Debugging with gdb and lldb

The following steps describe how to use gdb or lldb to debug both the Wasm guest and the host (i.e. the Wasmtime CLI or your Wasmtime-embedding program) at the same time:

  1. Compile your WebAssembly with debug info enabled, usually -g; for example:

    clang foo.c -g -o foo.wasm
  2. Run Wasmtime with the debug info enabled; this is -D debug-info from the CLI and Config::debug_info(true) in an embedding (e.g. see debugging in a Rust embedding). It's also recommended to use -O opt-level=0 for better inspection of local variables if desired.

  3. Use a supported debugger:

    lldb -- wasmtime run -D debug-info foo.wasm
    gdb --args wasmtime run -D debug-info -O opt-level=0 foo.wasm

If you run into trouble, the following discussions might help:

  • On MacOS with LLDB you may need to run: settings set plugin.jit-loader.gdb.enable on (#1953)

  • With LLDB, call __vmctx.set() to set the current context before calling any dereference operators (#1482):

    (lldb) p __vmctx->set()
    (lldb) p *foo
  • The address of the start of instance memory can be found in __vmctx->memory

  • On Windows you may experience degraded WASM compilation throughput due to the enablement of additional native heap checks when under the debugger by default. You can set the environment variable _NO_DEBUG_HEAP to 1 to disable them.