Linux kernel device driver proc file system notes

/*****************
 * proc file system*****************/

(1) Characteristics of the /proc file system and description of /proc files

The /proc file system is a special file system created by software. The kernel uses it to export information to the outside world. The /proc system only exists in memory and does not occupy external memory space.

Each file under /proc is bound to a kernel function, which dynamically generates the file's contents when the user reads the file. You can also modify kernel parameters by writing /proc files

File analysis under the /proc directory /proc/$pid Information directory about process $pid. Each process has a directory under /proc named after its process number. Example: $>strings -f /proc/[0-9]*/cmdline

• /proc/cmdline command line for kernel startup
• /proc/cpuinfo Processor information such as type, manufacturer, model, and performance.
• /proc/devices lists the major numbers of character and block devices, and the device names assigned to those numbers.
• /proc/dma shows the currently used DMA channels.
• /proc/filesystems lists the filesystem types available for use, usually those built into the kernel, but new types can be added via modules
• /proc/interrupts shows the interrupt number used, the interrupt name, and the number of times these interrupts have occurred since the system was started
• /proc/ioports Currently used I/O ports.
• /proc/kallsyms Kernel symbol table. After installing a new module, it will be reflected here
• /proc/kcore System physical memory image. Exactly the same size as physical memory, but not actually occupying that much memory; (Remember: nothing under /proc takes up any disk space unless copied to a file)
• /proc/kmsg Kernel output messages. Also sent to syslog.
• /proc/loadavg The average load of the system. The first three are the loads of the past 1 minute, 5 minutes, and 15 minutes, followed by the number of running tasks and the total number of tasks, and finally the process number of the last run.
• /proc/meminfo Memory usage information, including physical memory and swap.
• /proc/modules Which core modules are currently loaded.
• /proc/partitions The partition information of the hard disk currently mounted by the system
• /proc/pci system pci bus information
• /proc/net Network protocol status information.
• /proc/self A symbolic link to the process directory of programs that view /proc. When 2 processes look at /proc, they are different connections. This is mainly convenient for a program to get its own process directory.
• /proc/slabinfo Allocation information of slab cache in the system
• /proc/stat Some status information of the system
• /proc/swaps The swap area information used by the system
• /proc/sysrq-trigger is used to start the sysRq key $>echo 1 > sysrq-trigger
• /proc/uptime The length of time the system has been up and idle. For uptime use
• /proc/version Kernel version

(2) Implement a /proc file yourself

The header file <linux/proc_fs.h> needs to be included, and the function is defined in /fs/proc/generic.c

a. Create a file under /proc

Call create_proc_read_entry to create a new file under /proc

struct proc_dir_entry *create_proc_read_entry(
    const char *name,
    mode_t mode, 
    struct proc_dir_entry *base,
    read_proc_t *read_proc, 
    void * data)

b. Uninstall files under /proc

Use remove_proc_entry to uninstall proc files

void remove_proc_entry(
    const char *name, 
    struct proc_dir_entry *parent);

c. Define a function that returns data

When the process reads the /proc file, the kernel allocates a memory page (i.e. a memory block of PAGE_SIZE bytes), and the driver returns the data to be written to the user space through this memory page.

typedef int (read_proc_t)(char *page, char **start, off_t off,
int count, int *eof, void *data);

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