fread和fwrite同时对一个文件读写

这几天看到了fopen的参数设置。中文的那些真的是不能帮助精确理解。在网上发现了英文的，特附上：

FILE *fopen(const char *filename, const char *mode)
fopen opens the named file, and returns a stream, or NULL if the attempt fails. Legal values for mode include:
"r"
open text file for reading
"w"
create text file for writing; discard previous contents if any
"a"
append; open or create text file for writing at end of file
"r+"
open text file for update (i.e., reading and writing)
"w+"
create text file for update, discard previous contents if any
"a+"
append; open or create text file for update, writing at end
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. If the mode includes b after the initial letter, as in "rb" or "w+b", that indicates a binary file. Filenames are limited to FILENAME_MAX characters. At most FOPEN_MAX files may be open at once.

其中+号表示可读可写，即上面的“更新模式”。

对“更新模式”：可以同时读写，但是必须有清空缓冲区函数或者文件定位函数。意思是在读或写之间得调用文件定位的相关函数，比如fseek，rewind等等。

经我测试确实是这样。

int main()
{
FILE *fp = fopen("1.txt", "r+");
char Rbuf[5], Wbuf[5] = "4321";

fread(Rbuf, sizeof(char), 4, fp);
Rbuf[4] = '\0';

fseek(fp,ftell(fp),SEEK_SET);
fwrite(Wbuf, sizeof(char), 4, fp);
printf("%s\n", Rbuf);

fclose(fp);

return 0;
}

只有这样，你才能在读之后接着写进去文件，或者rewind定位后重头部写进去。其他，都不行根本写不进去。具体原因我也不明白，用ftell测试后也发现对的，但是还是写不进去。只有这样可以，可能是什么内部实现吧。

毕竟文件读写函数是封装之后的，还是按它的标准来。