user-0 提问: 一个以2进制方式读取bmp图片时的诡异现象

回复:
user: user-0
content: 好像明白了什么，我先仔细看看

user: user-1
content: 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.

user: user-1
content: 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

user: user-1
content: 每次在读写切换前，必须调用一次fseek

user: user-0
content: 引用 1 楼 zhao4zhong1 的回复:读和写位置在底层是用两个不同的变量。 我现在有点糊涂 1.fseek对读位置是否有效？从2楼的代码我判断出是时而有效时而无效，但对写位置是有效的，但网上说的都是fseek是将读写位置定位  2.ftell到底是告诉我们读位置还是写位置，如果这两个位置不一样的话。从我调试的过程中，我觉得是写位置，那如果要知道读位置应用那个函数？

user: user-0
content: 引用 1 楼 zhao4zhong1 的回复:读和写位置在底层是用两个不同的变量。 上楼的程序是怎么回事？ 还有就是网上查发现都没怎么提到读写位置，你说的在底层是不同变量是怎么回事，具体是什么？

user: user-0
content: 赵老师，我有调试了一番，出问题的代码改成这样： 		if (i == 3) 		{ 			//fseek(bmp, -1, SEEK_CUR); 			//tscb=fgetc(bmp); 			//int pos = ftell(bmp);// 			//tscb = fgetc(bmp); 			//int a = 1; 			for (int j = 0; j < 11; j++) 			{ 				tscb = fgetc(bmp); 				int a = 1; 			} 			//tscb = fgetc(bmp); 		} 结果是，tscb也就是读指针读的值是为偏移量4到14的值，这就说明fputc对读位置是会影响的，但最开始定位的语句没起作用  所以是fseek对读位置没影响，但如果没影响，这里两句话屏蔽不屏蔽  ，下面的tscb都应该是一样的结果，这是怎么回事？

user: user-1
content: 读和写位置在底层是用两个不同的变量。