使用 O_DIRECT 打开的文件中的 seek() 和 read()

Question

我正在尝试从文件中读取seek 和read，我的目标是所有读取都直接来自磁盘。为了做到这一点，我将open() 与O_DIRECT、lseek() 的文件添加到所需的偏移量，并尝试将read() 从磁盘中取出一个块。从磁盘读取时遇到错误：

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

int main(int argc, char *argv[])
{
        int disk_fd_read;
        off_t disk_off;
        int ret;

        int next_block;

        disk_fd_read = open(argv[1], O_RDONLY | O_DIRECT);
        if (disk_fd_read < 0) {
                printf("disk open error: %s\n", strerror(errno));
                exit(1);
        }

        disk_off = 100;

        disk_off = lseek(disk_fd_read, disk_off, SEEK_SET);
        if (disk_off != 100 || disk_off < 0) {
                printf("Error: could not seek %s\n", strerror(errno));
                exit (1);
        }
        printf("disk offset = %ld\n", disk_off);
        ret = read(disk_fd_read, &next_block, sizeof(uint64_t));
        /* 
        pread does not work either...
        ret = pread(disk_fd_read, &next_block, sizeof(uint64_t), disk_off);
        */
        if( ret == -1) {
                printf("error reading from device %s\n",strerror(errno));
                exit(1);
        }
        close(disk_fd_read);
}

/* RUN:
dd if=/dev/zero of=1Mfile bs=1M count=1
./a.out 1Mfile
disk offset = 100
error reading from device Invalid argument
*/

当我在打开文件时删除 O_DIRECT 时错误消失。从阅读手册页：

    EINVAL fd  is attached to an object which is unsuitable for reading; or the file was 
opened with the O_DIRECT flag, and either the address specified in buf, the value specified 
in count, or the current file offset is not suitably aligned.

这是否意味着 lseek 不支持 O_DIRECT？我们如何寻找不同的磁盘偏移量并直接从磁盘读取？

Answer 1

“适当对齐”是这里的关键。您需要确保您的偏移量是 4k（页面大小）对齐的。此外，大小需要是 4k 的倍数。