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);



        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);

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
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
printk("%pM %pm\n", mac, mac) will print:

2c:00:1d:00:1b:00 2c001d001b00
printk("%pI4 %pi4\n", ip, ip) will print:

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 <type> 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.

    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:


    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
        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

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

    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     //打印出IP struct in_addr

    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

    Passed by reference.

IPv4/IPv6 addresses (generic, with port, flowinfo, scope):

    %pIS        or 0001:0002:0003:0004:0005:0006:0007:0008
    %piS    or 00010002000300040005000600070008
    %pISc        or 1:2:3:4:5:6:7:8
    %pISpc    or [1:2:3:4:5:6:7:8]:12345

    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 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

    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

    Passed by reference.

    Further examples:

    %pISfc        or [1:2:3:4:5:6:7:8]/123456789
    %pISsc        or [1:2:3:4:5:6:7:8]%1234567890
    %pISpfc    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:


    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-> we used to use, %pd<n> 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:


    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
    %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 <>


posted @ 2017-07-13 16:46  sky-heaven  阅读(5965)  评论(0编辑  收藏  举报