How to determine, whether a file is open?

Question

My code needs to go through files in a directory, picking only those, which are currently opened (for writing) by any other process on the system.

The ideal solution would apply for all Unixes, but I'll settle for a Linux-only.

The program is written in Python, but I can add a custom C-function, if I have to -- I just need to know, what API is available for this...

One suggestion I found was to go through all file-descriptors under Linux /proc, resolving their links to see, if they point at the file of interest. But that seems rather heavy...

I know, for example, that opening a file increases its reference count -- filesystem will not deallocate blocks of an opened file even if it is deleted -- until it is closed -- the feature relied upon by tmpfile(3).

Perhaps, a user process can get access to these records in the kernel?

Answer 1

On Linux, /proc/<pid>/fd/ contains a list of symlinks to files held open by <pid>. This means you can quickly and easily build a list of files open at this moment in time by checking what they link to.

This isn't as "heavy" as you think. e.g. even running the bash while/read loop below only took about 1.5 seconds on my ancient AMD Phenom-II 1090T with ~1000 processes currently running.

In bash, you could build an associative array with something like:

declare -A openfiles

while IFS=$'\n' read l; do
  openfiles[$l]=1
done < <(find /proc/*/fd/ -type l -printf '%l\0' |
           grep -zvE '^(socket|pipe|anon_inode):' | sort -zu)

(This is just a simple example, completely unoptimised. It wouldn't be hard at all to optimise it)

and then check whether a file is open with:

if [ "${openfiles[full-path-to-file]}" == 1 ] ; then .... ; fi

In python, you could use os.walk() and os.readlink() to build a dict. Or use the proc, procfs or psutil module.

psutil is cross-platform and has an open_files() method which seems like it would be useful here.

Note: you could do this with lsof, but lsof is extraordinarily slow. It does a lot more than what you need for this job.

Answer 2

fnctl is what you are looking for. Its man page is quite elaborate. It tells you if an open file is read only or write as well and returns -1 for closed files. It is powerful and can give much more details like (flawed) lock mechanism.

#include <fcntl.h>

int
fcntl(int fd, int cmd, ...);

For your purpose, use the F_GETFL flag as in

int r, fd;
r = fcntl ( fd, F_GETFL );

if (r==-1)
printf("File %d is closed.\n", fd);
if (r>0)
printf("File %d is open.\n", fd);

If the return value is -1, the file is not open (or fd was not a valid file descriptor). If the return value is positive, fd describes an open file.