Linux Learning notes one of the 10--file I/O

Most file I/O in Unix systems requires only 5 functions: Open,read,write,lseek and close

The file descriptor file descriptor is a non-negative integer, and all open files are referenced by the file descriptor to the range of changes in the file descriptor is 0-open_max,open_max is the most open file per process

1. Open function

　　#include <fcntl.h>

int open (const char* pathname,

int flag,

mode_t mode

); Returns the file descriptor if successful, or 1 if an error occurs

There are 3 ways to open flag: o_rdonly read-only Open

O_wronly only Write Open

O_RDWR Read, write open

These three are mutually exclusive and cannot be used at the same time, the following constants are optional:

O_append each write operation to the end of the file

O_creat If the specified file does not exist, the file is created

O_EXCL returns 1 if the file to be created already exists, and modifies the value of errno

O_trunc if the file exists and opens in write/read-only mode, the entire contents of the file are emptied

O_noctty if the path name is pointing to the end device, do not use it as a control terminal.

O_nonblock if the pathname points to a fifo/block file/character file, the open and subsequent I/O of the file are set to non-blocking (nonblocking mode)

Most implementations define O_RDONLY as 0,o_wronly defined as 1,o_rdwr defined as 2

The mode parameter is used only when creating a new file to set the initial value of the file access permission.

S_irusr,s_iwuser,s_ixusr,s_irgrp,s_iwgrp,s_ixgrp,s_iroth,s_iwoth,s_ Ixoth. where r: Read, W: Write, X: Execute, USR: the user to which the file belongs, GRP: The group to which the file belongs, OTH: other users.

2. Create function

　　#include <fcntl.h>

int Create (const char* Pathname,

mode_t mode

); Returns a write-only open file descriptor if successful, or 1 if an error occurs

Equivalent to open (Pathname,o_wronly | O_create | o| Trunc,mode);

Create insufficient: It opens the created file as a write-only method, and when you need to write the file first, and then read the file, you can use open instead, such as:

Open (Pathname,o_rdwr | O_create | O_trunc,mode);

3. Read function

　　#include <unistd.h>

ssize_t Read (int fd,//File descriptor

void *buf,//data to be written to the kernel object

size_t Size//Space

); Returns the number of bytes read if successful, or 0 if the end of the file returns 1 if there is an error.

4. Write function

　　#include <unistd.h>

sszie_t Write (int fd,//File descriptor

const void *BUF,//data to be written to the kernel object

size_t size//write Data sizes

); Returns the number of bytes written if successful, or 1 if an error occurs

5. Lseek function

Lseek set its offset for an open file

　　#include <unistd.h>

off_t lseek (int fd,//positional file description symbol

off_t offset,//positioning position

int whence//Position reference point: File start position/file end position/file current position

); Returns a new file offset if successful, or 1 if an error occurs

Parameter whence:seek_set: Offset of the file at the beginning of the file offset bytes

Seek_cur: Current position plus offset

Seek_end: File end position plus offset

6. Close function

　　#include <unistd.h>

int close (int fd); Returns 0 if successful, or 1 if an error occurs

Closing a file also frees all record locks that the process adds to the file, and when a process terminates, the kernel automatically closes all of its open files

Linux Learning notes one of the 10--file I/O