How to open a file which overwrite existing content

Question

I try to open a file like this in linux. It will over-write an existing one if exits. That is what I want.

fout = open(out_file_name, O_WRONLY | O_CREAT, 644); 

However, if the existing is 1024 bytes, when I open in above way and write 800 new bytes. I still see the 224 bytes at the end of previous content.

How can I make it just have the 800 bytes that I have been written?

Answer 1

You want to use the O_TRUNC flag to open(), by OR-ing it with the existing flags you have above:

int fout = open(out_file_name, O_WRONLY | O_CREAT | O_TRUNC, 644); 

This will truncate the file. Below is the information in the man page for open(2).

   O_TRUNC           If the file already exists and is a regular file  and  the  open           mode  allows  writing  (i.e.,  is O_RDWR or O_WRONLY) it will be           truncated to length 0.  If the file is a FIFO or terminal device           file,  the  O_TRUNC  flag  is  ignored.  Otherwise the effect of           O_TRUNC is unspecified.