记一次文件读写遇到的bug

代码一：

#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<iostream>
#include<unistd.h>

using namespace std;

int main(){
	char* buf = new char[8];	//can't alloc 1024?
	int fd = open("./fileIO.txt", O_CREAT|O_RDWR);
	
	if(fd<0){
		cout<<"open( file failed!)"<<endl;
	}

	return 0;
}
           

结果，创建出来的文件出现了很危险的权限，S和T、

代码2：

#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<iostream>
#include<unistd.h>

using namespace std;

int main(){
	char* buf = new char[8];	//can't alloc 1024?
	int fd = open("./fileIO.txt", O_CREAT|O_RDWR);
	
	if(fd<0){
		cout<<"open( file failed!)"<<endl;
	}

	int m = write(fd,"test\n",5);
	cout<<m<<endl;

	int n = read(fd,buf,m);
	cout<<n<<endl;
	printf("%s\n",buf);
	close(fd);
	return 0;
}
           

结果：m有值，n为0，buf是确定是空的了。

后来，我觉得会不会是m是size_t的缘故，也只能死马当活马医了。

#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<iostream>
#include<unistd.h>

using namespace std;

int main(){
	char* buf = new char[8];	//can't alloc 1024?
	int fd = open("./fileIO.txt", O_CREAT|O_RDWR);
	
	if(fd<0){
		cout<<"open( file failed!)"<<endl;
	}

	int m = write(fd,"test\n",5);
	cout<<m<<endl;
	
	int n = read(fd,buf,5);
	cout<<n<<endl;
	printf("%s\n",buf);
	close(fd);
	return 0;
}
           

嘿嘿，m/n都有值了，但是依旧buf没东西、

sizeof(buf) 也有长度，但是就没东西。

终于意识到，前面打开文件写入的时候，是重头覆盖，文件内容已经被覆盖，但是新的内容还没被保存，文件操作句柄在write，所以就导致了去读读不出内容来。

#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<iostream>
#include<unistd.h>

using namespace std;

int main(){
	char* buf = new char[8];	//can't alloc 1024?
	int fd = open("./fileIO.txt", O_CREAT|O_RDWR);
	
	if(fd<0){
		cout<<"open( file failed!)"<<endl;
	}

	int m = write(fd,"test\n",5);
	cout<<m<<endl;
	close(fd);

	fd = open("./fileIO.txt", O_CREAT|O_RDWR);
	int n = read(fd,buf,5);
	cout<<n<<endl;
	printf("%s\n",buf);
	close(fd);
	return 0;
}
           

这样呢？

#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<iostream>
#include<unistd.h>

using namespace std;

int main(){
	char* buf = new char[8];	//can't alloc 1024?
	fd = open("./fileIO.txt", O_CREAT|O_RDWR|O_APPEND);
	
	if(fd<0){
		cout<<"open( file failed!)"<<endl;
	}

	int m = write(fd,"test\n",5);

	int n = read(fd,buf,5);
	cout<<n<<endl;
	printf("%s\n",buf);
	close(fd);
	return 0;
}