小总结

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