Logging in the wasmtime CLI

Wasmtime's libraries use Rust's log crate to log diagnostic information, and the wasmtime CLI executable uses tracing-subscriber for displaying this information on the console.

Basic logging is controlled by the WASMTIME_LOG environment variable. For example, To enable logging of WASI system calls, similar to the strace command on Linux, set WASMTIME_LOG=wasmtime_wasi=trace. For more information on specifying filters, see tracing-subscriber's EnvFilter docs.

$ WASMTIME_LOG=wasmtime_wasi=trace wasmtime hello.wasm [...] TRACE wiggle abi{module="wasi_snapshot_preview1" function="fd_write"} wasmtime_wasi::preview1::wasi_snapshot_preview1 > fd=Fd(1) iovs=*guest 0x14/1 Hello, world! TRACE wiggle abi{module="wasi_snapshot_preview1" function="fd_write"}: wasmtime_wasi::preview1::wasi_snapshot_preview1: result=Ok(14) TRACE wiggle abi{module="wasi_snapshot_preview1" function="proc_exit"}: wasmtime_wasi::preview1::wasi_snapshot_preview1: rval=1 TRACE wiggle abi{module="wasi_snapshot_preview1" function="proc_exit"}: wasmtime_wasi::preview1::wasi_snapshot_preview1: result=Exited with i32 exit status 1

Wasmtime can also redirect the log messages into log files, with the -D log-to-files option. It creates one file per thread within Wasmtime, with the files named wasmtime.dbg.*.

Additional environment variables that work with WASMTIME_LOG (not -D log-to-files):

  • WASMTIME_LOG_NO_CONTEXT: if set to 1, removes the time, level and target from output.