原理
#include <stdio.h>
#include <string.h>
int main()
{
const char *msg0="hello printf\n";
const char *msg1="hello fwrite\n";
const char *msg2="hello write\n";
printf("%s", msg0);
fwrite(msg1, strlen(msg0), 1, stdout);
write(1, msg2, strlen(msg2));return 0;
加上fork
#include <stdio.h>
#include <string.h>
int main()
{
const char *msg0="hello printf\n";
const char *msg1="hello fwrite\n";
const char *msg2="hello write\n";
printf("%s", msg0);
fwrite(msg1, strlen(msg0), 1, stdout);
write(1, msg2, strlen(msg2));
fork();
return 0;
为什么呢？
这个缓冲区在哪里呢？
在FILE这个结构体中除了有文件描述符，还定义有缓冲区
解释上面问题：
当我们重定向到文件里面，刷新策略就变成了全缓冲，到了fork之后，创建子进程，本来数据是父子进程共享的，但是程序退出，要把缓冲区的内容刷新出去，发生写时拷贝，这样就生成了两份，write是系统接口，不会进入缓冲区，也就不会有两份
模拟实现
简单模拟实现一个缓冲区的代码，供我们理解
mystdio.h
#ifndef __MYSTDIO_H__
#define __MYSTDIO_H__#include <string.h>#define SIZE 1024#define FLUSH_NOW 1
#define FLUSH_LINE 2
#define FLUSH_ALL 4typedef struct IO_FILE{int fileno;//文件描述符int flag; //用来判断是写还是读//char inbuffer[SIZE];//int in_pos;char outbuffer[SIZE]; // 用一下这个,输出缓冲区int out_pos;//输出缓冲区的大小
}_FILE;_FILE * _fopen(const char*filename, const char *flag);
int _fwrite(_FILE *fp, const char *s, int len);
void _fclose(_FILE *fp);#endif
mystdio.c
#include "Mystdio.h"
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <assert.h>#define FILE_MODE 0666// "w", "a", "r"
_FILE * _fopen(const char*filename, const char *flag)
{assert(filename);assert(flag);int f = 0;int fd = -1;if(strcmp(flag, "w") == 0) {f = (O_CREAT|O_WRONLY|O_TRUNC);fd = open(filename, f, FILE_MODE);}else if(strcmp(flag, "a") == 0) {f = (O_CREAT|O_WRONLY|O_APPEND);fd = open(filename, f, FILE_MODE);}else if(strcmp(flag, "r") == 0) {f = O_RDONLY;fd = open(filename, f);}else return NULL;if(fd == -1) return NULL;_FILE *fp = (_FILE*)malloc(sizeof(_FILE));if(fp == NULL) return NULL;fp->fileno = fd;//fp->flag = FLUSH_LINE;fp->flag = FLUSH_ALL;fp->out_pos = 0;return fp;
}// FILE中的缓冲区的意义是什么？？？？
int _fwrite(_FILE *fp, const char *s, int len)
{// "abcd\n"memcpy(&fp->outbuffer[fp->out_pos], s, len); // 没有做异常处理, 也不考虑局部问题fp->out_pos += len;if(fp->flag&FLUSH_NOW)//无缓冲{write(fp->fileno, fp->outbuffer, fp->out_pos);fp->out_pos = 0;}else if(fp->flag&FLUSH_LINE)//行缓冲{if(fp->outbuffer[fp->out_pos-1] == '\n'){ // 不考虑其他情况write(fp->fileno, fp->outbuffer, fp->out_pos);fp->out_pos = 0;}}else if(fp->flag & FLUSH_ALL)//全缓冲{if(fp->out_pos == SIZE){write(fp->fileno, fp->outbuffer, fp->out_pos);fp->out_pos = 0;}}return len;
}void _fflush(_FILE *fp)
{if(fp->out_pos > 0){write(fp->fileno, fp->outbuffer, fp->out_pos);fp->out_pos = 0;}
}void _fclose(_FILE *fp)
{if(fp == NULL) return;_fflush(fp);//进程退出也要刷新close(fp->fileno);free(fp);
}main.c
#include "Mystdio.h"
#include <unistd.h>#define myfile "test.txt"int main()
{_FILE *fp = _fopen(myfile, "a");if(fp == NULL) return 1;const char *msg = "hello world\n";int cnt = 10;while(cnt){_fwrite(fp, msg, strlen(msg));// fflush(fp);sleep(1);cnt--;}_fclose(fp);return 0;
}