64. How to get printk format specifiers right

Author:Randy Dunlap <rdunlap@infradead.org>
Author:Andrew Murray <amurray@mpc-data.co.uk>

64.1. Integer types

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.

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:

64.2. Symbols/Function Pointers

%pF     versatile_init+0x0/0x110
%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.

64.3. 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.

64.4. 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.

64.5. 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.

64.6. 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.

64.7. 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.

64.8. 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().

64.9. MAC/FDDI addresses

%pM     00:01:02:03:04:05
%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.

64.10. IPv4 addresses

%pI4    1.2.3.4
%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.

64.11. IPv6 addresses

%pI6    0001:0002:0003:0004:0005:0006:0007:0008
%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.

64.12. 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

64.13. 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.

64.14. 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<n> prints n last components. %pD does the same thing for struct file.

Passed by reference.

64.15. block_device names

%pg     sda, sda1 or loop0p1

For printing name of block_device pointers.

64.16. 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.

64.17. 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.

64.18. 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.

64.19. 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 <trace/events/mmflags.h>.

Passed by reference.

64.20. 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.