[C++知识库]how to use open function in c

open: Used to Open the file for reading, writing or both.

Syntax in C language 
	#include<sys/types.h>
	#include<sys/stat.h>
	#include <fcntl.h>  
	int open (const char* Path, int flags [, int mode ]); 
	
Parameters
	Path: 
		path to file which you want to use use absolute path begin with “/”, when you are not work in same directory of file.
		Use relative path which is only file name with extension, when you are work in same directory of file.
	
	flag:
		O_RDONLY: read only,
		O_WRONLY: write only,
		O_RDWR: read and write,
		O_CREAT: create file if it doesn’t exist, 	
		O_EXCL: prevent creation if it already exists
		
How it works in OS
	Find the existing file on disk
	Create file table entry
	Set first unused file descriptor to point to file table entry
	Return file descriptor used, -1 upon failure

// C program to illustrate 
// open system call 
#include<stdio.h> 
#include<fcntl.h> 
#include<errno.h> 
extern int errno; 
void main() 
{     
    // if file does not have in directory 
    // then file foo.txt is created. 
    int fd = open("foo.txt", O_RDONLY | O_CREAT); 
      
    printf("fd = %d/n", fd); 
} 

Output:
	fd = 3