Directory operations and virtual file systems

1.stat

Suppose a path is /opt/file, Then the search order is ：

① read out inode No 2 term （ root directory inode）, Find the root directory data block location

② Find the file name from the root directory data block opt The record of , Be alone from the record inode Number

③ read out opt The directory inode, Find out its data block location

④ from opt Find the file name in the data block of the directory file The record of , Read from the record inode Number

⑤ read out file Of documents inode

2.opendir(3)/readdir(3)/closedir(3)

Used to traverse the records in the directory data block .

opendir： Open a directory , Return to one Dir * The pointer represents the directory , similar FILE * Handle to pointer .

closedir： Close this handle , hold Dir * The pointer is passed to readdir Read the records in the directory data block , Return one point at a time struct dirent The pointer to , Repeated reading can traverse all records , After all the records are traversed readdir return NULL.

Structure struct dirent Definition ：

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

void printDir(char *dirname) {
    char pathname[1024];
    DIR *dir;
    struct dirent *dp;
    struct stat st;
    if (!(dir=opendir(dirname))) {
        perror("opendir");
        exit(1);
    }
    while (dp = readdir(dir))
    {
        if (!strcmp(dp->d_name, ".") || !strcmp(dp->d_name, "..")) {
            continue;
        }
        sprintf(pathname, "%s/%s", dirname, dp->d_name);
        if (stat(pathname, &st) < 0) {
            perror("stat");
            exit(1);
        }
        if (S_ISDIR(st.st_mode)) {
            printDir(pathname);
        }
        printf("%s\t", dp->d_name);
    }
    putchar(10);
    closedir(dir);
}

int main(int argc, char **argv) {
    if (argc < 2) {
        printf("usage:cmd + path");
        return 1;
    }

    printDir(argv[1]);
    return 0;
}

3.VFS

Virtual Filesystem

Linux Supports a wide variety of file system formats , Then these file systems can mount To a directory , Let's see a unified directory tree , Directories and files on various file systems ls The orders look the same , Read and write operations are the same , How this is done ？

Linux The kernel makes an abstraction layer on top of various file system formats , Make the document , Catalog , Concepts such as read / write access are called abstract layer concepts , So the systems look the same , This abstraction layer is the virtual file system .

4.dup and dup2

Can be used to assign an existing file descriptor , Make two file descriptors point to the same file Structure .

If two file descriptors point to the same file Structure ,File,Status,Flag And read and write locations are only saved in file Structure , also file The reference count of the structure is 2.

If 2 Time open The same file gets two file descriptors , Then each descriptor corresponds to a different file Structure , There can be different ones File,Status,Flag And reading and writing positions .

Distinguish between these two situations ：

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

int main(void) {
    int fd, save_fd;

    if ((fd = open("test.txt", O_RDWR)) < 0) {
        perror("open");
        exit(1);
    }
    save_fd = dup(1);
    dup2(fd, 1);
    close(fd);
    write(1, "123456789", 9);
    dup2(save_fd, 1);
    write(1, "123456789", 9);
    close(save_fd);
    return 0;
}