我正在尝试从文件中seek
和read
,我的目标是所有读操作直接来自磁盘。为了实现这一点,我使用O_DIRECT
选项打开文件,使用lseek()
函数定位到所需的偏移量,并尝试从磁盘中read()
一个块。然而在从磁盘中读取时遇到了错误:
#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
*/
当我在打开文件时去掉O_DIRECT选项,错误消失了。从read手册中可以看到:
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?我们如何从不同的磁盘偏移量进行寻址并直接从磁盘读取?
argc
以确保用户实际输入了命令行参数的情况下,永远不要超出argv [0]
的范围访问。 - user3629249