有时候，读写文件并不想要使用系统缓存（page cache），此时 direct 文件读写就派上了用场，使用方法：
（1）打开文件时，添加O_DIRECT参数：
需要定义_GNU_SOURCE，否则找不到O_DIRECT宏定义
示例片段：
#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); View Code （2）读写操作的传输数据大小和缓冲区地址都需要按照一定的规则对齐：
Linux下，对于不同的文件系统和内核版本，需要的对齐边界不同，也没有统一的接口可以获取到该边界值。
对于kernel 2.4版本：传输大小和缓冲区地址均需要按照访问文件系统的逻辑块大小对齐，比如文件系统的块大小是4K，buffer地址需要按照4K对齐，需要读写4K倍数的数据
对于kernel 2.6版本：传输大小和缓冲区地址按照目标存储设备的扇区大小（一般512）对齐
可使用memalign （malloc.h）来分配指定地址对齐的资源接口：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 512if (!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;
} View Code
引用Linus的话：
"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的详细描述，可以看linux open系统调用的文档：http://man7.org/linux/man-pages/man2/open.2.html
转载于:https://www.cnblogs.com/ziziwu/p/4126244.html