c++ - 读取二进制数据,seek 无法正常工作
问题描述
我在尝试从二进制文件中读取数据时遇到了一些问题,该文件的结构使得标题是前 1024 或 4069 个字节,之后的有效负载都是常规块。下面是我一直在使用的代码模型:
标题:
#pragma once
#include <stdio.h>
#include <iostream>
#include <fstream>
class BinaryFile
{
public:
long MagicNumber;
long HeaderSize;
long PayloadSize;
//... about 20 other header details
char* Padding; // unused area of the header that is reserved for future use
std::ifstream BinaryFileStream;
BinaryFile();
int Open_Binary_File(const char* path);
int Load_Payload_Into_Buffer(int payload_index, void* buffer);
};
以及我尝试使用的 C++ 代码:
#include "BinaryFile.h"
BinaryFile:BinaryFile()
{
MagicNumber = 0;
HeaderSize = 1024;
PayloadSize = 62830080;
// ... all the other header items, initalized with default values
std:ifstream BinaryFileStream;
Padding[360];
}
int BinaryFile::Open_Binary_File(const char* path) // This is the one that I would like to be using
{
BinaryFileStream.open(path, std::ifstream::binary);
BinaryFileStream.read((char*)&MagicNumber, (size_t) 4);
BinaryFileStream.read((char*)&HeaderSize, (size_t) 4);
// ... The rest of the header is placed into the object
BinaryFileStream.read((char*)&Padding, (size_t) 360);
// The file pointer should now be 1024 bytes into the file, at the end of the header, and at the start of the first payload
return 0;
}
int Load_Payload_Into_Buffer(int payload_index, void* buffer)
{
buffer = malloc(PayloadSize);
size_t offset = HeaderSize + static_cast<long long>(frame_index) * PayloadSize;
BinaryFileStream.seekg(offset, BinaryFileStream.beg);
BinaryFileStream.read((char*)buffer, PayloadSize);
return 0;
Error:
return 1;
}
以下是我尝试过的一些变体:
int BinaryFile::Open_Binary_File(const char* path)
{
BinaryFileStream.open(path, std::ifstream::binary);
BinaryFileStream.read((char*)&MagicNumber, (size_t) 4);
BinaryFileStream.read((char*)&HeaderSize, (size_t) 4);
// ... The rest of the header is placed into the object
BinaryFileStream.read((char*)&Padding, (size_t) 360);
BinaryFileStream.clear();
BinaryFileStream._Seekbeg.seekg((size_t)0, BinaryFileStream._Seekbeg);
BinaryFileStream.sync();
// The file pointer should now be 0 bytes into the file, at the start of the header
return 0;
}
int BinaryFile::Open_Binary_File(const char* path)
{
BinaryFileStream.open(path, std::ifstream::binary);
BinaryFileStream.read((char*)&MagicNumber, (size_t) 4);
BinaryFileStream.read((char*)&HeaderSize, (size_t) 4);
// ... The rest of the header is placed into the object
BinaryFileStream.read((char*)&Padding, (size_t) 360);
BinaryFileStream.clear();
BinaryFileStream.seekg((size_t)-1024);
BinaryFileStream.sync();
// The file pointer should now be 0 bytes into the file, at the start of the header
return 0;
}
我遇到的问题是,返回到缓冲区的有效负载包含一些下一个有效负载。文件指针的设置似乎不像我期望的那样工作。即如果我说
BinaryFileStream._Seekbeg.seekg(position, BinaryFileStream._Seekbeg)
我希望指针返回到文件的开头,然后沿着我所说的字节数寻找位置。有不同的方法可以做到这一点吗?还是有什么我想念的?
解决方案
事实证明,我需要做的是乘以HeaderSize
,CHAR_BIT
所以它看起来像这样:
int Load_Payload_Into_Buffer(int payload_index, void* buffer)
{
buffer = malloc(PayloadSize);
size_t offset = HeaderSize*CHAR_BIT + static_cast<long long>(frame_index) * PayloadSize;
BinaryFileStream.seekg(offset, BinaryFileStream.beg);
BinaryFileStream.read((char*)buffer, PayloadSize);
return 0;
Error:
return 1;
}
我认为是下一个有效载荷的一部分,实际上是相同的有效载荷,但在某个任意点拆分,并堆叠在它旁边。所以有效载荷被分割了大约。3/4,然后重新排列,原来的最后一个季度现在在第一个位置,原来的前 3/4 现在在第二个位置。
希望这是有道理的,并会在未来帮助某人!