``, can be passed to this format specifier. 308 309The additional ``p``, ``f``, and ``s`` specifiers are used to specify port 310(IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ``:`` prefix, 311flowinfo a ``/`` and scope a ``%``, each followed by the actual value. 312 313In case of an IPv6 address the compressed IPv6 address as described by 314http://tools.ietf.org/html/rfc5952 is being used if the additional 315specifier ``c`` is given. The IPv6 address is surrounded by ``[``, ``]`` in 316case of additional specifiers ``p``, ``f`` or ``s`` as suggested by 317https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07 318 319In case of IPv4 addresses, the additional ``h``, ``n``, ``b``, and ``l`` 320specifiers can be used as well and are ignored in case of an IPv6 321address. 322 323Passed by reference. 324 325Further examples:: 326 327 %pISfc 1.2.3.4 or [1:2:3:4:5:6:7:8]/123456789 328 %pISsc 1.2.3.4 or [1:2:3:4:5:6:7:8]%1234567890 329 %pISpfc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345/123456789 330 331UUID/GUID addresses 332------------------- 333 334:: 335 336 %pUb 00010203-0405-0607-0809-0a0b0c0d0e0f 337 %pUB 00010203-0405-0607-0809-0A0B0C0D0E0F 338 %pUl 03020100-0504-0706-0809-0a0b0c0e0e0f 339 %pUL 03020100-0504-0706-0809-0A0B0C0E0E0F 340 341For printing 16-byte UUID/GUIDs addresses. The additional ``l``, ``L``, 342``b`` and ``B`` specifiers are used to specify a little endian order in 343lower (l) or upper case (L) hex notation - and big endian order in lower (b) 344or upper case (B) hex notation. 345 346Where no additional specifiers are used the default big endian 347order with lower case hex notation will be printed. 348 349Passed by reference. 350 351dentry names 352------------ 353 354:: 355 356 %pd{,2,3,4} 357 %pD{,2,3,4} 358 359For printing dentry name; if we race with :c:func:`d_move`, the name might 360be a mix of old and new ones, but it won't oops. %pd dentry is a safer 361equivalent of %s dentry->d_name.name we used to use, %pd<n> prints ``n`` 362last components. %pD does the same thing for struct file. 363 364Passed by reference. 365 366block_device names 367------------------ 368 369:: 370 371 %pg sda, sda1 or loop0p1 372 373For printing name of block_device pointers. 374 375struct va_format 376---------------- 377 378:: 379 380 %pV 381 382For printing struct va_format structures. These contain a format string 383and va_list as follows:: 384 385 struct va_format { 386 const char *fmt; 387 va_list *va; 388 }; 389 390Implements a "recursive vsnprintf". 391 392Do not use this feature without some mechanism to verify the 393correctness of the format string and va_list arguments. 394 395Passed by reference. 396 397Device tree nodes 398----------------- 399 400:: 401 402 %pOF[fnpPcCF] 403 404 405For printing device tree node structures. Default behaviour is 406equivalent to %pOFf. 407 408 - f - device node full_name 409 - n - device node name 410 - p - device node phandle 411 - P - device node path spec (name + @unit) 412 - F - device node flags 413 - c - major compatible string 414 - C - full compatible string 415 416The separator when using multiple arguments is ':' 417 418Examples:: 419 420 %pOF /foo/bar@0 - Node full name 421 %pOFf /foo/bar@0 - Same as above 422 %pOFfp /foo/bar@0:10 - Node full name + phandle 423 %pOFfcF /foo/bar@0:foo,device:--P- - Node full name + 424 major compatible string + 425 node flags 426 D - dynamic 427 d - detached 428 P - Populated 429 B - Populated bus 430 431Passed by reference. 432 433Fwnode handles 434-------------- 435 436:: 437 438 %pfw[fP] 439 440For printing information on fwnode handles. The default is to print the full 441node name, including the path. The modifiers are functionally equivalent to 442%pOF above. 443 444 - f - full name of the node, including the path 445 - P - the name of the node including an address (if there is one) 446 447Examples (ACPI):: 448 449 %pfwf \_SB.PCI0.CIO2.port@1.endpoint@0 - Full node name 450 %pfwP endpoint@0 - Node name 451 452Examples (OF):: 453 454 %pfwf /ocp@68000000/i2c@48072000/camera@10/port/endpoint - Full name 455 %pfwP endpoint - Node name 456 457Time and date (struct rtc_time) 458------------------------------- 459 460:: 461 462 %ptR YYYY-mm-ddTHH:MM:SS 463 %ptRd YYYY-mm-dd 464 %ptRt HH:MM:SS 465 %ptR[dt][r] 466 467For printing date and time as represented by struct rtc_time structure in 468human readable format. 469 470By default year will be incremented by 1900 and month by 1. Use %ptRr (raw) 471to suppress this behaviour. 472 473Passed by reference. 474 475struct clk 476---------- 477 478:: 479 480 %pC pll1 481 %pCn pll1 482 483For printing struct clk structures. %pC and %pCn print the name of the clock 484(Common Clock Framework) or a unique 32-bit ID (legacy clock framework). 485 486Passed by reference. 487 488bitmap and its derivatives such as cpumask and nodemask 489------------------------------------------------------- 490 491:: 492 493 %*pb 0779 494 %*pbl 0,3-6,8-10 495 496For printing bitmap and its derivatives such as cpumask and nodemask, 497%*pb outputs the bitmap with field width as the number of bits and %*pbl 498output the bitmap as range list with field width as the number of bits. 499 500Passed by reference. 501 502Flags bitfields such as page flags, gfp_flags 503--------------------------------------------- 504 505:: 506 507 %pGp referenced|uptodate|lru|active|private 508 %pGg GFP_USER|GFP_DMA32|GFP_NOWARN 509 %pGv read|exec|mayread|maywrite|mayexec|denywrite 510 511For printing flags bitfields as a collection of symbolic constants that 512would construct the value. The type of flags is given by the third 513character. Currently supported are [p]age flags, [v]ma_flags (both 514expect ``unsigned long *``) and [g]fp_flags (expects ``gfp_t *``). The flag 515names and print order depends on the particular type. 516 517Note that this format should not be used directly in the 518:c:func:`TP_printk()` part of a tracepoint. Instead, use the show_*_flags() 519functions from <trace/events/mmflags.h>. 520 521Passed by reference. 522 523Network device features 524----------------------- 525 526:: 527 528 %pNF 0x000000000000c000 529 530For printing netdev_features_t. 531 532Passed by reference. 533 534Thanks 535====== 536 537If you add other %p extensions, please extend <lib/test_printf.c> with 538one or more test cases, if at all feasible. 539 540Thank you for your cooperation and attention. 541