Linux Direct 文件讀寫（文件DIO）

有時候，讀寫文件並不想要使用系統緩存（page cache），此時 direct 文件讀寫就派上了用場，使用方法：

 

（1）打開文件時，添加O_DIRECT參數：

須要定義_GNU_SOURCE，不然找不到O_DIRECT宏定義

示例片斷：

#define _GNU_SOURCE
  
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
 
int fd = open("test.out", O_RDWR | O_CREAT | O_DIRECT, 0644);

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（2）讀寫操做的傳輸數據大小和緩衝區地址都須要按照必定的規則對齊：

Linux下，對於不一樣的文件系統和內核版本，須要的對齊邊界不一樣，也沒有統一的接口能夠獲取到該邊界值。

對於kernel 2.4版本：傳輸大小和緩衝區地址均須要按照訪問文件系統的邏輯塊大小對齊，好比文件系統的塊大小是4K，buffer地址須要按照4K對齊，須要讀寫4K倍數的數據

對於kernel 2.6版本：傳輸大小和緩衝區地址按照目標存儲設備的扇區大小（通常512）對齊

 

可以使用memalign （malloc.h）來分配指定地址對齊的資源接口：void *memalign(size_t boundary, size_t size);

 

完整示例程序： 

 

#define _GNU_SOURCE
 
#include <sys/types.h>
#include <fcntl.h>
#include <malloc.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
 
int main(void) {
        char hello_str[] = "Hello World!";
        void *write_buffer;
        void *read_buffer;
        int fd; 
        int ret = 0;
     
        fd = open("test.out", O_RDWR | O_CREAT | O_DIRECT, 0644);
        if (fd < 0) {
                printf("Failed to open file\n");
                return fd; 
        }   
     
        /* allocate a 1024 bytes buffer */
        write_buffer = memalign(512, 512 * 2); // align by 512
        if (!write_buffer) {
                printf("Failed to alloc write buffer\n");
                ret = -ENOMEM;
                goto bad_write_buffer;
        }   
 
        memcpy(write_buffer, hello_str, sizeof(hello_str));
 
        ret = write(fd, write_buffer, 512 * 2); 
        if (ret < 0) {
                printf("Failed to write file\n"); 
                goto bad_write;
        }   
 
        lseek(fd, 0, SEEK_SET); // read  previous write data
     
        read_buffer = memalign(512, 512 * 2);
        if (!read_buffer) {
                printf("Failed to alloc read buffer\n");
                ret = -ENOMEM;
                goto bad_read_buffer;
        }
 
        ret = read(fd, read_buffer, 512 * 2);
        if (ret <0) {
                printf("Failed to read file\n");
                goto bad_read;
        }
 
        printf("read from file : %s\n", read_buffer);
 
bad_read:
        free(read_buffer);
bad_read_buffer:
bad_write:
        free(write_buffer);
bad_write_buffer:
        close(fd);
        return ret;
}

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引用Linus的話：

"The thing that has always disturbed me about O_DIRECT is that the whole interface is just stupid, and was probably designed by a deranged monkey on some serious mind-controlling substances."—Linus（O_DIRECT就是一坨屎）

 

PS:

O_DIRECT的詳細描述，能夠看linux open系統調用的文檔：http://man7.org/linux/man-pages/man2/open.2.html