读写数据。 [英] reading and writing data.

问题描述

我正在尝试编写一个用于读取和写入数据的程序。它使用指针。



读取文件mm.jpg并将其写入jj.jpg（此处未指定）。但是jj.jpg是0字节，而mm.jpg是54字节。



我修改了整个程序。 void main（）仅用于检查这些函数是否正常工作。



实际上，它包含有标题作为其他程序的一部分。

I am trying to write a program for reading and writing data. It uses pointers.

It reads a file mm.jpg to and writes it to jj.jpg (Not specified here). But jj.jpg is 0 bytes while mm.jpg 54 bytes.

I have modified the whole program. void main() is here just to check whether these function work correctly.

Actually , it is to be included has header as part of an other program.

long sizeoffile(char filename[])
{
	fstream fil(filename,ios::in);

	fil.seekg(ios::beg,ios::end);
	

	long ans1 = fil.tellg();
	
	return ans1;
}


bool readfile(char filename[],char * inputdat)
{
	ifstream getit(filename,ios::in|ios::binary);
	
	long filsize = sizeoffile(filename);
	
	if(!getit)
	{	
		return false;
	}
	
	
	inputdat = new char[filsize];
	
	if(!inputdat)
	{
		
		return false;
	}
	getit.read((char*)inputdat,(filsize));
	getit.close(); 
	
	return true;
}

bool writefile(char filename[],char * outputdat,long buffsize)
{
	ofstream putit(filename,ios::out|ios::binary);
	
	if(!outputdat)
	{
		
		return false;
	}
	
	if(!putit)
	{
		
;		return false;
	}
	
	putit.write((char*)outputdat,(buffsize));
	putit.close();
	
	return true;
}

void main()
{
	char * strdat = NULL;	
	if(!(readfile("mm.jpg",strdat)))
		cout<<"Unable to read file.";
	
		if(!writefile("jj.jpg",strdat,sizeoffile("mm.jpg")))
			cout<<"Unable to write file.";
			
			delete []strdat;
		
		
	
}

推荐答案

• 您正在混合文件大小变量的数据类型。不要使用 double 因为它会导致精度损失。
• 您正在尝试从输入文件中读取比存在更多的数据：
  
  

• You are mixing datatypes for your file size variables. Do not use double as it can lead to loss of precision.
• You are trying to read more data from the input file than exists:
  

	getit.read((char*)inputdat,(filsize+sizeof(inputdat)));
// buffer is only filsize characters long

同样，您尝试编写的数据多于读入的数据。

请注意 sizeof（inputdat）和 sizeof（outputdat）总是恒定的 - 字符指针的大小（32位为4位，64位为8位）。

您还使用的是 writefile 函数中的输出文件名，可能为零;因此你不会写任何数据。

Similarly you are trying to write more data than was read in.

Note that sizeof(inputdat) and sizeof(outputdat) will alway be constant - the size of a character pointer (4 on 32-bit, 8 on 64-bit).

You are also using the size of the output filename in the writefile function, which is likely to be zero; hence you don''t write any data.

我发现了我之前错过的问题。您需要根据以下注释修改代码：

I have found the problem, which I missed before. You need to modify your code as per the comments below:

// inputdat, needs to be the address of the pointer, rather than the buffer.
long readfile(char filename[],char ** inputdat)
{
	ifstream getit(filename,ios::in|ios::binary);
	
	if(!getit)
	{
		return 0;
	}
	long filsize = sizeoffile(filename,B);
	
	
	// save the buffer address in the original pointer not
        // the temporary parameter variable
	*inputdat = new char[filsize];
	
	if(!*inputdat)
	{
		return 0;
	}
        // read only the number of bytes in the file
	getit.read((char*)*inputdat,(filsize));
	getit.close(); 
	
        // return the size of the input file (and buffer)
	return filsize;
}

// use the size of the input file for the amount to write
bool writefile(char filename[],char * outputdat, long filesize)
{
	ofstream putit(filename,ios::out|ios::binary);
        // no need to call filesize, since the output file does not exist
	
	if(!putit)
	{
		return false;
	}

        // write the correct number of bytes	
	putit.write((char*)outputdat,(filesize));
	putit.close();
	
	return true;
}


int main()
{
	char* buffer = NULL;
	long filesize;
        // send the address of the buffer pointer to the read function
	filesize = readfile("TestIn.bmp", &buffer);
        
        // if some data was read in
	if (filesize > 0)
	{
                // write the number of bytes read in to the buffer
		writefile("TestOut.bmp", buffer, filesize);
	}

	return 0;
}

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