File system and directory operations

1.mmap

• addr： If addr Parameter is NULL, The kernel selects the appropriate address in the process address space to establish the mapping . If addr No NULL, Then give the kernel a hint , What address should I start mapping from , The kernel will choose addr Start mapping at a suitable address above . After the mapping is established , The real mapping first address can be obtained through the return value .

• len： The length of the part of the file to be mapped .off Parameters are mapped from where in the file , Must be an integer multiple of the page size .

• filedes： Descriptor of the file .

• prot： Yes 4 Species value ：①PROT_EXEC： This section of the map is executable , Such as mapping shared libraries  ②PROT_READ： This section of the map is readable ③PROT_WRITE： This section of the mapping can be written as ④PROT_NONE： This segment of the map is not accessible .

• flag：MAP_SHARED: Multiple processes share the same file mapping ,MAP_PRIVATE: Multiple processes do not share the same file mapping  

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>

int main(void) {
    int fd = open("test.txt", O_RDWR);
    if (fd < 0) {
        perror("open file");
        exit(1);
    }
    int *p = mmap(NULL, 6, PROT_WRITE, MAP_SHARED, fd, 0);
    ((int *)((char *)p) + 1)[0] = 0x30313233;
    munmap(p, 6);
    return 0;
}

2. Virtual file system VFS

【1】ext2 file system

The smallest unit of storage in a file system is a block , The size of a block is determined during formatting , Such as ：mke2fs Of -b Option to set the block size to 1024,2048 or 4096 byte .

• Boot block ： Size is 1KB, from PC Standard setting , Used to store disk partition information and startup information , This block cannot be used by any file system occasional reads .

• Superblock ： Describe the file system information of the entire partition , Such as ： Block size , File system version number , The last time mount And so on . A superblock has a copy at the beginning of each block group .

• Block group descriptor table ： It consists of many block group descriptors , How many block groups the whole partition is divided into corresponds to how many block group descriptors . Each block group descriptor stores the description information of a block group , Include inode Where does the table start , Where does the data block begin , Idle inode And how many data blocks there are . The block group descriptor table has a copy at the beginning of each block group , This information is important , So there are multiple copies .

• inode Bitmap ： Similar to block bitmap , Itself occupies a block , Each of them bit It means a inode Whether it is free or not .

• inode surface ： file type （ routine , Catalog , A symbolic link ）, jurisdiction , file size , establish / modify / Access time and other information exist inode in , Each file has a inode.

• Data blocks ：

        ① General documents ： The data of the file has data blocks .

        ② Catalog ： All file names and directories in this directory have data blocks .（ The file name is saved in the data block of the directory , Other information is saved in the file inode in ）.

        ③ A symbolic link ： If the destination pathname is short, it will be saved directly in inode in , To find... Faster , Otherwise, allocate a data block to save .

        ④ Device file 、FIFO,socket And so on ： No data blocks , The primary and secondary equipment numbers of the equipment file are saved in inode.

【2】stat（2）

Read the file inode, And then put inode Fill in a for various file attributes in struct stat Structure is passed to the caller .

stat(1) The order is based on stat Functionally implemented .

stat You need to find according to the incoming file path inode.

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
    struct stat st;
    if (argc < 2) {
        printf("usage:cmd + filename,dirname\n");
        exit(1);
    }

    lstat(argv[1], &st);
    switch (st.st_mode & S_IFMT)
    {
    case S_IFREG:
        printf("Is regular file\n");
        break;
    case S_IFDIR:
        printf("Is directory\n");
        break;
    case S_IFCHR:
        printf("Is charactor device\n");
        break;
    default:
        printf("Is other file type\n");
        break;
    }
    return 0;
}