C API

Core concepts

There are two central objects in the event ring C API. They are:

1. struct monad_event_ring - represents an event ring whose shared memory segments have been mapped into the address space of the current process; the primary thing the client does with this object is use it to initialize iterators that point into the event ring, using the monad_event_ring_init_iterator function
2. struct monad_event_iterator - the star of the show: this iterator object is used to read sequential events. The iterator’s try_next operation copies the current event descriptor (if it is available) and if successful, advances the iterator. Conceptually, it behaves like the expression descriptor = *i++, if an event descriptor is ready immediately (it does nothing otherwise)

The easiest way to understand the API is to compile and run the included eventwatch example program. This program dumps ASCII representations of execution events to stdout, as they are written by a execution daemon running on the same host.

In eventwatch, the event descriptors are fully decoded, but the event payloads are only shown in hexdump form, because this simple program that does not include pretty-printing logic for all event payload types. The program is only 250 lines of code, and reading through it should explain how the various API calls fit together.

The SDK also includes C++20 std::formatter specializations which can fully decode event payloads into human-readable form. These are used by the monad-event-cli utility program.

Using the API in your project

libmonad_event is designed for third party integration, so it does not have any library dependencies aside from a recent version of glibc. This also means it has no dependency on the rest of the monad repository or on its build system: the sole requirement is a C compiler supporting C23.

The “Getting start” guide to building the C example program discusses several ways to use the SDK library as a third-party dependency in your code. Alternatively, the source files that make up the library target can be copied into your own codebase. A Rust client library is also available.

API overview

Event ring APIs

API	Purpose
monad_event_ring_mmap	Given a file descriptor to an open event ring file, map its shared memory segments into the current process, initializing a struct monad_event_ring
monad_event_ring_init_iterator	Given a pointer to a struct monad_event_ring, initialize an iterator that can read from the event ring
monad_event_ring_try_copy	Given a specific sequence number, try to copy the event descriptor for it, if it hasn’t been overwritten
monad_event_ring_payload_peek	Get a zero-copy pointer to an event payload
monad_event_ring_payload_check	Check if an event payload referred to by a zero-copy pointer has been overwritten
monad_event_ring_memcpy	memcpy the event payload to a buffer, succeeding only if the payload is not expired
monad_event_ring_get_last_error	Return a human-readable string describing the last error that occurred on this thread

All functions which can fail will return an errno(3) domain error code diagnosing the reason for failure. The function monad_event_ring_get_last_error can be called to provide a human-readable string explanation of what failed.

Event iterator APIs

API	Purpose
monad_event_iterator_try_next	If an event descriptor if is available, copy it and advance the iterator; behaves like *i++, but only if *i is ready
monad_event_iterator_try_copy	Copy the event descriptor at the current iteration point, without advancing the iterator
monad_event_iterator_reset	Reset the iterator to point to the most recently produced event descriptor; used for gap recovery
monad_exec_iter_consensus_prev	Rewinds an iterator to the previous consensus event (BLOCK_START, BLOCK_QC, BLOCK_FINALIZED, or BLOCK_VERIFIED)
monad_exec_iter_block_number_prev	Rewinds an iterator to the previous consensus event for the given block number
monad_exec_iter_block_id_prev	Rewinds an iterator to the previous consensus event for the given block ID
monad_exec_iter_rewind_for_simple_replay	Rewinds an iterator to replay events you may have missed, based on the last finalized block you saw

Event ring utility APIs

API	Purpose
monad_event_ring_check_content_type	Check if the binary layouts of event definitions used by the library match what is recorded in a mapped event ring
monad_event_ring_find_writer_pids	Find processes that have opened an event ring file descriptor for writing; used for detecting publisher exit
monad_check_path_supports_map_hugetlb	Check if a path is on a filesystem that allows its files to be mmap’ed with MAP_HUGETLB
monad_event_open_hugetlbfs_dir_fd	Open the default hugetlbfs directory where event ring files are created1
monad_event_resolve_ring_file	If a path contains no / character (i.e., if it is a “pure” filename), resolve it relative to some default event ring directory2
monad_event_is_snapshot_file	Check if a path refers to an event ring snapshot file
monad_event_decompress_snapshot_fd	Decompress the event ring snapshot contained in the given file descriptor
monad_event_decompress_snapshot_mem	Decompress the event ring snapshot contained in the given memory buffer

Library organization

Event ring files in libmonad_event:

File	Contains
event_ring.{h,c}	Definitions of core shared memory structures for event rings, and the API that initializes and mmaps event ring files
event_iterator.h	Defines the basic event iterator object and its API
event_iterator_inline.h	Definitions of the event_iterator.h functions, all of which are inlined for performance reasons
event_metadata.h	Structures that describe event metadata (string names of events, descriptions of events, etc.)
exec_iter_help.h	API for rewinding the the iterator to point to block executions or consensus events

Execution event files in libmonad_event:

File	Contains
base_ctypes.h	Definitions of basic vocabulary types common in Ethereum data (e.g., 256 bit integer types, etc).
eth_ctypes.h	Definitions of structures used in the Ethereum virtual machine
exec_event_ctypes.h	Definition of execution event payload structures, and the event type enumeration enum monad_exec_event_type
exec_event_ctypes_metadata.c	Defines static metadata about execution events, and the schema hash value array
monad_ctypes.h	Definitions of Monad blockchain extensions to Ethereum

Supporting files in libmonad_event:

File	Contains
event_ring_util.{h,c}	Convenience functions that are useful in most event ring programs, but which are not part of the core API
format_err.{h,c}	Helper utility from the execution codebase used to implement the monad_event_ring_get_last_error() function
srcloc.h	Helper utility used with the format_err.h API, for capturing source code locations in C

Other files in the SDK:

File	Contents
eventwatch.c	A sample program that shows how to use the API
*_fmt.hpp files	Files ending in _fmt.hpp are used with C++ <format> and contain std::formatter specializations for SDK types
hex.hpp	<format> hexdump utility used by the _fmt.hpp files to dump uint8_t[] values

Footnotes

1. By default, this returns the a path on a hugetlbfs mount, as computed by libhugetlbfs ↩

2. If compiling with MONAD_EVENT_USE_LIBHUGETLBFS=OFF, a default event ring directory must be specified; see here for details ↩