What does os.O_TRUNC do?

Question

func OpenFile(name string, flag int, perm FileMode) (*File, error)

f, err := os.OpenFile("notes.txt", os.O_RDWR|os.O_CREATE|O_TRUNC, 0755)

Does "O_TRUNC" empty the entire file before writing? What do they mean with "truncates"?

Also, does the function ioutil.WriteFile() empty the entire file before writing?

score 11 · Accepted Answer · edited Jun 06 '20 at 18:54

11

There was some confusion on previous definitions of os.O_TRUNC using the verbiage "if possible" - see here.

Today the docs read:

O_TRUNC  int = syscall.O_TRUNC  // truncate regular writable file when opened.

So

Does "O_TRUNC" empty the entire file before writing ?

Yes. It essentially clobbers the file's contents - if the file path exists already (and is a file or symlink to an existing file).

... WriteFile truncates it before writing.

edited Jun 06 '20 at 18:54

David Jones

answered Nov 03 '19 at 15:07

colm.anseo

2

For ordinary files the file is truncated on opening, which is before writing. And may, in the general case, be arbitrarily long before writing. You can truncate a file by opening it with O_TRUNC and closing it without writing anything. – David Jones Jun 06 '20 at 18:54

1 Answers1