C++ 如何将一个.txt文件清空,有哪个函数可以调用吗？我打开一个已经有内容的文件，同时清空里面原有内容

[天地一扁舟]

有函数可以调用吗？还是用ios::trunc

[qq120848369]

truncate/ftruncate 或者fopen用"w"以及衍生参数打开。

[漫步者、]

fopen中的w模式

[就是那个党伟]

w应该是清空重写吧 a是追加，r是只读。 w即可

[赵4老师]

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 2^31–2, 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 SetEndOfFile The SetEndOfFile function moves the end-of-file (EOF) position for the specified file to the current position of the file pointer. BOOL SetEndOfFile( HANDLE hFile // handle of file whose EOF is to be set ); Parameters hFile Handle to the file to have its EOF position moved. The file handle must have been created with GENERIC_WRITE access to the file. Return Values If the function succeeds, the return value is nonzero. If the function fails, the return value is zero. To get extended error information, call GetLastError. Remarks This function can be used to truncate or extend a file. If the file is extended, the contents of the file between the old EOF position and the new position are not defined. If you called CreateFileMapping to create a file-mapping object for hFile, you must first call UnmapViewOfFile to unmap all views and call CloseHandle to close the file-mapping object before you can call SetEndOfFile. 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, CloseHandle, CreateFile, CreateFileMapping, UnmapViewOfFile

[沙尘暗影]

引用 4 楼 zhao4zhong1 的回复:

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 2^31–2, 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 SetEndOfFile The SetEndOfFile function moves the end-of-file (EOF) position for the specified file to the current position of the file pointer. BOOL SetEndOfFile( HANDLE hFile // handle of file whose EOF is to be set ); Parameters hFile Handle to the file to have its EOF position moved. The file handle must have been created with GENERIC_WRITE access to the file. Return Values If the function succeeds, the return value is nonzero. If the function fails, the return value is zero. To get extended error information, call GetLastError. Remarks This function can be used to truncate or extend a file. If the file is extended, the contents of the file between the old EOF position and the new position are not defined. If you called CreateFileMapping to create a file-mapping object for hFile, you must first call UnmapViewOfFile to unmap all views and call CloseHandle to close the file-mapping object before you can call SetEndOfFile. 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, CloseHandle, CreateFile, CreateFileMapping, UnmapViewOfFile

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