I have a project using circuits_i2c and I want to be able to run the project on my PC with circuits_sim. Following the documentation I wound up with this in my config.exs (qemu is the MIX_TARGET I want to use the simulated i2c bus for:
if config_target() == :qemu do
config :circuits_i2c, default_backend: CircuitsSim.I2C.Backend
config :circuits_sim,
config: [
{CircuitsSim.Device.MCP23008, bus_name: "i2c-0", address: 0x20},
]
end
This works for iex and I can see the MIX_TARGET take effect:
✦ ❯ iex -S mix
Generated circuits_sim_test app
Erlang/OTP 26 [erts-14.2.3] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]
Interactive Elixir (1.16.2) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> Circuits.I2C.bus_names
[]
✦ ❯ MIX_TARGET=qemu iex -S mix
Erlang/OTP 26 [erts-14.2.3] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]
Interactive Elixir (1.16.2) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> Circuits.I2C.bus_names
["i2c-0"]
iex(2)>
However, for releases built like this:
✦ ❯ MIX_TARGET=qemu mix release
Running the resulting release gives the following error, where circuits_i1c appears to try to load the NIF despite being apparently configured to use the circuits_sim backend:
✦ ❯ _build/qemu_dev/rel/circuits_sim_test/bin/circuits_sim_test start_iex
=CRASH REPORT==== 13-Aug-2025::18:14:17.622342 ===
crasher:
initial call: kernel:init/1
pid: <0.675.0>
registered_name: []
exception exit: {on_load_function_failed,'Elixir.Circuits.I2C.Nif',
{error,
{load_failed,
"Failed to load NIF library: '/home/larry/projects/circuitsSimIssue/circuits_sim_test/_build/qemu_dev/rel/circuits_sim_test/lib/circuits_i2c-2.1.0/priv/i2c_nif.so: cannot open shared object file: No such file or directory'"}}}
in function init:run_on_load_handlers/0
in call from kernel:init/1 (kernel.erl, line 227)
ancestors: [kernel_sup,<0.668.0>]
message_queue_len: 0
messages: []
links: [<0.670.0>]
dictionary: []
trap_exit: false
status: running
heap_size: 610
stack_size: 28
reductions: 234
neighbours:
=SUPERVISOR REPORT==== 13-Aug-2025::18:14:17.622504 ===
supervisor: {local,kernel_sup}
errorContext: start_error
reason: {on_load_function_failed,'Elixir.Circuits.I2C.Nif',
{error,
{load_failed,
"Failed to load NIF library: '/home/larry/projects/circuitsSimIssue/circuits_sim_test/_build/qemu_dev/rel/circuits_sim_test/lib/circuits_i2c-2.1.0/priv/i2c_nif.so: cannot open shared object file: No such file or directory'"}}}
offender: [{pid,undefined},
{id,on_load},
{mfargs,{proc_lib,start_link,[kernel,init,[on_load]]}},
{restart_type,transient},
{significant,false},
{shutdown,2000},
{child_type,worker}]
=CRASH REPORT==== 13-Aug-2025::18:14:17.623120 ===
crasher:
initial call: application_master:init/4
pid: <0.667.0>
registered_name: []
exception exit: {{shutdown,
{failed_to_start_child,on_load,
{on_load_function_failed,'Elixir.Circuits.I2C.Nif',
{error,
{load_failed,
"Failed to load NIF library: '/home/larry/projects/circuitsSimIssue/circuits_sim_test/_build/qemu_dev/rel/circuits_sim_test/lib/circuits_i2c-2.1.0/priv/i2c_nif.so: cannot open shared object file: No such file or directory'"}}}}},
{kernel,start,[normal,[]]}}
in function application_master:init/4 (application_master.erl, line 142)
ancestors: [<0.666.0>]
message_queue_len: 1
messages: [{'EXIT',<0.668.0>,normal}]
links: [<0.666.0>,<0.665.0>]
dictionary: []
trap_exit: true
status: running
heap_size: 987
stack_size: 28
reductions: 208
neighbours:
=INFO REPORT==== 13-Aug-2025::18:14:17.623386 ===
application: kernel
exited: {{shutdown,
{failed_to_start_child,on_load,
{on_load_function_failed,'Elixir.Circuits.I2C.Nif',
{error,
{load_failed,
"Failed to load NIF library: '/home/larry/projects/circuitsSimIssue/circuits_sim_test/_build/qemu_dev/rel/circuits_sim_test/lib/circuits_i2c-2.1.0/priv/i2c_nif.so: cannot open shared object file: No such file or directory'"}}}}},
{kernel,start,[normal,[]]}}
type: permanent
Kernel pid terminated (application_controller) ("{application_start_failure,kernel,{{shutdown,{failed_to_start_child,on_load,{on_load_function_failed,'Elixir.Circuits.I2C.Nif',{error,{load_failed,\"Failed to load NIF library: '/home/larry/projects/circuitsSimIssue/circuits_sim_test/_build/qemu_dev/rel/circuits_sim_test/lib/circuits_i2c-2.1.0/priv/i2c_nif.so: cannot open shared object file: No such file or directory'\"}}}}},{kernel,start,[normal,[]]}}}")
Crash dump is being written to: erl_crash.dump...done
I want to be able to build a release using circuits_sim, because I am testing more than just this elixir app in qemu and my higher level nix build that produces that VM needs a release as input. Is there a way to make that work?
