fstream文件修改
请教各位一下fstream文件操作的相关问题，FILE相关知识点就免了。
最近在写一个文件加密的小程序，涉及到文件的修改，有几个问题要请教一下各位：
1，我现在能做到的是先只读打开该文件，读取后关闭，再trunc打开重写文件，这样通过字符串修改可以修改文件，但问题来了，一个操作打开两次文件，我同学说这样开销太大了。
2，网上基本搜到的做法都是分读写两次打开的，我现在尝试一次读写打开，直接通过指针位移从头覆盖内容，但现在问题卡在，我进行解码操作后，由于原文比密文长度短，导致有密文尾巴无法覆盖，但我不会单纯的修改。
希望各位分享相关一下经验，谢谢！
[解决办法]
打开文件毕竟次数有限，按照你的第一种方式，主要操作在缓冲区进行，尽量减少读写文件的次数就好了。
[解决办法]
lz的问题，fstream要缩短文件长度好像没有办法吧，记得似乎只能重新打开
或者我不知道，请lx了解的朋友讲解一下
[解决办法]
1、一次打开文件进行读写，是没有问题的
2、把长文件截断为短文件对于fstream还不知道如何操作,FILE是没有问题的
定义person类并实例化对象，将对象的数据信息写入到二进制文件中，然后再读取文件中的信息并显示出来。
#include<iostream>
#include<iomanip>
#include<fstream>
using namespace std;
class person
{
public:
char name[10]; // 姓名
char profession[10]; // 职业
int age; // 年龄
double pay; // 月工资
person(){}
person(char *name,char *profession,int age,double pay)
{
strcpy(this->name,name);
strcpy(this->profession,profession);
this->age=age;
this->pay=pay;
}
};
void main()
{
person lx("刘翔","运动员",28,2132.48);
person dq("董卿","主持人",38,260000);
fstream f("d:\\Result.txt",ios::in
[解决办法]
ios::out
[解决办法]
ios::binary);
if( !f ) return;
f.write((char*)&lx,sizeof(lx));
f.write((char*)&dq,sizeof(dq));
f.seekp(0);
person p;
while(true)
{
f.read((char *)(&p),sizeof(person));
if(f.fail())break;
cout<<setw(10)<<p.name;
cout<<setw(10)<<p.profession;
cout<<setw(5)<<p.age;
cout<<fixed<<setprecision(2)<<setw(14)<<p.pay;
cout<<endl;
}
f.close();
}
[解决办法]
linux ftruncate
windows SetEndOfFile
------解决方案--------------------
_chsize
Changes the file size.
int _chsize( int handle, long size );
[解决办法]
先
SetFilePointer
The SetFilePointer function moves the file pointer of an open file.
DWORD SetFilePointer(
HANDLE hFile, // handle of file
LONG lDistanceToMove, // number of bytes to move file pointer
PLONG lpDistanceToMoveHigh,
// pointer to high-order DWORD of
// distance to move
DWORD dwMoveMethod // how to move
);
Parameters
hFile
Handle to the file whose file pointer is to be moved. The file handle must have been created with GENERIC_READ or GENERIC_WRITE access to the file.
lDistanceToMove
Low-order 32 bits of a signed value that specifies the number of bytes to move the file pointer. If lpDistanceToMoveHigh is not NULL, lpDistanceToMoveHigh and lDistanceToMove form a single 64-bit signed value that specifies the distance to move. If lpDistanceToMoveHigh is NULL, lDistanceToMove is a 32-bit signed value. A positive value for lDistanceToMove moves the file pointer forward in the file, and a negative value moves the file pointer backward.
lpDistanceToMoveHigh
Pointer to the high-order 32 bits of the signed 64-bit distance to move. If you do not need the high-order 32 bits, this pointer may be NULL. When non-NULL, this parameter also receives the high-order DWORD of the new value of the file pointer. For more information, see the Remarks section later in this topic.
dwMoveMethod
Starting point for the file pointer move. This parameter can be one of the following values. Value Meaning
FILE_BEGIN The starting point is zero or the beginning of the file.
FILE_CURRENT The starting point is the current value of the file pointer.
FILE_END The starting point is the current end-of-file position.
Return Values
If the SetFilePointer function succeeds and lpDistanceToMoveHigh is NULL, the return value is the low-order DWORD of the new file pointer. If lpDistanceToMoveHigh is not NULL, the function returns the low order DWORD of the new file pointer, and puts the high-order DWORD of the new file pointer into the LONG pointed to by that parameter.
If the function fails and lpDistanceToMoveHigh is NULL, the return value is 0xFFFFFFFF. To get extended error information, call GetLastError.
If the function fails, and lpDistanceToMoveHigh is non-NULL, the return value is 0xFFFFFFFF. However, because 0xFFFFFFFF is a valid value for the low-order DWORD of the new file pointer, you must check GetLastError to determine whether an error occurred. If an error occurred, GetLastError returns a value other than NO_ERROR. For a code example that illustrates this point, see the Remarks section later in this topic.
If the new file pointer would have been a negative value, the function fails, the file pointer is not moved, and the code returned by GetLastError is ERROR_NEGATIVE_SEEK.
Remarks
You cannot use the SetFilePointer function with a handle to a nonseeking device, such as a pipe or a communications device. To determine the file type for hFile, use the GetFileType function.
To determine the present position of a file pointer, see Retrieving a File Pointer.
You should be careful when setting the file pointer in a multithreaded application. For example, an application whose threads share a file handle, update the file pointer, and read from the file must protect this sequence by using a critical section object or mutex object. For more information about these objects, see Critical Section Objects and Mutex Objects.
If the hFile file handle was opened with the FILE_FLAG_NO_BUFFERING flag set, an application can move the file pointer only to sector-aligned positions. A sector-aligned position is a position that is a whole number multiple of the volume's sector size. An application can obtain a volume's sector size by calling the GetDiskFreeSpace function. If an application calls SetFilePointer with distance-to-move values that result in a position that is not sector-aligned and a handle that was opened with FILE_FLAG_NO_BUFFERING, the function fails, and GetLastError returns ERROR_INVALID_PARAMETER.
Note that it is not an error to set the file pointer to a position beyond the end of the file. The size of the file does not increase until you call the SetEndOfFile, WriteFile, or WriteFileEx function. A write operation increases the size of the file to the file pointer position plus the size of the buffer written, leaving the intervening bytes uninitialized.
If the return value is 0xFFFFFFFF and if lpDistanceToMoveHigh is non-NULL, an application must call GetLastError to determine whether the function has succeeded or failed. The following sample code illustrates this point:
//
// Case One: calling the function with lpDistanceToMoveHigh == NULL
// Try to move hFile's file pointer some distance
dwPtr = SetFilePointer (hFile, lDistance, NULL, FILE_BEGIN) ;
if (dwPtr == 0xFFFFFFFF) // Test for failure
{
// Obtain the error code
dwError = GetLastError() ;
// Deal with failure
// . . .
} // End of error handler
//
// Case Two: calling the function with lpDistanceToMoveHigh != NULL
// Try to move hFile's file pointer some huge distance
dwPtrLow = SetFilePointer (hFile, lDistLow, & lDistHigh, FILE_BEGIN) ;
// Test for failure
if (dwPtrLow == 0xFFFFFFFF && (dwError = GetLastError()) != NO_ERROR )
{
// Deal with failure
// . . .
} // End of error handler
The parameter lpDistanceToMoveHigh is used to manipulate huge files. If it is set to NULL, then lDistanceToMove has a maximum value of 2^312, or 2 gigabytes less two. This is because all file pointer values are signed values. Therefore if there is even a small change that the file will grow to that size, you should treat the file as a huge file and work with 64-bit file pointers. With file compression on NTFS, and sparse files, it is possible to have files that large even if the underlying volume is not that large.
If lpDistanceToMoveHigh is not NULL, then lpDistanceToMoveHigh and lDistanceToMove form a single 64-bit signed value. The lDistanceToMove parameter is treated as the low-order 32 bits of the value, and lpDistanceToMoveHigh as the upper 32 bits. Thus, lpDistanceToMoveHigh is a sign extension of lDistanceToMove.
To move the file pointer from zero to 2 gigabytes, lpDistanceToMoveHigh can be either NULL or a sign extension of lDistanceToMove. To move the pointer more than 2 gigabytes, use lpDistanceToMoveHigh and lDistanceToMove as a single 64-bit quantity. For example, to move in the range from 2 gigabytes to 4 gigabytes set the contents of lpDistanceToMoveHigh to zero, or to 1 for a negative sign extension of lDistanceToMove.
To work with 64-bit file pointers, you can declare a LONG, treat it as the upper half of the 64-bit file pointer, and pass its address in lpDistanceToMoveHigh. This means you have to treat two different variables as a logical unit, which is error-prone. The problems can be ameliorated by using the LARGE_INTEGER structure to create a 64-bit value and passing the two 32-bit values by means of the appropriate elements of the union.
It is conceptually simpler and better design to use a function to hide the interface to SetFilePointer. To do so, use something like this:
__int64 myFileSeek (HANDLE hf, __int64 distance, DWORD MoveMethod)
{
LARGE_INTEGER li;
li.QuadPart = distance;
li.LowPart = SetFilePointer (hf, li.LowPart, &li.HighPart, MoveMethod);
if (li.LowPart == 0xFFFFFFFF && GetLastError() != NO_ERROR)
{
li.QuadPart = -1;
}
return li.QuadPart;
}
Note You can use SetFilePointer to determine the length of a file. To do this, use FILE_END for dwMoveMethod and seek to location zero. The file offset returned is the length of the file. However, this practice can have unintended side effects, such as failure to save the current file pointer so that the program can return to that location. It is simpler and safer to use GetFileSize instead.
You can also use the SetFilePointer function to query the current file pointer position. To do this, specify a move method of FILE_CURRENT and a distance of zero.
QuickInfo
Windows NT: Requires version 3.1 or later.
Windows: Requires Windows 95 or later.
Windows CE: Requires version 1.0 or later.
Header: Declared in winbase.h.
Import Library: Use kernel32.lib.
See Also
File I/O Overview, File Functions, GetDiskFreeSpace, GetFileSize, GetFileType, ReadFile, ReadFileEx, ReadFileVlm, WriteFile, WriteFileEx, WriteFileVlm
再
SetEndOfFile