Ubuntu compiles kernel modules, and the content is reflected in the system log

1.Linux login interface

1. Check the current file directory:

After connecting to the Linux system through Xshell

Enter the command: ls

2. Create a new code/kernel folder

2. Write code

1. Create hello_module.c

Command: vim hello_module.c

2. Press i to enter edit mode and enter the following code

 //hello_module.c #include <linux/module.h> #include <linux/kernel.h> #include <linux/init.h> ​ static int __init hello_init(void){ printk("This is hello_module, welcome to Linux kernel \n"); return 0; } ​ static void __exit hello_exit(void){ printk("see you next time!\n"); } ​ module_init(hello_init); module_exit(hello_exit); ​ MODULE_LICENSE("GPL"); MODULE_AUTHOR("Mr Q"); MODULE_DESCRIPTION("hello kernel module"); MODULE_ALIAS("hello");

The above code is explained as follows:
(1) #include <linux/module.h>: Required. The module.h header file contains the structure definition of the module and the version control of the module. Any module program must include this header file;
(2) #include <linux/kernel.h>: kernel.h contains commonly used kernel functions, such as the printk() function in the above program;
(3) #include <linux/init.h>: Required. init.h contains declarations of module_init() and module_exit() functions;
(4) module_init(): required. Module loading function, when loading a module, this function is automatically executed to perform initialization operations;
(5) module_exit(): required. Module uninstallation function, which is automatically executed when the module is uninstalled to perform cleanup operations; (6) MODULE_LICENSE(): indicates the software license agreement accepted by the module code. The Linux kernel is an open source project using GPL V2, which requires all individuals or organizations that use and modify the Linux kernel code to make the modified source code public. This is a mandatory open source agreement, so generally, when writing driver code, it is necessary to explicitly declare and follow this agreement, otherwise the kernel UI will issue a contaminated warning;
(7) MODULE_AUTHOR(): describes the author information of the module;
(8) MODULE_DESCRIPTION(): briefly describe the purpose and function of the module;
(9) MODULE_ALIAS(): Alias ​​provided for user controls;
(10) printk(): kernel output function, which prints the contents of the system file "/var/log/kern.log" by default.

3. Save and exit, and press ESC to view the file directory;
:wq

3. Write the Makefile

vim Makefile

 obj-m := hello_module.o
 ​
 KERNELBUILD := /lib/modules/$(shell uname -r)/build
 CURRENT_PATH := $(shell pwd)
 ​
 all:
     make -C $(KERNELBUILD) M=$(CURRENT_PATH) modules
 ​
 clean:
         make -C $(KERNELBUILD) M=$(CURRENT_PATH) clean

The above code is explained as follows:
(1) obj-m := <module name>.o: defines the name of the module to be generated (2) KERNELBUILD := /lib/modules/$(shell uname -r)/build: KERNELBUILD is a custom name used to point to the kernel compilation directory of the running Linux, where the "uname -r" identifier shows the corresponding kernel version;
(3) CURRENT_PATH := $(shell pwd): CURRENT_PATH is a custom name used to point to the current directory;
(4) all: actions executed by compilation (5) clean: actions required by make clean. “make clean” is used to clear the object files (files with the suffix “.o”) and executable files generated by the last make command.

:wq save and exit

4. Compile:

Enter the command: make

You can see the compiled files

Check compiled modules

You can also use the modinfo command to further check:

5. Insert module

Insert the module using the insmod command. After the insertion is complete, you can use the lsmod command to check whether the current module has been loaded into the system:

The first one is.

After the system loads the module, it will also create a new directory named after the module under the "/sys/module" directory:

6. View log output

Since prink() uses the default output level in this demonstration, you can view the output results through the "dmesg" or "tail /var/log/kern.log" command.

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