# printk * [Printk](#printk) * [Message logging](#message-logging) * [Specifiers](#specifiers) * [Printk Index](#printk-index) * [Subsystem specific prefix](#subsystem-specific-prefix) * [References](#references) ## Printk * `printk()` is one of the most widely known functions in the Linux kernel * standard tool we have for printing messages and usually the **most basic way** of tracing and debugging * All printk() messages are printed to the kernel log buffer, which is a **ring buffer** exported to userspace through `/dev/kmsg`. The usual way to read it is using `dmesg`. ### Message logging * `printk()` can specify a log level : `printk(KERN_INFO "Message: %s\n", arg);` | Name | String | Alias function | | ------------- | ------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | KERN\_EMERG | “0” | [`pr_emerg()`](https://docs.kernel.org/core-api/printk-basics.html#c.pr_emerg) | | KERN\_ALERT | “1” | [`pr_alert()`](https://docs.kernel.org/core-api/printk-basics.html#c.pr_alert) | | KERN\_CRIT | “2” | [`pr_crit()`](https://docs.kernel.org/core-api/printk-basics.html#c.pr_crit) | | KERN\_ERR | “3” | [`pr_err()`](https://docs.kernel.org/core-api/printk-basics.html#c.pr_err) | | KERN\_WARNING | “4” | [`pr_warn()`](https://docs.kernel.org/core-api/printk-basics.html#c.pr_warn) | | KERN\_NOTICE | “5” | [`pr_notice()`](https://docs.kernel.org/core-api/printk-basics.html#c.pr_notice) | | KERN\_INFO | “6” | [`pr_info()`](https://docs.kernel.org/core-api/printk-basics.html#c.pr_info) | | KERN\_DEBUG | “7” | [`pr_debug()`](https://docs.kernel.org/core-api/printk-basics.html#c.pr_debug) and [`pr_devel()`](https://docs.kernel.org/core-api/printk-basics.html#c.pr_devel) if DEBUG is defined | | KERN\_DEFAULT | “” | | | KERN\_CONT | “c” | [`pr_cont()`](https://docs.kernel.org/core-api/printk-basics.html#c.pr_cont) | * To check the current *console\_loglevel* : (shows *current*, *default*, *minimum* and *boot-time-default* log levels) ```shell $ cat /proc/sys/kernel/printk 4 4 1 7 ``` * This can be changed using `# echo 8 > /proc/sys/kernel/printk` or `# dmesg -n 5`. * `pr_info`, `pr_debug` are macros that help to print logs at different levels ### Specifiers There are many print specifiers present, it’s better to check these specifiers before using printk. These are available at : [Printk Formats - Kernel Docs](https://docs.kernel.org/core-api/printk-formats.html). The common types are : * Integer Types (signed char, unsigned char, int, unsigned short int, etc.) * Pointer Types * Plain Pointers * Error Pointers * Symbol/Function Pointers (Eg : `%pS versatile_init+0x0/0x110`) * Probe pointers from BPF/tracing * Kernel Pointers * Unmodified address * Pointer Differences * Struct Resources * Physical addresses (`%pa[p] 0x01234567 or 0x0123456789abcdef`) * DMA addresses (`%pad 0x01234567 or 0x0123456789abcdef`) * Raw buffer * MAC addresses (`%pM 00:01:02:03:04:05`) * IPv4/v6 Addresses (`%pI4 1.2.3.4`) * UID/GUID (`%pUb 00010203-0405-0607-0809-0a0b0c0d0e0f`) * Dentrys (`%pd{,2,3,4}`) * And many more … ### Printk Index > The kernel messages are evolving together with the code. As a result, particular kernel messages are not `KABI` and never will be! (See : [Kernel ABI doc](/linux-kernel-notes/misc/kabi.md) for KABI) * The printk index helps to find changes in the message formats. Also it helps to track the strings back to the kernel sources and the related commit. * The index of printk formats are split in into separate files. ``` /sys/kernel/debug/printk/index/vmlinux /sys/kernel/debug/printk/index/ext4 /sys/kernel/debug/printk/index/scsi_mod ``` The content is inspired by the dynamic debug interface and looks like: ```shell $> head -1 /sys/kernel/debug/printk/index/vmlinux; shuf -n 5 vmlinux # filename:line function "format" <5> block/blk-settings.c:661 disk_stack_limits "%s: Warning: Device %s is misaligned\n" <4> kernel/trace/trace.c:8296 trace_create_file "Could not create tracefs '%s' entry\n" <6> arch/x86/kernel/hpet.c:144 _hpet_print_config "hpet: %s(%d):\n" <6> init/do_mounts.c:605 prepare_namespace "Waiting for root device %s...\n" <6> drivers/acpi/osl.c:1410 acpi_no_auto_serialize_setup "ACPI: auto-serialization disabled\n" ``` ### Subsystem specific prefix * The macro `pr_fmt()` macro allows to define a prefix that is printed before the string generated by the related `printk()` calls. ```c #define pr_fmt(fmt) "ACPI: OSL: " fmt static int __init acpi_no_auto_serialize_setup(char *str) { acpi_gbl_auto_serialize_methods = FALSE; pr_info("Auto-serialization disabled\n"); return 1; } ``` This results in the following `printk` index entry: ``` <6> drivers/acpi/osl.c:1410 acpi_no_auto_serialize_setup "ACPI: auto-serialization disabled\n" ``` ### References * [Printk Basics - Kernel Docs](https://docs.kernel.org/core-api/printk-basics.html) * [Printk Formats - Kernel Docs](https://docs.kernel.org/core-api/printk-formats.html) * [Printk Index - Kernel Docs](https://docs.kernel.org/core-api/printk-index.html) --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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