打开文件的时候用“a+”模式的话不可以同时使用读和写么？
如果用这个模式打开的话，只读数据或者只写数据确实挺方便。但是如果读之后再写的话好像全乱套了。是不是这个打开模式只会在第一次读或者写的时候设置文件位置指针，而不是每次都能读的时候从文件的开头开始，写的时候从文件的末尾开始，并且读写之后递增或者递减文件位置指针。感觉好像是不能的，因为只有一个文件位置指针。
[解决办法]
......书上已经非常清楚地描述了以这种状态打开文件的特性，仔细看看书吧。
[解决办法]
不会用两个变量分别记录读和写的位置？
read_pos=fseek(...)
write_pos=fseek(...)
[解决办法]
楼主说的情况，应该和文件系统的实现有关系
不知道楼主的应用是不是有文件系统。。。
不过楼主大可以单步跟进去，看看到底是怎么回事？
那样更加简明，也更加直接
[解决办法]
fopen, _wfopen
Open a file.
FILE *fopen( const char *filename, const char *mode );
FILE *_wfopen( const wchar_t *filename, const wchar_t *mode );
Function Required Header Compatibility
fopen <stdio.h> ANSI, Win 95, Win NT
_wfopen <stdio.h> or <wchar.h> Win NT
For additional compatibility information, see Compatibility in the Introduction.
Libraries
LIBC.LIB Single thread static library, retail version
LIBCMT.LIB Multithread static library, retail version
MSVCRT.LIB Import library for MSVCRT.DLL, retail version
The c, n, and t mode options are Microsoft extensions for fopen and _fdopen and should not be used where ANSI portability is desired.
Return Value
Each of these functions returns a pointer to the open file. A null pointer value indicates an error.
Parameters
filename
Filename
mode
Type of access permitted
Remarks
The fopen function opens the file specified by filename. _wfopen is a wide-character version of fopen; the arguments to _wfopen are wide-character strings. _wfopen and fopen behave identically otherwise.
Generic-Text Routine Mappings
TCHAR.H Routine _UNICODE & _MBCS Not Defined _MBCS Defined _UNICODE Defined
_tfopen fopen fopen _wfopen
The character string mode specifies the type of access requested for the file, as follows:
"r"
Opens for reading. If the file does not exist or cannot be found, the fopen call fails.
"w"
Opens an empty file for writing. If the given file exists, its contents are destroyed.
"a"
Opens for writing at the end of the file (appending) without removing the EOF marker before writing new data to the file; creates the file first if it doesn’t exist.
"r+"
Opens for both reading and writing. (The file must exist.)
"w+"
Opens an empty file for both reading and writing. If the given file exists, its contents are destroyed.
"a+"
Opens for reading and appending; the appending operation includes the removal of the EOF marker before new data is written to the file and the EOF marker is restored after writing is complete; creates the file first if it doesn’t exist.
When a file is opened with the "a" or "a+" access type, all write operations occur at the end of the file. The file pointer can be repositioned using fseek or rewind, but is always moved back to the end of the file before any write operation is carried out. Thus, existing data cannot be overwritten.
The "a" mode does not remove the EOF marker before appending to the file. After appending has occurred, the MS-DOS TYPE command only shows data up to the original EOF marker and not any data appended to the file. The "a+" mode does remove the EOF marker before appending to the file. After appending, the MS-DOS TYPE command shows all data in the file. The "a+" mode is required for appending to a stream file that is terminated with the CTRL+Z EOF marker.
When the "r+", "w+", or "a+" access type is specified, both reading and writing are allowed (the file is said to be open for “update”). However, when you switch between reading and writing, there must be an intervening fflush, fsetpos, fseek, or rewind operation. The current position can be specified for the fsetpos or fseek operation, if desired.
In addition to the above values, the following characters can be included in mode to specify the translation mode for newline characters:
t
Open in text (translated) mode. In this mode, CTRL+Z is interpreted as an end-of-file character on input. In files opened for reading/writing with "a+", fopen checks for a CTRL+Z at the end of the file and removes it, if possible. This is done because using fseek and ftell to move within a file that ends with a CTRL+Z, may cause fseek to behave improperly near the end of the file.
Also, in text mode, carriage return–linefeed combinations are translated into single linefeeds on input, and linefeed characters are translated to carriage return–linefeed combinations on output. When a Unicode stream-I/O function operates in text mode (the default), the source or destination stream is assumed to be a sequence of multibyte characters. Therefore, the Unicode stream-input functions convert multibyte characters to wide characters (as if by a call to the mbtowc function). For the same reason, the Unicode stream-output functions convert wide characters to multibyte characters (as if by a call to the wctomb function).
b
Open in binary (untranslated) mode; translations involving carriage-return and linefeed characters are suppressed.
If t or b is not given in mode, the default translation mode is defined by the global variable _fmode. If t or b is prefixed to the argument, the function fails and returns NULL.
For more information about using text and binary modes in Unicode and multibyte stream-I/O, see Text and Binary Mode File I/O and Unicode Stream I/O in Text and Binary Modes.
c
Enable the commit flag for the associated filename so that the contents of the file buffer are written directly to disk if either fflush or _flushall is called.
n
Reset the commit flag for the associated filename to “no-commit.” This is the default. It also overrides the global commit flag if you link your program with COMMODE.OBJ. The global commit flag default is “no-commit” unless you explicitly link your program with COMMODE.OBJ.
Valid characters for the mode string used in fopen and _fdopen correspond to oflag arguments used in _open and _sopen, as follows.
Characters in mode String Equivalent oflag Value for _open/_sopen
a _O_WRONLY | _O_APPEND (usually _O_WRONLY | _O_CREAT | _O_APPEND)
a+ _O_RDWR | _O_APPEND (usually _O_RDWR | _O_APPEND | _O_CREAT )
r _O_RDONLY
r+ _O_RDWR
w _O_WRONLY (usually _O_WRONLY | _O_CREAT | _O_TRUNC)
w+ _O_RDWR (usually _O_RDWR | _O_CREAT | _O_TRUNC)
b _O_BINARY
t _O_TEXT
c None
n None
Example
/* FOPEN.C: This program opens files named "data"
* and "data2".It uses fclose to close "data" and
* _fcloseall to close all remaining files.
*/
#include <stdio.h>
FILE *stream, *stream2;
void main( void )
{
int numclosed;
/* Open for read (will fail if file "data" does not exist) */
if( (stream = fopen( "data", "r" )) == NULL )
printf( "The file 'data' was not opened\n" );
else
printf( "The file 'data' was opened\n" );
/* Open for write */
if( (stream2 = fopen( "data2", "w+" )) == NULL )
printf( "The file 'data2' was not opened\n" );
else
printf( "The file 'data2' was opened\n" );
/* Close stream */
if( fclose( stream ) )
printf( "The file 'data' was not closed\n" );
/* All other files are closed: */
numclosed = _fcloseall( );
printf( "Number of files closed by _fcloseall: %u\n", numclosed );
}
Output
The file 'data' was opened
The file 'data2' was opened
Number of files closed by _fcloseall: 1
Stream I/O Routines
See Also fclose, _fdopen, ferror, _fileno, freopen, _open, _setmode
[解决办法]
查MSDN是Windows程序员必须掌握的技能之一。
英语也是一门计算机语言的说。
[解决办法]
Update mode permits reading and writing the same file;
fflush or a file-positioning function must be called between a read and a write or vice versa.
[解决办法]
r+ 以可读写方式打开文件，该文件必须存在。
读写文件以后，文件内的流指针就会停在当前读写的位置。
最好使用fseek定位文件的指针。
fseek重定位流(数据流/文件)上的文件内部位置指针
注意：不是定位文件指针，文件指针指向文件/流。位置指针指向文件内部的字节位置，随着文件的读取会移动，文件指针如果不重新赋值将不会改变指向别的文件。
[解决办法]
可以实现对文件的读写。
[解决办法]
