文件读取方式的疑惑
在ANSI C标准中只采用缓冲文件系统，也就是既用缓冲文件系统处理文本文件也用它处理二进制文件。而它的缓冲区的大小由各个具体的C版本确定，一般为512字节。我的理解是，当文件大小大于缓冲区大小时，就读入缓冲区大小的内容，当需要继续往下操作时，将再次访问硬盘，读取下一个缓冲区大小的数据。不知是否正确。我想问的是，若像我所想的这样，那是否有什么方法可以重新设置缓冲区的大小呢？
[解决办法]
setvbuf
Controls stream buffering and buffer size.
int setvbuf( FILE *stream, char *buffer, int mode, size_t size );
Routine Required Header Compatibility
setvbuf <stdio.h> ANSI, Win 95, 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
Return Value
setvbuf returns 0 if successful, or a nonzero value if an illegal type or buffer size is specified.
Parameters
stream
Pointer to FILE structure
buffer
User-allocated buffer
mode
Mode of buffering
size
Buffer size in bytes. Allowable range: 2 < size < 32768. Internally, the value supplied for size is rounded down to the nearest multiple of 2.
Remarks
The setvbuf function allows the program to control both buffering and buffer size for stream. stream must refer to an open file that has not undergone an I/O operation since it was opened. The array pointed to by buffer is used as the buffer, unless it is NULL, in which case setvbuf uses an automatically allocated buffer of length size/2 * 2 bytes.
The mode must be _IOFBF, _IOLBF, or _IONBF. If mode is _IOFBF or _IOLBF, then size is used as the size of the buffer. If mode is _IONBF, the stream is unbuffered and size and buffer are ignored. Values for mode and their meanings are:
_IOFBF
Full buffering; that is, buffer is used as the buffer and size is used as the size of the buffer. If buffer is NULL, an automatically allocated buffer size bytes long is used.
_IOLBF
With MS-DOS, the same as _IOFBF.
_IONBF
No buffer is used, regardless of buffer or size.
Example
/* SETVBUF.C: This program opens two streams: stream1
* and stream2. It then uses setvbuf to give stream1 a
* user-defined buffer of 1024 bytes and stream2 no buffer.
*/
#include <stdio.h>
void main( void )
{
char buf[1024];
FILE *stream1, *stream2;
if( ((stream1 = fopen( "data1", "a" )) != NULL) &&
((stream2 = fopen( "data2", "w" )) != NULL) )
{
if( setvbuf( stream1, buf, _IOFBF, sizeof( buf ) ) != 0 )
printf( "Incorrect type or size of buffer for stream1\n" );
else
printf( "'stream1' now has a buffer of 1024 bytes\n" );
if( setvbuf( stream2, NULL, _IONBF, 0 ) != 0 )
printf( "Incorrect type or size of buffer for stream2\n" );
else
printf( "'stream2' now has no buffer\n" );
_fcloseall();
}
}
Output
'stream1' now has a buffer of 1024 bytes
'stream2' now has no buffer
Stream I/O Routines
See Also fclose, fflush, fopen, setbuf
