# How to debug a silent crash?
**URL:** https://discourse.julialang.org/t/how-to-debug-a-silent-crash/136383
**Category:** General Usage
**Tags:** crash
**Created:** [March 25, 2026, 3:16pm UTC](https://discourse.julialang.org/t/how-to-debug-a-silent-crash/136383 "2026-03-25T15:16:10Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@AwesomeQuest](https://discourse.julialang.org/u/AwesomeQuest)
#### Post date: [March 25, 2026, 3:16pm UTC](https://discourse.julialang.org/t/how-to-debug-a-silent-crash/136383/1 "2026-03-25T15:16:10Z")
I’m writing a script which calls a C library and the program is just stopping prematurely with no output.
How do I debug this?
A little context:
I’m passing a Julia function as a callback to the SimpleBLE C library that is called when data is received from a Bluetooth device. The callback is called once and seems to execute successfully and a few seconds later the process stops.
The callback contains an `unsafe_wrap` to read the incoming data, then calls `println` to display it and finally calls the library’s free function on the received data.
---
### Author:  [@obsidianjulua](https://discourse.julialang.org/u/obsidianjulua)
#### Post date: [March 25, 2026, 5:38pm UTC](https://discourse.julialang.org/t/how-to-debug-a-silent-crash/136383/2 "2026-03-25T17:38:45Z")
Is julia garbage collecting the callback closure? You may need to setup some GC preserve, or flush the c buffer in order for julia to print thd output
---
### Author:  [@AwesomeQuest](https://discourse.julialang.org/u/AwesomeQuest)
#### Post date: [March 25, 2026, 7:22pm UTC](https://discourse.julialang.org/t/how-to-debug-a-silent-crash/136383/3 "2026-03-25T19:22:52Z")
Do you mean the callback itself can get garbage collected after C uses it exactly once?
Here is the offending function
```julia-auto
# The following functions call on each other in sequence
# This reformats the callback to only need to take one argument
function peripheral_notify(handle, service, characteristic, callback)
function c_callback(handle, service, characteristic, data, data_length, userdata)
jdata = unsafe_wrap(Vector{UInt8}, data, data_length)
callback(jdata)
simpleble_free(data)
end
peripheral_notify(handle, service, characteristic, c_callback, C_NULL)
end
# This converts the callback into a c function
function peripheral_notify(handle, service, characteristic, callback, userdata)
c_callback = @cfunction($callback, Cvoid, (SBLEPeripheral, SBLEUUID, SBLEUUID, Ptr{UInt8},Csize_t, Ptr{Cvoid}))
_peripheral_notify(handle, service, characteristic, c_callback, userdata)
end
# This calls ccall
_peripheral_notify(handle, service, characteristic, callback, userdata) =
@ccall simpleblepath.simpleble_peripheral_notify(
handle::SBLEPeripheral,
service::SBLEUUID,
characteristic::SBLEUUID,
callback::Ptr{Cvoid},
userdata::Ptr{Cvoid})::SimpleBLE_Error
```
Adding a `GC.@preserve` for the callbacks does not seem to fix it.
---
### Author:  [@AwesomeQuest](https://discourse.julialang.org/u/AwesomeQuest)
#### Post date: [March 25, 2026, 7:33pm UTC](https://discourse.julialang.org/t/how-to-debug-a-silent-crash/136383/4 "2026-03-25T19:33:57Z")
Ok, removing the `simpleble_free` seems to have solved the issue, but the docs say I should free the memory given to callbacks so now I’m confused.
---
### Author:  [@obsidianjulua](https://discourse.julialang.org/u/obsidianjulua)
#### Post date: [March 25, 2026, 8:08pm UTC](https://discourse.julialang.org/t/how-to-debug-a-silent-crash/136383/5 "2026-03-25T20:08:05Z")
stash the cfunction pointer somewhere the GC can see it. $callback in @cfunction
```llm
const _active_callbacks = Dict{Ptr{Cvoid}, Base.CFunction}()
function peripheral_notify(handle, service, characteristic, callback, userdata)
c_callback = @cfunction($callback, Cvoid, (SBLEPeripheral, SBLEUUID, SBLEUUID, Ptr{UInt8}, Csize_t, Ptr{Cvoid}))
_active_callbacks[handle] = c_callback # prevent GC
_peripheral_notify(handle, service, characteristic, c_callback, userdata)
end
```
---
### Author:  [@obsidianjulua](https://discourse.julialang.org/u/obsidianjulua)
#### Post date: [March 25, 2026, 8:15pm UTC](https://discourse.julialang.org/t/how-to-debug-a-silent-crash/136383/7 "2026-03-25T20:15:52Z")
Actually you might be freeing the memory twice, and causing a heap corruption, why not freeing it works.
---
### Author:  [@AwesomeQuest](https://discourse.julialang.org/u/AwesomeQuest)
#### Post date: [March 25, 2026, 8:17pm UTC](https://discourse.julialang.org/t/how-to-debug-a-silent-crash/136383/8 "2026-03-25T20:17:10Z")
Does not seem to have fixed it. But it seems like good practice so I’m keeping it.
---
### Author:  [@AwesomeQuest](https://discourse.julialang.org/u/AwesomeQuest)
#### Post date: [March 25, 2026, 8:17pm UTC](https://discourse.julialang.org/t/how-to-debug-a-silent-crash/136383/9 "2026-03-25T20:17:40Z")
I’m making a MWE to send to the author to ask him.
---
### Author:  [@obsidianjulua](https://discourse.julialang.org/u/obsidianjulua)
#### Post date: [March 31, 2026, 3:35pm UTC](https://discourse.julialang.org/t/how-to-debug-a-silent-crash/136383/10 "2026-03-31T15:35:07Z")
1. Function returns a pointer — who owns it? Factory functions return owned pointers (caller must free). Accessor functions return borrowed pointers (caller must NOT free). Interior pointers have lifetime tied to a parent.
2. Function takes a pointer argument — does it consume it? Most functions borrow their arguments. Some sink functions take ownership (callee frees). The caller needs to know so it doesn’t double-free.
# obsidian\_notes