# Understanding \`mkpidlock\` and why it deletes my files

**URL:** https://discourse.julialang.org/t/understanding-mkpidlock-and-why-it-deletes-my-files/120915
**Category:** General Usage
**Tags:** question
**Created:** [October 4, 2024, 1:51pm UTC](https://discourse.julialang.org/t/understanding-mkpidlock-and-why-it-deletes-my-files/120915 "2024-10-04T13:51:39Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![phx](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/phx/32/219940_2.png) [@phx](https://discourse.julialang.org/u/phx)
#### Post date: [October 4, 2024, 1:51pm UTC](https://discourse.julialang.org/t/understanding-mkpidlock-and-why-it-deletes-my-files/120915/1 "2024-10-04T13:51:39Z")

</div>

I have multiple instances of a script that will need to write on a single file, so I was looking into locking the access to said file by using `mkpidlock`. Eventually I’ll want to use it with HDF5 files (which I know I can’t with `mkpidlock` as it is) so I’m trying to understand its inner workings.  
I think there’s something fundamental I’m not understanding, since the following behaviour surprises me:

1. Create a plain text file, `echo "hello" > test.txt`
2. Start Julia, loading the function: `julia -ie 'using FileWatching.Pidfile: mkpidlock'`
3. Open the file with it, but without doing anything with it:

```julia
mkpidlock("test.txt"; stale_age=2, wait=true, mode=0o666) do
       () -> nothing
       end

```

The `do` block executes after some seconds, warning me that it’s attempting to remove a probably state pidfile. I find it odd, since I just created it.  
The most surprising thing is however that after the `do` block ends the file just disappears. I though I did nothing on it! What happened?  
I dug into the code for `mkpidlock` and found out that a likely culprit is the `tryrmopenfile` called by `open_exclusive`, in `FileWatching/src/pidfile.jl`, but I don’t understand why the file should get deleted. And also the reason for the initial waiting period in the first place. What am I missing here? Is the function erasing the file I created because there was a stale pidfile?

---

<div class="post-metadata">

### Author: ![Federico\_Marotta](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/federico_marotta/32/33810_2.png) [@Federico\_Marotta](https://discourse.julialang.org/u/Federico_Marotta)
#### Post date: [March 6, 2025, 4:27pm UTC](https://discourse.julialang.org/t/understanding-mkpidlock-and-why-it-deletes-my-files/120915/2 "2025-03-06T16:27:14Z")

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I had the same problem and came to the conclusion that `mkpidlock()` is not supposed to lock the file passed as first argument. Rather, it creates a file that acts as a lock. The fact that the file exists signals other processes that the lock is already taken by someone. I think the reason behind this is that it is very difficult to implement a cross-platform file lock for Linux, Windos, MacOS, etc. So we should use it like so:

```julia
mkpidlock("test.txt.lock") do
    open("test.txt", "a") do io
        write(io, "Meow")
    end
end

```