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cranelift/docs/cli-logging.md

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Logging in the wasmtime CLI

Wasmtime's libraries use Rust's log crate to log diagnostic information, and the wasmtime CLI executable uses pretty_env_logger by default for logging this information to the console.

Basic logging is controlled by the RUST_LOG environment variable. For example, To enable logging of WASI system calls, similar to the strace command on Linux, set RUST_LOG=wasi_common=trace.

$ RUST_LOG=wasi_common=trace wasmtime hello.wasm
[...]
 TRACE wasi_common::hostcalls_impl::fs                       > fd_write(fd=1, iovs_ptr=0x10408, iovs_len=1, nwritten=0x10404)
Hello, world!
 TRACE wasi_common::hostcalls_impl::fs                       >      | *nwritten=14
 TRACE wasi_common::hostcalls                                >      | errno=ESUCCESS (No error occurred. System call completed successfully.)
 TRACE wasi_common::hostcalls_impl::misc                     > proc_exit(rval=1)

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