O_DIRECT方式读取文件示例

来源：互联网发布：mac上的开发软件编辑：程序博客网时间：2024/05/29 19:33
#include <fcntl.h>#include <stdio.h>#include <stdlib.h>#include <sys/stat.h>#include <sys/types.h>#include <sys/time.h>#include <unistd.h>#include <glog/logging.h>#include <gflags/gflags.h>// 读文件类class CFileReader{public:CFileReader(): _buffer(NULL){}~CFileReader(){free(_buffer);}bool open(const char* filepath){// 以O_DIRECT方式打开文件int fd = ::open(filepath, O_RDONLY | O_DIRECT);if (-1 == fd){LOG(ERROR) << "open " << filepath << " error: " << strerror(errno);return false;}// 取得文件大小，以便一次性将文件读取出来struct stat st;if (-1 == fstat(fd, &st)){LOG(ERROR) << "stat " << filepath << " error: " << strerror(errno);close(fd);return false;}// 分配足以容纳整个文件的Buffer// 由于以O_DIRECT方式读取，所以需要按页对齐size_t size = st.st_size + (getpagesize() - st.st_size%getpagesize());                posix_memalign((void**)&_buffer, getpagesize(), size);if (NULL == _buffer){LOG(ERROR) << "malloc failed";close(fd);return false;}// 将整个文件读取_buffer中int bytes_read = read(fd, _buffer, size);if (-1 == bytes_read){                        LOG(ERROR) << "read " << filepath << " error: " << strerror(errno);close(fd);free(_buffer);_buffer = NULL;return false;}else if (bytes_read != size){// 两组测试输出数据：// FileSize(212000000) => AlignSize(212000768) -> RealSize(212000000)// FileSize(2120000000) => AlignSize(2120003584) -> RealSize(2120000000)printf("FileSize(%d) => AlignSize(%d) -> RealSize(%d)\n", st.st_size, size, bytes_read);}return true;}// 从文件中读取一个节点数据// offset：偏移量// return：返回指向记录的指针template <class P>const P* get_record(uint64_t offset) const{return reinterpret_cast<P*>(_buffer + offset);}// 取得文件所有的记录template <class P>const P** get_all_record() const{return reinterpret_cast<P**>(_buffer);}private:char* _buffer;};// 用于计时class TimeWatcher{public:TimeWatcher(const std::string& tip): _tip(tip){struct timeval now;gettimeofday(&now, NULL);_now_msec = (now.tv_sec * 1000) + (now.tv_usec / 1000);}~TimeWatcher(){struct timeval now;gettimeofday(&now, NULL);time_t cur_msec = (now.tv_sec * 1000) + (now.tv_usec / 1000);LOG(INFO) << _tip << " spend " << cur_msec - _now_msec << "ms";}private:std::string _tip;time_t _now_msec;};struct User{int32_t age;int32_t hight;int32_t weight;char bitmap[50*4];};// 用于生成测试文件bool make_test_file(const char* filepath, int num_records){int fd = open(filepath, O_WRONLY | O_CREAT, S_IRUSR | S_IWUSR);if (-1 == fd){LOG(ERROR) << "open " << filepath << " error: " << strerror(errno);return false;}User user;TimeWatcher time_watcher("write");for (int i=0; i<num_records; ++i){user.age = i;user.hight = i + i;user.weight = i * i;if (-1 == write(fd, &user, sizeof(user))){LOG(ERROR) << "write " << filepath << " error: " << strerror(errno);close(fd);return false;}}close(fd);return true;}// 模拟随机读取void random_read(const CFileReader& file_reader, int num_random){int *index = new int[num_random];for (int i=0; i<num_random; ++i){srandom(time(NULL));index[i] = random() % num_random;}TimeWatcher time_watcher("randmon read");for (int i=0; i<num_random; ++i){file_reader.get_record<struct User>(index[i]);}}// 执行测试void test(){int num_records1 = 1000000;int num_records2 = 10000000;std::string file1 = "./file_1000000";std::string file2 = "./file_10000000";if (make_test_file(file1.c_str(), 1000000)         && make_test_file(file2.c_str(), 10000000)){printf("to read, press ENTER to continue ...\n");getchar();CFileReader file_reader1;{TimeWatcher time_watcher("open");if (!file_reader1.open(file1.c_str())){return;}}random_read(file_reader1, 1000000);random_read(file_reader1, 3000000);CFileReader file_reader2;{TimeWatcher time_watcher("open");if (!file_reader2.open(file2.c_str())){return;}}random_read(file_reader2, 1000000);random_read(file_reader2, 3000000);}}int main(int argc, char* argv[]){test();return 0;}
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