Guest Programming (ceno_rt)

The ceno_rt crate provides the runtime environment for guest programs running inside the Ceno ZKVM. It offers essential functionalities for interacting with the host and the ZKVM environment.

Execution Environment

A guest program’s execution begins at the _start symbol, which is defined in ceno_rt. This entry point sets up the global pointer and stack, and then calls the standard Rust main function.

After main returns, or to exit explicitly, the program can call ceno_rt::halt(exit_code). An exit_code of 0 indicates success.

Key features of ceno_rt include:

Memory-Mapped I/O (mmio)

The ceno_rt::mmio module provides a way to read data provided by the host through memory-mapped regions.

• Hints: These are private inputs from the host. You can read them using ceno_rt::mmio::read_slice() to get a byte slice or ceno_rt::mmio::read::<T>() to deserialize a specific type.
• Public I/O: The ceno_rt::mmio::commit function is used to reveal public outputs. It verifies that the output produced by the guest matches the expected output provided by the host.

Standard I/O (io)

The ceno_rt::io module contains IOWriter for writing data. In debug builds, a global IOWriter instance is available through ceno_rt::io::info_out(). You can use the debug_print! and debug_println! macros for logging during development.

Dynamic Memory (allocator)

For dynamic memory needs, ceno_rt::allocator provides a simple allocator.

System Calls (syscalls)

The ceno_rt::syscalls module offers low-level functions to access precompiled operations for performance-critical tasks. For more details, see the “Accelerated Operations with Precompiles” chapter.

Weakly Linked Functions

ceno_rt defines several weakly linked functions (e.g., sys_write, sys_alloc_words, sys_rand) that provide default implementations for certain system-level operations. These can be overridden by the host environment.

When writing a guest program, you will typically include ceno_rt as a dependency in your Cargo.toml.