The difference between open and fopen

1. Function declaration 、 Parameters 、 Return value 、 The header file

 Function declaration :
  FILE *fopen(const char *filename, const char *mode)
 Parameters :
  1. filename --  This is a  C  character string , Contains the name of the file to open .
  2. mode --  This is a  C  character string , Including file access mode , The pattern is as follows ：
	  "r" 	 Open a file for reading . The file must exist .
	  "w" 	 Create an empty file for writing . If the file name is the same as an existing file , The contents of the existing file will be deleted , The file is treated as a new empty file .
	  "a" 	 Append to a file . The write operation appends data to the end of the file . If the file doesn't exist , Create file .
	  "r+" 	 Open a file to update , Can be read or written to . The file must exist .
	  "w+" 	 Create an empty file for reading and writing .
	  "a+" 	 Open a file for reading and appending .
 Return value :
   This function returns a  FILE  The pointer . Otherwise return to  NULL, And set the global variable  errno  To identify errors .
 The header file :
  #include <stdio.h>

open

 Function declaration :
  int open( const char * pathname,int flags);// Open an existing file 
  int open( const char * pathname,int flags, mode_t mode);// Open a file that doesn't exist 
 Parameters :
 - pathname: The file path string to open .
 - flag:flags The parameter has a series of constant values to choose from , You can select multiple constants at the same time and connect them with bitwise OR operators , So the macro definitions of these constants are based on O_ start , Express or.
	  (1) Will options , One of the following three constants must be specified , And only one... Is allowed to be specified .
		 O_RDONLY  Read only open 
		 O_WRONLY  Just write open 
		 O_RDWR  Read and write open 
	  (2) optional , At the same time specified 0 One or more , And the required options are bitwise OR 
		 O_CREAT  If this file does not exist, create it . When using this option, you need to provide the third parameter mode, Indicates the access rights of the file .
		 O_EXCL  If you also specify O_CREAT, And the file already exists , Error return .
		 O_TRUNC  If the file already exists , And open in write only or read-write mode , Then its length is truncated （Truncate） by 0 byte .
		O_NONBLOCK  For device files , With O_NONBLOCK Mode open can do non blocking I/O（Nonblock I/O）.
 - mode: Specify the file permissions when creating a new file 
		7( Readable, writable, and executable )、4( Can be read )、2( Can write )、1( Executable )
 Return value :
   If successful, the descriptor of the open file is returned , Failure returns -1.
 The header file :
  #include <fcntl.h>
  #include <sys/types.h>
  #include <sys/stat.h>

2. About the difference between buffers ( The main )

• fopen In user mode, there is a cache , It has been used. FILE This structure holds buffered data . It's going on read and write Reduce the switching between user mode and kernel mode .
• open There is no cache , Each read operation obtains data directly from the file system . It's going on read and write You need to switch between kernel mode and user mode every time .
• If you access files sequentially ,fopen A series of functions is better than calling open Series fast ; If you randomly access files open than fopen fast .

3. Different levels

• open It's a system call , The file handle is returned ( File descriptor ), The handle of a file is the index of the file in the file description sub table .open It's a system function , It's not portable .
• fopen yes ANSIC Standard C Library functions , Returns a pointer to the file structure . Different kernels should be called in different systems api, portable .
• linux The system function in is open,fopen Is its encapsulation function ,fopen Implementation of to call open.

4. Different scenarios

• It's usually used fopen Open a normal file , use open Open device file .