多线程下载中写文件的问题
几个线程同时写文件时，使用mmap好还是使用普通的fwrite好？
mmap写文件 要用 msync同步数据，那文件先是缓存在哪？虚拟内存？

fwrite 是在什么时候同步数据呢？只有在关闭文件时？如果这样，我想边下载边把文件存入硬盘的话 不是要频繁的打开 关闭文件？否则文件一直在缓冲区？


[解决办法]
fflush
Flushes a stream.

int fflush( FILE *stream );

Function Required Header Compatibility
fflush <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

fflush returns 0 if the buffer was successfully flushed. The value 0 is also returned in cases in which the specified stream has no buffer or is open for reading only. A return value of EOF indicates an error.

Note If fflush returns EOF, data may have been lost due to a write failure. When setting up a critical error handler, it is safest to turn buffering off with the setvbuf function or to use low-level I/O routines such as _open, _close, and _write instead of the stream I/O functions.

Parameter

stream

Pointer to FILE structure

Remarks

The fflush function flushes a stream. If the file associated with stream is open for output, fflush writes to that file the contents of the buffer associated with the stream. If the stream is open for input, fflush clears the contents of the buffer. fflush negates the effect of any prior call to ungetc against stream. Also, fflush(NULL) flushes all streams opened for output. The stream remains open after the call. fflush has no effect on an unbuffered stream.

Buffers are normally maintained by the operating system, which determines the optimal time to write the data automatically to disk: when a buffer is full, when a stream is closed, or when a program terminates normally without closing the stream. The commit-to-disk feature of the run-time library lets you ensure that critical data is written directly to disk rather than to the operating-system buffers. Without rewriting an existing program, you can enable this feature by linking the program’s object files with COMMODE.OBJ. In the resulting executable file, calls to _flushall write the contents of all buffers to disk. Only _flushall and fflush are affected by COMMODE.OBJ.

For information about controlling the commit-to-disk feature, see Stream I/O, fopen, and _fdopen.

Example

/* FFLUSH.C */

#include <stdio.h>
#include <conio.h>

void main( void )
{
int integer;
char string[81];

/* Read each word as a string. */
printf( "Enter a sentence of four words with scanf: " );
for( integer = 0; integer < 4; integer++ )
{
scanf( "%s ", string );
printf( "%s\n ", string );
}

/* You must flush the input buffer before using gets. */
fflush( stdin );
printf( "Enter the same sentence with gets: " );
gets( string );
printf( "%s\n ", string );
}


Output

Enter a sentence of four words with scanf: This is a test
This
is
a
test
Enter the same sentence with gets: This is a test
This is a test


Stream I/O Routines

See Also fclose, _flushall, setvbuf

[解决办法]
不要搞“几个线程同时写一个文件”这种傻事。
------解决方案--------------------

可以用流来操作和处理，先把文件分割，然后再合并文件。
[解决办法]
可以预先分配文件空间，而后指定每个线程可以读写的区域，互不相干，这样也就不需要考虑同步问题了。