内核通过 printk() 输出的信息具有日志级别，日志级别是通过在 printk() 输出的字符串前加一个带尖括号的整数来控制的，如 printk("<6>Hello, world!/n");。内核中共提供了八种不同的日志级别，在 linux/kernel.h 中有相应的宏对应。 #define KERN_EMERG "<0>" /* system is unusable */ #define KERN_ALERT "<1>" /* action must be taken immediately */ #define KERN_CRIT "<2>" /* critical conditions */ #define KERN_ERR "<3>" /* error conditions */ #define KERN_WARNING "<4>" /* warning conditions */ #define KERN_NOTICE "<5>" /* normal but significant */ #define KERN_INFO "<6>" /* informational */ #define KERN_DEBUG "<7>" /* debug-level messages */ 所以 printk() 可以这样用：printk(KERN_INFO "Hello, world!/n");。 未指定日志级别的 printk() 采用的默认级别是 DEFAULT_MESSAGE_LOGLEVEL，这个宏在 kernel/printk.c 中被定义为整数 4，即对应KERN_WARNING。 在 /proc/sys/kernel/printk 会显示4个数值（可由 echo 修改），分别表示当前控制台日志级别、未明确指定日志级别的默认消息日志级别、最小（最高）允许设置的控制台日志级别、引导时默认的日志级别。当 printk() 中的消息日志级别小于当前控制台日志级别时，printk 的信息（要有/n符）就会在控制台上显示。但无论当前控制台日志级别是何值，通过 /proc/kmsg （或使用dmesg）总能查看。另外如果配置好并运行了 syslogd 或 klogd，没有在控制台上显示的 printk 的信息也会追加到 /var/log/messages.log 中。 char myname[] = "chinacodec/n"; printk(KERN_INFO "Hello, world %s!/n", myname); 可用的格式字符 --------------------------------------------------------- int %d or %x unsigned int %u or %x long %ld or %lx unsigned long %lu or %lx long long %lld or %llx unsigned long long %llu or %llx size_t %zu or %zx ssize_t %zd or %zx u64 SHOULD be printed with %llu/%llx, (unsigned long long): printk("%llu", (unsigned long long)u64_var); s64 SHOULD be printed with %lld/%llx, (long long): printk("%lld", (long long)s64_var); 可参考 http://lxr.linux.no/#linux+v2.6.34/Documentation/printk-formats.txt 打印裸指针(raw pointer)用 %p,%p除了可以用来打印指针外还可以打印其它的信息 %pF可打印函数指针的函数名和偏移地址，%pf只打印函数指针的函数名，不打印偏移地址。 如 printk("%pf %pF\n", ptr, ptr) will print: module_start module_start+0x0/0x62 [hello] 但是为了支持这个功能你需要开启CONFIG_KALLSYMS 选项 %pM打印冒号分隔的MAC地址，%pm打印MAC地址的16进制无分隔 如 printk("%pM %pm\n", mac, mac) will print: 2c:00:1d:00:1b:00 2c001d001b00 %I4打印无前导0的IPv4地址，%i4打印冒号分隔的IPv4地址 %i4打印无前导0的IPv6地址，%i6打印无冒号分隔的IPv6地址 如 printk("%pI4 %pi4\n", ip, ip) will print: 127.0.0.1 127.000.000.001 其它的特殊格式字符参见 http://lxr.linux.no/#linux+v2.6.34/lib/vsprintf.c#L930 参考 http://www.embedded-bits.co.uk/2010/printk-format-specifiers/

https://www.kernel.org/doc/Documentation/printk-formats.txt If variable is of Type, use printk format specifier: --------------------------------------------------------- int %d or %x unsigned int %u or %x long %ld or %lx unsigned long %lu or %lx long long %lld or %llx unsigned long long %llu or %llx size_t %zu or %zx ssize_t %zd or %zx s32 %d or %x u32 %u or %x s64 %lld or %llx u64 %llu or %llx If  is dependent on a config option for its size (e.g., sector_t, blkcnt_t) or is architecture-dependent for its size (e.g., tcflag_t), use a format specifier of its largest possible type and explicitly cast to it. Example: printk("test: sector number/total blocks: %llu/%llu\n", (unsigned long long)sector, (unsigned long long)blockcount); Reminder: sizeof() result is of type size_t. The kernel's printf does not support %n. For obvious reasons, floating point formats (%e, %f, %g, %a) are also not recognized. Use of any unsupported specifier or length qualifier results in a WARN and early return from vsnprintf. Raw pointer value SHOULD be printed with %p. The kernel supports the following extended format specifiers for pointer types: Symbols/Function Pointers: %pF versatile_init+0x0/0x110 //打印出函数。 %pf versatile_init %pS versatile_init+0x0/0x110 %pSR versatile_init+0x9/0x110 (with __builtin_extract_return_addr() translation) %ps versatile_init %pB prev_fn_of_versatile_init+0x88/0x88 For printing symbols and function pointers. The 'S' and 's' specifiers result in the symbol name with ('S') or without ('s') offsets. Where this is used on a kernel without KALLSYMS - the symbol address is printed instead. The 'B' specifier results in the symbol name with offsets and should be used when printing stack backtraces. The specifier takes into consideration the effect of compiler optimisations which may occur when tail-call's are used and marked with the noreturn GCC attribute. On ia64, ppc64 and parisc64 architectures function pointers are actually function descriptors which must first be resolved. The 'F' and 'f' specifiers perform this resolution and then provide the same functionality as the 'S' and 's' specifiers. Kernel Pointers: %pK 0x01234567 or 0x0123456789abcdef For printing kernel pointers which should be hidden from unprivileged users. The behaviour of %pK depends on the kptr_restrict sysctl - see Documentation/sysctl/kernel.txt for more details. Struct Resources: %pr [mem 0x60000000-0x6fffffff flags 0x2200] or [mem 0x0000000060000000-0x000000006fffffff flags 0x2200] %pR [mem 0x60000000-0x6fffffff pref] or [mem 0x0000000060000000-0x000000006fffffff pref] For printing struct resources. The 'R' and 'r' specifiers result in a printed resource with ('R') or without ('r') a decoded flags member. Passed by reference. Physical addresses types phys_addr_t: %pa[p] 0x01234567 or 0x0123456789abcdef For printing a phys_addr_t type (and its derivatives, such as resource_size_t) which can vary based on build options, regardless of the width of the CPU data path. Passed by reference. DMA addresses types dma_addr_t: %pad 0x01234567 or 0x0123456789abcdef For printing a dma_addr_t type which can vary based on build options, regardless of the width of the CPU data path. Passed by reference. Raw buffer as an escaped string: %*pE[achnops] For printing raw buffer as an escaped string. For the following buffer 1b 62 20 5c 43 07 22 90 0d 5d few examples show how the conversion would be done (the result string without surrounding quotes): %*pE "\eb \C\a"\220\r]" %*pEhp "\x1bb \C\x07"\x90\x0d]" %*pEa "\e\142\040\\\103\a\042\220\r\135" The conversion rules are applied according to an optional combination of flags (see string_escape_mem() kernel documentation for the details): a - ESCAPE_ANY c - ESCAPE_SPECIAL h - ESCAPE_HEX n - ESCAPE_NULL o - ESCAPE_OCTAL p - ESCAPE_NP s - ESCAPE_SPACE By default ESCAPE_ANY_NP is used. ESCAPE_ANY_NP is the sane choice for many cases, in particularly for printing SSIDs. If field width is omitted the 1 byte only will be escaped. Raw buffer as a hex string: %*ph 00 01 02 ... 3f %*phC 00:01:02: ... :3f %*phD 00-01-02- ... -3f %*phN 000102 ... 3f For printing a small buffers (up to 64 bytes long) as a hex string with certain separator. For the larger buffers consider to use print_hex_dump(). MAC/FDDI addresses: %pM 00:01:02:03:04:05 //打印出mac. %pMR 05:04:03:02:01:00 %pMF 00-01-02-03-04-05 %pm 000102030405 %pmR 050403020100 For printing 6-byte MAC/FDDI addresses in hex notation. The 'M' and 'm' specifiers result in a printed address with ('M') or without ('m') byte separators. The default byte separator is the colon (':'). Where FDDI addresses are concerned the 'F' specifier can be used after the 'M' specifier to use dash ('-') separators instead of the default separator. For Bluetooth addresses the 'R' specifier shall be used after the 'M' specifier to use reversed byte order suitable for visual interpretation of Bluetooth addresses which are in the little endian order. Passed by reference. IPv4 addresses: %pI4 1.2.3.4 //打印出IP struct in_addr %pi4 001.002.003.004 %p[Ii]4[hnbl] For printing IPv4 dot-separated decimal addresses. The 'I4' and 'i4' specifiers result in a printed address with ('i4') or without ('I4') leading zeros. The additional 'h', 'n', 'b', and 'l' specifiers are used to specify host, network, big or little endian order addresses respectively. Where no specifier is provided the default network/big endian order is used. Passed by reference. IPv6 addresses: %pI6 0001:0002:0003:0004:0005:0006:0007:0008 //打印出IP struct in_addr %pi6 00010002000300040005000600070008 %pI6c 1:2:3:4:5:6:7:8 For printing IPv6 network-order 16-bit hex addresses. The 'I6' and 'i6' specifiers result in a printed address with ('I6') or without ('i6') colon-separators. Leading zeros are always used. The additional 'c' specifier can be used with the 'I' specifier to print a compressed IPv6 address as described by http://tools.ietf.org/html/rfc5952 Passed by reference. IPv4/IPv6 addresses (generic, with port, flowinfo, scope): %pIS 1.2.3.4 or 0001:0002:0003:0004:0005:0006:0007:0008 %piS 001.002.003.004 or 00010002000300040005000600070008 %pISc 1.2.3.4 or 1:2:3:4:5:6:7:8 %pISpc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345 %p[Ii]S[pfschnbl] For printing an IP address without the need to distinguish whether it's of type AF_INET or AF_INET6, a pointer to a valid 'struct sockaddr', specified through 'IS' or 'iS', can be passed to this format specifier. The additional 'p', 'f', and 's' specifiers are used to specify port (IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ':' prefix, flowinfo a '/' and scope a '%', each followed by the actual value. In case of an IPv6 address the compressed IPv6 address as described by http://tools.ietf.org/html/rfc5952 is being used if the additional specifier 'c' is given. The IPv6 address is surrounded by '[', ']' in case of additional specifiers 'p', 'f' or 's' as suggested by https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07 In case of IPv4 addresses, the additional 'h', 'n', 'b', and 'l' specifiers can be used as well and are ignored in case of an IPv6 address. Passed by reference. Further examples: %pISfc 1.2.3.4 or [1:2:3:4:5:6:7:8]/123456789 %pISsc 1.2.3.4 or [1:2:3:4:5:6:7:8]%1234567890 %pISpfc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345/123456789 UUID/GUID addresses: %pUb 00010203-0405-0607-0809-0a0b0c0d0e0f %pUB 00010203-0405-0607-0809-0A0B0C0D0E0F %pUl 03020100-0504-0706-0809-0a0b0c0e0e0f %pUL 03020100-0504-0706-0809-0A0B0C0E0E0F For printing 16-byte UUID/GUIDs addresses. The additional 'l', 'L', 'b' and 'B' specifiers are used to specify a little endian order in lower ('l') or upper case ('L') hex characters - and big endian order in lower ('b') or upper case ('B') hex characters. Where no additional specifiers are used the default big endian order with lower case hex characters will be printed. Passed by reference. dentry names: %pd{,2,3,4} %pD{,2,3,4} For printing dentry name; if we race with d_move(), the name might be a mix of old and new ones, but it won't oops. %pd dentry is a safer equivalent of %s dentry->d_name.name we used to use, %pd prints n last components. %pD does the same thing for struct file. Passed by reference. block_device names: %pg sda, sda1 or loop0p1 For printing name of block_device pointers. struct va_format: %pV For printing struct va_format structures. These contain a format string and va_list as follows: struct va_format { const char *fmt; va_list *va; }; Implements a "recursive vsnprintf". Do not use this feature without some mechanism to verify the correctness of the format string and va_list arguments. Passed by reference. struct clk: %pC pll1 %pCn pll1 %pCr 1560000000 For printing struct clk structures. '%pC' and '%pCn' print the name (Common Clock Framework) or address (legacy clock framework) of the structure; '%pCr' prints the current clock rate. Passed by reference. bitmap and its derivatives such as cpumask and nodemask: %*pb 0779 %*pbl 0,3-6,8-10 For printing bitmap and its derivatives such as cpumask and nodemask, %*pb output the bitmap with field width as the number of bits and %*pbl output the bitmap as range list with field width as the number of bits. Passed by reference. Flags bitfields such as page flags, gfp_flags: %pGp referenced|uptodate|lru|active|private %pGg GFP_USER|GFP_DMA32|GFP_NOWARN %pGv read|exec|mayread|maywrite|mayexec|denywrite For printing flags bitfields as a collection of symbolic constants that would construct the value. The type of flags is given by the third character. Currently supported are [p]age flags, [v]ma_flags (both expect unsigned long *) and [g]fp_flags (expects gfp_t *). The flag names and print order depends on the particular type. Note that this format should not be used directly in TP_printk() part of a tracepoint. Instead, use the show_*_flags() functions from . Passed by reference. Network device features: %pNF 0x000000000000c000 For printing netdev_features_t. Passed by reference. If you add other %p extensions, please extend lib/test_printf.c with one or more test cases, if at all feasible. Thank you for your cooperation and attention. By Randy Dunlap  and Andrew Murray 

*博客内容为网友个人发布，仅代表博主个人观点，如有侵权请联系工作人员删除。