CODEWITHSUNDEEP
clinux
tristan
tristan

Reputation: 4322

How to check if an opened file has been moved or removed by another process

I have a process using C on Linux OS that writes data to a file. It uses open()/write() functions and I've been wondering if another process rm'd or mv'd the file. How can my process find out and recreate the file?

Upvotes: 3

Views: 970

Answers (2)

Jonathan Leffler
Jonathan Leffler

Reputation: 754890

You can use fstat() to get the information about the open file. If the st_nlink field is zero, the file has been removed from the file system (possibly by being moved to a different file system, but there's no real way for you to determine that). There's a decent chance you have the only remaining reference to that file - though there might be other processes also holding it open. The disk space won't be released until the last process with an open file descriptor for the file finally closes the file.

If the st_nlink field is still positive, then your file still has a name somewhere out in the file system. You then need to use stat() to determine whether the st_dev and st_ino fields for the given file name match the same fields from the file descriptor. If the name still exists and has the same device and inode number, then it is 'the same' file (though the contents may have changed). If there's a difference, then the open file is different from the file specified by name.

Note that if you want to be sure that the given name is not a symbolic link to a moved copy of the file, then you would have to use lstat() on the file when you open it (to ensure it isn't a symlink at that point), and again when you check the file (instead of using stat()).

Upvotes: 6

Lelouch Lamperouge
Lelouch Lamperouge

Reputation: 8421

You can use the stat call to do this.

struct stat st;
if(stat("/tmp",&st) == 0)
        printf(" /tmp is present\n");
else
    /* Write code to create the file */

Upvotes: 0

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