# Runtime Platform Abstraction Layer (PAL)

The ExecuTorch _Platform Abstraction Layer_ (PAL) provides a way for execution
environments to override operations like:
- Getting the current time.
- Printing a log statement.
- Panicking the process/system.

The PAL function declarations are in
[`executorch/runtime/platform/platform.h`](https://github.com/pytorch/executorch/blob/main/runtime/platform/platform.h).

## Overriding the default PAL

The default PAL implementation is in
[`executorch/runtime/platform/target/Posix.cpp`](https://github.com/pytorch/executorch/blob/main/runtime/platform/target/Posix.cpp).
It uses `std::chrono::steady_clock` for the time, prints log messages to
`stderr`, and makes other default assumptions.

But, if they don't work for your system, you can override the default PAL by:
- Including
  [`executorch/runtime/platform/platform.h`](https://github.com/pytorch/executorch/blob/main/runtime/platform/platform.h)
  in one of your application's `.c` or `.cpp` files.
- Defining an implementation of one or more of the `et_pal_*()` functions.

No build system changes necessary. The default PAL functions are weak symbols,
so providing your own strong-symbol definition will override them at link time.

## Minimal PAL

If you run into build problems because your system doesn't support the functions
called by `Posix.cpp`, you can instead use the no-op minimal PAL at
[`executorch/runtime/platform/target/Minimal.cpp`](https://github.com/pytorch/executorch/blob/main/runtime/platform/target/Minimal.cpp)
by building with the `buck2` flag `-c executorch.pal_default=minimal`. This will
avoid calling `fprintf()`, `std::chrono::steady_clock`, and anything else that
`Posix.cpp` uses. But since the `Minimal.cpp` `et_pal_*()` functions are no-ops,
you will need to override all of them.