# Writing on external temporary files with MPI.jl
**URL:** https://discourse.julialang.org/t/writing-on-external-temporary-files-with-mpi-jl/137485
**Category:** General Usage
**Created:** [June 7, 2026, 8:13pm UTC](https://discourse.julialang.org/t/writing-on-external-temporary-files-with-mpi-jl/137485 "2026-06-07T20:13:50Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@phx](https://discourse.julialang.org/u/phx)
#### Post date: [June 7, 2026, 8:13pm UTC](https://discourse.julialang.org/t/writing-on-external-temporary-files-with-mpi-jl/137485/1 "2026-06-07T20:13:50Z")
Hi all. I’m learning how to use MPI with Julia, specifically the MPI.jl package.
I’m trying to understand how to print on external temporary files, and I can’t make it work.
This is my current minimal non-working example script, called `mpi_test.jl`:
```julia
using MPI
MPI.Init()
let
comm = MPI.COMM_WORLD
prank = MPI.Comm_rank(comm)
io_file, _ = mktemp()
io_file = MPI.bcast(io_file, comm)
f = MPI.File.open(comm, io_file; write=true)
MPI.File.write(f, "$prank: hello!")
io_files = MPI.gather(io_file, comm)
prank == 0 && for p in 0:MPI.Comm_size(comm)-1
println(p, ": ", io_files[p+1])
end
end
```
which I run with `mpiexecjl -np 4 julia --project mpi_test.jl`.
The output is
```bash
0: /tmp/jl_FokE6l
1: /tmp/jl_FokE6l
2: /tmp/jl_FokE6l
3: /tmp/jl_FokE6l
```
but the files are not actually there:
```bash
cat: /tmp/jl_FokE6l: No such file or directory
```
If I write to an ordinary file, say `io_file = "test.txt"` instead of using `mktemp()`, the file is correctly created in the working directory and contains the given string.
With `mktemp()` instead, no file is created at all in the `/tmp` folder.
Can someone explain what I’m missing here?
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [June 7, 2026, 9:59pm UTC](https://discourse.julialang.org/t/writing-on-external-temporary-files-with-mpi-jl/137485/2 "2026-06-07T21:59:40Z")
- Did you mean to call `tempname()` instead of `mktemp()`, since you want `MPI.File.open` to be the function that actually creates the file?
- Note that there is no point in calling `tempname()` on anything except for `prank == 0`, since that is where your are broadcasting (`bcast`) from.
- Did you forget to call `close(f)`?
(I assume you are running on a single node, so that all the MPI processes have access to a single `/tmp` filesystem?)
---
### Author:  [@phx](https://discourse.julialang.org/u/phx)
#### Post date: [June 7, 2026, 11:31pm UTC](https://discourse.julialang.org/t/writing-on-external-temporary-files-with-mpi-jl/137485/3 "2026-06-07T23:31:54Z")
> - Did you mean to call `tempname()` instead of `mktemp()`, since you want `MPI.File.open` to be the function that actually creates the file?
Huh, I never realised `mktemp()` already opened the file. I guess I always used it wrong.
> - Note that there is no point in calling `tempname()` on anything except for `prank == 0`, since that is where your are broadcasting (`bcast`) from.
Yeah I know, it doesn’t matter for this simple example, I just found it simpler to write.
> - Did you forget to call `close(f)`?
I deliberately left it out, I assumed it didn’t change the outcome.
> - (I assume you are running on a single node, so that all the MPI processes have access to a single `/tmp` filesystem?)
Yes, I’m running it on a single node, on my laptop.
Anyway I switched from `mktemp` to `tempname` and `close`d the file at the end, but the behaviour didn’t change.
---
### Author:  [@phx](https://discourse.julialang.org/u/phx)
#### Post date: [June 7, 2026, 11:34pm UTC](https://discourse.julialang.org/t/writing-on-external-temporary-files-with-mpi-jl/137485/4 "2026-06-07T23:34:37Z")
Well, I’m just dumb, I found out why this happens, and it has nothing to do with MPI: the temporary files were deleted by Julia when the script ended because I didn’t supply the `cleanup=false` argument to `mktemp()`.