1. 不可以给一个指针赋一个空指针，例如：
int* p1;
int* p2;
p2 = p1;
2. fflush函数
NAME
fflush - flush a stream
SYNOPSIS
#include <stdio.h>
int fflush(FILE *stream);
DESCRIPTION
The function fflush() forces a write of all user-space buffered data
for the given output or update stream via the stream’s underlying write
function. The open status of the stream is unaffected.
If the stream argument is NULL, fflush() flushes all open output
streams.
For a non-locking counterpart, see unlocked_stdio(3).
RETURN VALUE
Upon successful completion 0 is returned. Otherwise, EOF is returned
and the global variable errno is set to indicate the error.
ERRORS
EBADF Stream is not an open stream, or is not open for writing.
The function fflush() may also fail and set errno for any of the errors
specified for the routine write(2).
NOTES
Note that fflush() only flushes the user space buffers provided by the
C library. To ensure that the data is physically stored on disk the
kernel buffers must be flushed too, e.g. with sync(2) or fsync(2).
int i；
for (i=0;i<10;i++)
{
printf("%d",i);
// fflush(stdout);
sleep(1);
}
有flush的时候 是一秒输出一个数字 没flush的时候是10秒后输出10个字符
结果当然一样 都是：0123456789
过程不一样啊