刚刚遇到的问题，一直以为使用C标准库的fopen函数时，mode参数中’b’的作用可有可无，今天遇到了一点麻烦，所以才感觉到了有什么区别。
Linux下的换行符是‘\n’,windows 下的是’\r\n’。
这个问题在Linux不会出现，在windows 下才会。
现有一个windows下的文本文件。
如果打开文件的时候mode参数没有‘b’，则认为是以字符文件的形式打开，在使用fread读取文件内容的时候，遇到了’\r\n’这样的两个字节换行符，fread会把它变成一个字符 ‘\n’， 忽略了’\r’，这样，fread的返回值–实际读取字节数要比文件定位符的跳转的数量小。比如：读取了50个字节的内容，有5行（4个换行符），那么fread返回值就是50-4=46。
如果用带有‘b’的二进制模式打开，这个问题就不会有了，因为它会把’\r’单独作为一个字符对待。
#include <stdio.h>
int main( void )
{
FILE *stream;
char list[4000];
int i, numread, numwritten;
stream=fopen("C:\\t1.cpp","rb");
if (!stream)
{
printf("Open file error\n");
return 0;
}
numread = fread( list, sizeof( char ), 2000, stream );
printf( "Number of items read = %d\n", numread );
numread = fread( list, sizeof( char ), 20, stream );
printf( "Number of items read = %d\n", numread );
fclose( stream );
return 1;
}参考
FILE *fopen(
const char *filename,
const char *mode
);
FILE *_wfopen(
const wchar_t *filename,
const wchar_t *mode
);
Parameters
filename
Filename.
mode
Type of access permitted.
Return Value
Each of these functions returns a pointer to the open file. A null pointer value indicates an error. If filename or mode is NULL or an empty string, these functions trigger the invalid parameter handler, as described in Parameter Validation. If execution is allowed to continue, these functions return NULL and set errno to EINVAL.
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 (see Link Options).
N
Specifies that the file is not inherited by child processes.
S
Specifies that caching is optimized for, but not restricted to, sequential access from disk.
R
Specifies that caching is optimized for, but not restricted to, random access from disk.
T
Specifies a file as temporary. If possible, it is not flushed to disk.
D
Specifies a file as temporary. It is deleted when the last file pointer is closed.
ccs=ENCODING
Specifies the coded character set to use (UTF-8, UTF-16LE, or UNICODE) for this file. Leave unspecified if you want ANSI encoding. This option is available in Visual C++ 2005 and later.
Valid characters for the mode string used in fopen and _fdopen correspond to oflag arguments used in _open and _sopen, as follows.