printk is a nature and basic tool for debugging kernel. Sometimes it is the only tool we have. Here are some tips of using printk.
1) printk formats
Documentation/printk-formats.txt introduces many useful printk formats. I use %p family the most:
2) print_hex_dump
Sometimes I have to create memory dumps. You can use a simple for loop to do that, but Linux kernel provides a better way - print_hex_dump.
For example:
Function prototype:
3) pr_alert family
Kernel provides some wrapper macros the different printk levels. I prefer to use the macros because they are more easier to read and less characters to type.
1) printk formats
Documentation/printk-formats.txt introduces many useful printk formats. I use %p family the most:
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
%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
2) print_hex_dump
Sometimes I have to create memory dumps. You can use a simple for loop to do that, but Linux kernel provides a better way - print_hex_dump.
For example:
Function prototype:
static inline void print_hex_dump(const char *level, const char *prefix_str,
int prefix_type, int rowsize, int groupsize,
const void *buf, size_t len, bool ascii)
print_hex_dump(KERN_ALERT, "mem: ", DUMP_PREFIX_ADDRESS,
16, 1, p, 512, 1);
output:
mem: ddc86680: 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 ................ mem: ddc86690: 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 ................ mem: ddc866a0: 12 12 12 12 60 6b c8 dd 16 80 99 e0 fa 8e 2a c1 ....`k........*. mem: ddc866b0: 16 80 99 e0 ce 92 2a c1 16 80 99 e0 f2 c1 1b c1 ......*......... mem: ddc866c0: 16 80 99 e0 4c 8b 0a c1 4c 8b 0a c1 61 80 99 e0 ....L...L...a... mem: ddc866d0: 16 80 99 e0 61 80 99 e0 16 80 99 e0 61 80 99 e0 ....a.......a... mem: ddc866e0: 75 80 99 e0 48 01 00 c1 2b 36 05 c1 00 00 00 00 u...H...+6...... mem: ddc866f0: 4a 0c 00 00 99 ad 06 00 6d 35 05 c1 9e 8b 2a c1 J.......m5....*. mem: ddc86700: 6d 35 05 c1 48 8c 2a c1 6d 35 05 c1 ee 89 0a c1 m5..H.*.m5...... mem: ddc86710: ee 89 0a c1 e4 0a 14 c1 e4 0a 14 c1 ee 89 0a c1 ................ mem: ddc86720: ee 89 0a c1 6d 35 05 c1 6d 35 05 c1 6d 35 05 c1 ....m5..m5..m5.. mem: ddc86730: a7 39 05 c1 ef b8 2a c1 00 00 00 00 00 00 00 00 .9....*......... mem: ddc86740: 4a 0c 00 00 97 ad 06 00 5a 5a 5a 5a 5a 5a 5a 5a J.......ZZZZZZZZ mem: ddc86750: 14 dc 46 dd 14 dc 46 dd 00 00 00 00 6b 6b 6b 6b ..F...F.....kkkk mem: ddc86760: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5 kkkkkkkkkkkkkkk. mem: ddc86770: cc cc cc cc c0 69 c8 dd a0 83 20 c1 fa 8e 2a c1 .....i.... ...*.
3) pr_alert family
Kernel provides some wrapper macros the different printk levels. I prefer to use the macros because they are more easier to read and less characters to type.
#define pr_emerg(fmt, ...) \
printk(KERN_EMERG pr_fmt(fmt), ##__VA_ARGS__)
#define pr_alert(fmt, ...) \
printk(KERN_ALERT pr_fmt(fmt), ##__VA_ARGS__)
#define pr_crit(fmt, ...) \
printk(KERN_CRIT pr_fmt(fmt), ##__VA_ARGS__)
#define pr_err(fmt, ...) \
printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
#define pr_warning(fmt, ...) \
printk(KERN_WARNING pr_fmt(fmt), ##__VA_ARGS__)
#define pr_warn pr_warning
#define pr_notice(fmt, ...) \
printk(KERN_NOTICE pr_fmt(fmt), ##__VA_ARGS__)
#define pr_info(fmt, ...) \
printk(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__)
#define pr_cont(fmt, ...) \
printk(KERN_CONT fmt, ##__VA_ARGS__)
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