System file interface open

System file interface open Introduce

man Manual introduction

Parameters 1： File pathname , Generally, the current path is opened , Enter the file name directly

a key ：
Parameters 2：flags – Indicates how you want to open , There are several commonly used

O_TRUNC ---  Empty files   open truncated
O_WRONLY  ---  Just write open  open write only
O_RDONLY  ---  Read only open   open read only
O_APPEND ---  Append open    open append
O_CREAT  ---  If the file does not exist , Create a new 

notes ： The options and c The options given by language are different ,c Linguistic w The option is to clear and then write ,a The option is to append , It's not here .

The additional write here should include O_WRONLY and O_APPEND
The clear write here should contain O_TRUNC and O_WRONLY

---

Use... Between options | separate .
Like this ：

reason ： We can have a look at linux How the kernel defines open flags Of

You can find , The kernel is implemented with bit segments flags Of .
in other words , every last bit Each bit represents a permission .os See what permissions are opened , Just look flag The number of this variable bit Is it 1 that will do

Parameters 3：mode. If you choose O_CREAT Option and actually created a file , that mode This is the initial permission of this file .
such as ：0644 namely rw-r–r--