Linux中的匿名inode是什么？

Question

我做了一个关于"匿名inode"的谷歌搜索,它似乎与epoll有关......但究竟是什么呢？

Answer 1

至少在某些情况下,匿名inode是没有附加目录条目的inode.创建此类inode的最简单方法是:

int fd = open( "/tmp/file", O_CREAT | O_RDWR, 0666 );
unlink( "/tmp/file" );
// Note that the descriptor fd now points to an inode that has no filesystem entry; you
// can still write to it, fstat() it, etc. but you can't find it in the filesystem.

Answer 2

open 同 O_TMPFILE

这将是匿名inode的一个很好的定义:它在给定目录中创建一个没有任何名称的inode,它根本不会出现ls.

然后,当您关闭描述符时,文件将被删除.

它是在Linux 3.11中添加的.

#define _GNU_SOURCE
#include <assert.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

int main(void) {
    char buf[]  = { 'a', 'b', 'c', 'd' };
    char buf2[]  = { 'e', 'f', 'g', 'h' };
    int f, ret;
    size_t off;

    /* write */
    f = open(".", O_TMPFILE | O_RDWR, S_IRUSR | S_IWUSR);
    ret = write(f, buf, sizeof(buf));

    /* Interactivelly check if anything changed on directory. It hasn't. */
    /*puts("hit enter to continue");*/
    /*getchar();*/

    /* read */
    lseek(f, 0, SEEK_SET);
    off = 0;
    while ((ret = read(f, buf2 + off, sizeof(buf) - off))) {
        off += ret;
    }
    close(f);
    assert(!memcmp(buf, buf2, sizeof(buf)));

    return EXIT_SUCCESS;
}

在Ubuntu 17.04,Linux 4.10,glibc 2.24中测试,运行:

gcc -o main.out -std=c99 -Wall -Wextra -pedantic  main.c
./main.out

O_TMPFILE Linux内核功能

如果您正在处理内核模块,这可能是一个定义.

你称之为:

#define _GNU_SOURCE
#include <fcntl.h>
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

int main(void) {
    int f = open(".", O_TMPFILE | O_RDWR, S_IRUSR | S_IWUSR);
    struct stat s;
    fstat(f, &s);
    printf("pid %jd\n", (intmax_t)getpid());
    printf("inode %jd\n", (intmax_t)s.st_ino);
    getchar();
}

并返回/proc/PID/fd用户,例如来自inode.

现在用户有一个anon_inode_getfd与关联任意的fd和ioctl,而当fd关闭时,一切都被释放.

此方法很有用,例如,如果您想要多个file_operations系统调用,但不想创建多个设备文件,这会进一步污染inode:您只需创建额外的fds.

使用QEMU Buildroot的最小可运行示例:

• https://github.com/cirosantilli/linux-kernel-module-cheat/blob/de6c179fc0cf122789f0fe85cc69b847a1f4fe9c/kernel_module/anonymous_inode.c
• https://github.com/cirosantilli/linux-kernel-module-cheat/blob/de6c179fc0cf122789f0fe85cc69b847a1f4fe9c/rootfs_overlay/anonymous_inode.sh

pid 15952     
inode 44698689