标准IO缓冲机制

参考资料：

　　https://q16964777.iteye.com/blog/2228244

　　知道缓冲有几种模式：无缓冲、行缓冲、全缓冲。通过判断FILTE中的 _flags 的判断可以知道究竟是那种缓冲模式。

#include <stdio.h>
int stream_attribute(FILE *fp)
{
if(fp->_flags & _IO_UNBUFFERED)
{
printf("The IO type is unbuffered\n");
}else if(fp->_flags & _IO_LINE_BUF){
printf("The IO type is line buf\n");
}else{
printf("The IO type is full buf\n");
}
printf("The IO size : %d\n",fp->_IO_buf_end - fp->_IO_buf_base);
return ;
}
int main()
{
FILE *fp;
stream_attribute(stdin);
printf("___________________________________\n\n");
stream_attribute(stdout);
printf("___________________________________\n\n");
stream_attribute(stderr);
printf("___________________________________\n\n");
if((fp = fopen("test.txt","w+")) == NULL)
{
perror("fail to fopen");
}
stream_attribute(fp);
return ;
}

我们修改一下代码再看
#include <stdio.h>
int stream_attribute(FILE *fp)
{
if(fp->_flags & _IO_UNBUFFERED)
{
printf("The IO type is unbuffered\n");
}else if(fp->_flags & _IO_LINE_BUF){
printf("The IO type is line buf\n");
}else{
printf("The IO type is full buf\n");
}
printf("The IO size : %d\n",fp->_IO_buf_end - fp->_IO_buf_base);
return ;
}
int main()
{
FILE *fp;
getchar();
stream_attribute(stdin);
printf("___________________________________\n\n");
stream_attribute(stdout);
printf("___________________________________\n\n");
stream_attribute(stderr);
printf("___________________________________\n\n");
if((fp = fopen("test.txt","w+")) == NULL)
{
perror("fail to fopen");
}
printf("before write:\n");
stream_attribute(fp);
fputc('a',fp);
printf("after write:\n");
stream_attribute(fp);
return ;
}

　　另外要清楚，缓冲区是在执行读写操作之后才分配的。