深究标准IO的缓存

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(void)
{
  char buf[5];
  FILE *myfile = stdin;
  fgets(buf, 5, myfile);
  fputs(buf, myfile);
  
  return 0;
}

struct _IO_FILE {
int _flags;
#define _IO_file_flags _flags



char* _IO_read_ptr;
char* _IO_read_end;
char* _IO_read_base;
char* _IO_write_base;
char* _IO_write_ptr;
char* _IO_write_end;
char* _IO_buf_base;
char* _IO_buf_end;

char *_IO_save_base;
char *_IO_backup_base;
char *_IO_save_end;

struct _IO_marker *_markers;

struct _IO_FILE *_chain;

int _fileno;
};

1.
char* _IO_read_ptr;
char* _IO_read_end;
char* _IO_read_base;
2.
char* _IO_write_base;
char* _IO_write_ptr;
char* _IO_write_end;
3.
char* _IO_buf_base;
char* _IO_buf_end;

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(void)
{
  char buf[5];
  FILE *myfile =stdin;
  printf("before reading/n");
  printf("read buffer base %p/n", myfile->_IO_read_base);
  printf("read buffer length %d/n", myfile->_IO_read_end - myfile->_IO_read_base);
  printf("write buffer base %p/n", myfile->_IO_write_base);
  printf("write buffer length %d/n", myfile->_IO_write_end - myfile->_IO_write_base);
  printf("buf buffer base %p/n", myfile->_IO_buf_base);
  printf("buf buffer length %d/n", myfile->_IO_buf_end - myfile->_IO_buf_base);
  printf("/n");
  fgets(buf, 5, myfile);
  fputs(buf, myfile);
  printf("/n");
  printf("after reading/n");
  printf("read buffer base %p/n", myfile->_IO_read_base);
  printf("read buffer length %d/n", myfile->_IO_read_end - myfile->_IO_read_base);
  printf("write buffer base %p/n", myfile->_IO_write_base);
  printf("write buffer length %d/n", myfile->_IO_write_end - myfile->_IO_write_base);
  printf("buf buffer base %p/n", myfile->_IO_buf_base);
  printf("buf buffer length %d/n", myfile->_IO_buf_end - myfile->_IO_buf_base);

  return 0;
}

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(void)
{
  char buf[5];
  char *cur;
  FILE *myfile;
  myfile = fopen("bbb.txt", "r");
  printf("before reading, myfile->_IO_read_ptr: %d/n", myfile->_IO_read_ptr - myfile->_IO_read_base);
  fgets(buf, 5, myfile); //仅仅读4个字符
  cur = myfile->_IO_read_base;
  while (cur <</span> myfile->_IO_read_end) //实际上读满了这个缓冲区
  {
    printf("%c",*cur);
    cur++;
  }
  printf("/nafter reading, myfile->_IO_read_ptr: %d/n", myfile->_IO_read_ptr - myfile->_IO_read_base);
  return 0;
}

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(void)
{
  char buf[2048]={0};
  int i;
  FILE *myfile;
  myfile = fopen("aaa.txt", "r+");
  i= 0;
  while (i<</span>2048)
  {
    fwrite(buf+i, 1, 512, myfile);
    i +=512;
    //注释掉这句则可以写入aaa.txt
    myfile->_IO_write_ptr = myfile->_IO_write_base;
    printf("%p write buffer base/n", myfile->_IO_write_base);
    printf("%p buf buffer base /n", myfile->_IO_buf_base);
    printf("%p read buffer base /n", myfile->_IO_read_base);
    printf("%p write buffer ptr /n", myfile->_IO_write_ptr);
    printf("/n");
  }
  return 0;
}

#include <stdlib.h>
#include <stdio.h>

int main(void)
{
  char buf[5];
  char buf2[10];
  
  fgets(buf, 5, stdin); //第一次输入时,超过5个字符

  puts(stdin->_IO_read_ptr);//本句说明整行会被一次全部读入缓冲区,

                                         //而非仅仅上面需要的个字符
  stdin->_IO_read_ptr = stdin->_IO_read_end; //标准I/O会认为缓冲区已空,再次调用read
                                             //注释掉,再看看效果
  printf("/n");
  puts(buf);
  
  fgets(buf2, 10, stdin);
  puts(buf2);
  
  return 0;
}

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

char buf[5]={'1','2', '3', '4', '5'}; //最后一个不要是/n,是/n的话,标准I/O会自动flush的
                                                    //这是行缓冲跟全缓冲的重要区别

void writeLog(FILE *ftmp)
{
  fprintf(ftmp, "%p write buffer base/n", stdout->_IO_write_base);
  fprintf(ftmp, "%p buf buffer base /n", stdout->_IO_buf_base);
  fprintf(ftmp, "%p read buffer base /n", stdout->_IO_read_base);
  fprintf(ftmp, "%p write buffer ptr /n", stdout->_IO_write_ptr);
  fprintf(ftmp, "/n");
}

int main(void)
{
  int i;
  FILE *ftmp;
  ftmp = fopen("ccc.txt", "w");
  i= 0;
  while (i<</span>4)
  {
    fwrite(buf, 1, 5, stdout);
    i++;
    *stdout->_IO_write_ptr++ = '/n';//可以单独把这句打开,看看效果
    //getchar();//getchar()会标准I/O将缓冲区输出
    //打开下面的注释,你就会发现屏幕上什么输出也没有
    //stdout->_IO_write_ptr = stdout->_IO_write_base;
    writeLog(ftmp); //这个只是为了查看缓冲区指针的变化  
  }
  return 0;
}

#include <</span>stdlib.h>
#include <</span>stdio.h>
#include <</span>sys/types.h>
#include <</span>sys/stat.h>
#include <</span>fcntl.h>

int main(void)
{
  fputs("stderr", stderr);
  printf("%d/n", stderr->_IO_buf_end - stderr->_IO_buf_base);

  return 0;
}

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(void)
{
  char buf[5];
  char buf2[10];

  fgets(buf, sizeof(buf), stdin);//输入要于4个,少于13个字符才能看出效果
  puts(buf);

  //交替注释下面两行
  //stdin->_IO_read_end = stdin->_IO_read_ptr+1;

  stdin->_IO_read_end = stdin->_IO_read_ptr + sizeof(buf2)-1;
   
  fgets(buf2, sizeof(buf2), stdin);
  puts(buf2);
  if (feof(stdin))
    printf("input end/n");
  return 0;
}