开源鸿蒙 procfs

2022-08-09
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procfs

Basic Concepts

The proc filesystem (procfs) is a virtual file system that displays process or other system information in a file-like structure. It is more convenient to obtain system information in file operation mode than API calling mode.

Working Principles

In the OpenHarmony kernel, procfs is automatically mounted to the /proc directory during startup. Only the kernel module can create file nodes to provide the query service.

Development Guidelines

To create a procfs file, you need to use ProcMkdir to create a directory and use CreateProcEntry to create a file. The development of the file node function is to hook the read and write functions to the file created by CreateProcEntry. When the procfs file is read or written, the hooked functions will be called to implement custom functions.

Development Example

The following describes how to create the /proc/hello/world file to implement the following functions:

Create a file in /proc/hello/world.
Read the file. When the file is read, “HelloWorld!” is returned.
Write the file and print the data written in the file.

#include "proc_fs.h"

static int TestRead(struct SeqBuf *buf, void *arg)
{
    LosBufPrintf(buf, "Hello World! \n"); /* Print "Hello World!" to the buffer. The data in the buffer will be returned to the read result. */
    return 0;
}

static int TestWrite(struct ProcFile *pf, const char *buffer, size_t buflen, loff_t *ppos)
{
    if ((buffer == NULL) || (buflen <= 0)) {
        return -EINVAL;
    }

    PRINTK("your input is: %s\n", buffer); /* Different from the read API, the write API prints the data only to the console. */
    return buflen;
}
static const struct ProcFileOperations HELLO_WORLD_OPS = {
    .read = TestRead,
    .write = TestWrite,
};

void HelloWorldInit(void)
{
    /* Create the hello directory. */
    struct ProcDirEntry *dir = ProcMkdir("hello", NULL);
    if (dir == NULL) {
        PRINT_ERR("create dir failed!\n");
        return;
    }

    /* Create the world file. */
    struct ProcDirEntry *entry = CreateProcEntry("world", 0, dir);
    if (entry == NULL) {
        PRINT_ERR("create entry failed!\n");
        return;
    }

    /* Hook the custom read and write APIs to the file. */
    entry->procFileOps = &HELLO_WORLD_OPS;
}

Verification

After the OS startup, run the following commands in the shell:

OHOS # cat /proc/hello/world
OHOS # Hello World!
OHOS # echo "yo" > /proc/hello/world
OHOS # your input is: yo

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