flatc is not found but is present

🐛 Describe the bug

flatc is not found when trying to export

Export code:

@torch.no_grad()
def to_executorch(model, dst_file: Path):
    inputs = model.example_input
    model = model.eval()
    exported_program = torch.export.export(model, tuple(inputs.values()), strict=True)
    executorch_program = to_edge_transform_and_lower(
        exported_program,
        partitioner=[XnnpackPartitioner()],
    ).to_executorch()

    dst_file.write_bytes(executorch_program.buffer)

Traceback

Traceback (most recent call last):
  File ".venv/lib/python3.11/site-packages/executorch/exir/program/_program.py", line 115, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/executorch/exir/program/_program.py", line 1379, in to_edge_transform_and_lower
    edge_manager = edge_manager.to_backend(method_to_partitioner)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/executorch/exir/program/_program.py", line 115, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/executorch/exir/program/_program.py", line 1681, in to_backend
    new_edge_programs = to_backend(method_to_programs_and_partitioners)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/redeyed/.local/share/uv/python/cpython-3.11.11-linux-x86_64-gnu/lib/python3.11/functools.py", line 909, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/executorch/exir/backend/backend_api.py", line 762, in _
    lower_all_submodules_to_backend(
  File ".venv/lib/python3.11/site-packages/executorch/exir/backend/backend_api.py", line 591, in lower_all_submodules_to_backend
    backend_name_to_subclass[backend_id].preprocess_multimethod(
  File ".venv/lib/python3.11/site-packages/executorch/exir/backend/backend_details.py", line 145, in preprocess_multimethod
    preprocess_result = cls.preprocess(program, compile_spec_for_program)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/executorch/backends/xnnpack/xnnpack_preprocess.py", line 189, in preprocess
    processed_bytes=serialize_xnnpack_binary(
                    ^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/executorch/backends/xnnpack/serialization/xnnpack_graph_serialize.py", line 347, in serialize_xnnpack_binary
    flatbuffer_payload = convert_to_flatbuffer(xnnpack_graph)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/executorch/backends/xnnpack/serialization/xnnpack_graph_serialize.py", line 328, in convert_to_flatbuffer
    _flatc_compile(d, schema_path, json_path)
  File ".venv/lib/python3.11/site-packages/executorch/exir/_serialize/_flatbuffer.py", line 216, in _flatc_compile
    _run_flatc(
  File ".venv/lib/python3.11/site-packages/executorch/exir/_serialize/_flatbuffer.py", line 202, in _run_flatc
    subprocess.run([flatc_path] + list(args), check=True)
  File "/home/redeyed/.local/share/uv/python/cpython-3.11.11-linux-x86_64-gnu/lib/python3.11/subprocess.py", line 548, in run
    with Popen(*popenargs, **kwargs) as process:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/redeyed/.local/share/uv/python/cpython-3.11.11-linux-x86_64-gnu/lib/python3.11/subprocess.py", line 1026, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/home/redeyed/.local/share/uv/python/cpython-3.11.11-linux-x86_64-gnu/lib/python3.11/subprocess.py", line 1955, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'flatc'

Workaround

I tried to find flatc and found it

find .venv/lib/python3.11/site-packages/executorch/ -name flatc
.venv/lib/python3.11/site-packages/executorch/data/bin/flatc

So I exported it

export FLATC_EXECUTABLE=$PWD/.venv/lib/python3.11/site-packages/executorch/data/bin/flatc
uv run ./my_script.py

ENV

executorch 1.1.0
pytorch-lightning 2.6.0
pytorch-tokenizers 1.1.0
torch 2.10.0+cu128
torchao 0.15.0
torchdata 0.11.0
torchmetrics 1.8.2
torchvision 0.25.0+cu128
vector-quantize-pytorch 1.27.20

OS: Ubuntu 24.04

Versions

uv run collect_env.py
Collecting environment information...
PyTorch version: 2.10.0+cu128
Is debug build: False
CUDA used to build PyTorch: 12.8
ROCM used to build PyTorch: N/A

OS: Ubuntu 24.04.3 LTS (x86_64)
GCC version: (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
Clang version: 18.1.3 (1ubuntu1)
CMake version: version 4.2.1
Libc version: glibc-2.39

Python version: 3.11.11 (main, Mar 17 2025, 21:02:09) [Clang 20.1.0 ] (64-bit runtime)
Python platform: Linux-6.8.0-90-generic-x86_64-with-glibc2.39
Is CUDA available: True
CUDA runtime version: Could not collect
CUDA_MODULE_LOADING set to:
GPU models and configuration:
GPU 0: NVIDIA GeForce RTX 5070 Ti
GPU 1: NVIDIA GeForce RTX 2060 SUPER

Nvidia driver version: 580.126.09
cuDNN version: Could not collect
Is XPU available: False
HIP runtime version: N/A
MIOpen runtime version: N/A
Is XNNPACK available: True
Caching allocator config: N/A

CPU:
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Address sizes: 39 bits physical, 48 bits virtual
Byte Order: Little Endian
CPU(s): 16
On-line CPU(s) list: 0-15
Vendor ID: GenuineIntel
Model name: Intel(R) Core(TM) i9-9900KF CPU @ 3.60GHz
CPU family: 6
Model: 158
Thread(s) per core: 2
Core(s) per socket: 8
Socket(s): 1
Stepping: 13
CPU(s) scaling MHz: 42%
CPU max MHz: 5000,0000
CPU min MHz: 800,0000
BogoMIPS: 7200,00
Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault ssbd ibrs ibpb stibp ibrs_enhanced tpr_shadow flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid mpx rdseed adx smap clflushopt intel_pt xsaveopt xsavec xgetbv1 xsaves dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp_epp vnmi md_clear flush_l1d arch_capabilities ibpb_exit_to_user
Virtualization: VT-x
L1d cache: 256 KiB (8 instances)
L1i cache: 256 KiB (8 instances)
L2 cache: 2 MiB (8 instances)
L3 cache: 16 MiB (1 instance)
NUMA node(s): 1
NUMA node0 CPU(s): 0-15
Vulnerability Gather data sampling: Mitigation; Microcode
Vulnerability Itlb multihit: KVM: Mitigation: VMX disabled
Vulnerability L1tf: Not affected
Vulnerability Mds: Not affected
Vulnerability Meltdown: Not affected
Vulnerability Mmio stale data: Mitigation; Clear CPU buffers; SMT vulnerable
Vulnerability Reg file data sampling: Not affected
Vulnerability Retbleed: Mitigation; Enhanced IBRS
Vulnerability Spec rstack overflow: Not affected
Vulnerability Spec store bypass: Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1: Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2: Mitigation; Enhanced / Automatic IBRS; IBPB conditional; RSB filling; PBRSB-eIBRS SW sequence; BHI SW loop, KVM SW loop
Vulnerability Srbds: Mitigation; Microcode
Vulnerability Tsx async abort: Mitigation; TSX disabled
Vulnerability Vmscape: Mitigation; IBPB before exit to userspace

Versions of relevant libraries:
[pip3] Could not collect
[conda] Could not collect

cc @JacobSzwejbka @angelayi @GregoryComer @digantdesai @cbilgin

Start in executorch/exir/_serialize/_flatbuffer.py, especially the _run_flatc and _flatc_compile path shown in the traceback, then inspect the bundled executable at executorch/data/bin/flatc. Reproduce the export with XnnpackPartitioner and verify that a normal installation finds the bundled flatc without requiring FLATC_EXECUTABLE; preserve the existing environment-variable workaround if applicable.