# A bug in thread?
**URL:** https://discourse.julialang.org/t/a-bug-in-thread/99835
**Category:** General Usage
**Tags:** question
**Created:** [June 4, 2023, 7:32am UTC](https://discourse.julialang.org/t/a-bug-in-thread/99835 "2023-06-04T07:32:15Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@zhijie\_sun](https://discourse.julialang.org/u/zhijie_sun)
#### Post date: [June 4, 2023, 7:32am UTC](https://discourse.julialang.org/t/a-bug-in-thread/99835/1 "2023-06-04T07:32:15Z")
I find if I use `Threads.threadid()` and `sleep` together, the code will give wrong result. Also give wrong result when I replace `sleep` by `lock`.
The code shouldn’t output anything, but it did output, that is why I say give wrong result.
Below is my code and output, I run in julia 1.8.5 and by `julia --threads 4 xxx.jl`
```julia
using Base.Threads
lk = ReentrantLock()
n_thread = 4
dat = Array{Int64, 1}(undef, n_thread)
count::Int64 = 0
atom_count = Threads.Atomic{Int64}(0)
for i in 1:4
global count = 0
atom_count[] = 0
fill!(dat, 0)
Threads.@threads for j in 1:100
thread_id = Threads.threadid()
old_num = 0
dat[thread_id] = 0
for k in 1:10
num = rand(1:100)
dat[thread_id] = num
old_num = num
# lock(lk)
# try
# count += 1
# finally
# unlock(lk)
# end
sleep(0.001)
if dat[thread_id] != old_num
println("$thread_id $k")
end
end
Threads.atomic_add!(atom_count, 1)
end
println("=======================")
# while(atom_count[] < 100)
# end
end
```
```julia
=======================
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
1 10
=======================
1 1
3 1
3 2
1 2
3 3
1 3
1 4
3 4
1 4
1 5
1 5
3 5
3 6
1 7
1 7
3 7
1 9
3 8
3 9
1 10
3 10
=======================
1 1
2 1
1 2
2 2
1 3
1 4
2 3
1 5
2 4
2 5
1 6
2 6
1 7
1 8
2 7
2 8
1 9
1 10
2 9
2 10
=======================
```
---
### Author:  [@fatteneder](https://discourse.julialang.org/u/fatteneder)
#### Post date: [June 4, 2023, 8:13am UTC](https://discourse.julialang.org/t/a-bug-in-thread/99835/2 "2023-06-04T08:13:21Z")
This is caused by running `@threads` with the `:dynamic` scheduler, which is the default. If you change the threaded loop to `Threads.@threads :static for j in 1:100` then the problem goes away.
See the extended docs for more information (type `?? Threads.@threads` in the REPL). In particular, the docs mention this here
```julia
For example, the above conditions imply that:
• The lock taken in an iteration must be released within the same iteration.
• Communicating between iterations using blocking primitives like Channels
is incorrect.
• Write only to locations not shared across iterations (unless a lock or
atomic operation is used).
• The value of threadid() may change even within a single iteration.
```
And
```julia
:static scheduler creates one task per thread and divides the iterations equally
among them, assigning each task specifically to each thread. In particular, the
value of threadid() is guaranteed to be constant within one iteration. Specifying
:static is an error if used from inside another @threads loop or from a thread other
than 1.
```
---
### Author:  [@zhijie\_sun](https://discourse.julialang.org/u/zhijie_sun)
#### Post date: [June 4, 2023, 11:09am UTC](https://discourse.julialang.org/t/a-bug-in-thread/99835/3 "2023-06-04T11:09:40Z")
Oh, thank you!