This page is a placeholder. All examples on this page are currently AI-generated and are not correct. This documentation will be completed in the future with accurate, tested examples.
Overview
Opcodes: 0xa0 (LOG0) to 0xa4 (LOG4)`
Introduced: Frontier (EVM genesis)
The LOG family of instructions emits event logs that external systems (off-chain indexers, monitoring services) can capture and process. Each instruction encodes a fixed number of topics (indexed parameters) and flexible data, enabling efficient event filtering without on-chain computation.
Instruction Set
| Opcode | Name | Topics | Stack Items |
|---|
| 0xa0 | LOG0 | 0 | 2 (offset, length) |
| 0xa1 | LOG1 | 1 | 3 (offset, length, topic0) |
| 0xa2 | LOG2 | 2 | 4 (offset, length, topic0, topic1) |
| 0xa3 | LOG3 | 3 | 5 (offset, length, topic0, topic1, topic2) |
| 0xa4 | LOG4 | 4 | 6 (offset, length, topic0, topic1, topic2, topic3) |
Gas Cost
All LOG instructions cost:
375 gas (base)
+ 375 gas per topic
+ 8 gas per byte of data
- LOG0 with empty data: 375 gas
- LOG1 with 32 bytes: 375 + 375 + 256 = 1006 gas
- LOG4 with 64 bytes: 375 + (4 × 375) + 512 = 2387 gas
Memory expansion costs apply when reading data beyond current allocation.
Key Constraints
EIP-214 (Static Call Protection):
LOG instructions cannot execute in static call context. Attempting to log during a STATICCALL reverts with StaticCallViolation.
// This will revert
function badLog() external view {
// emit event - reverts in view functions
}
Common Usage
Event Indexing
event Transfer(address indexed from, address indexed to, uint256 value);
function transfer(address to, uint256 amount) public {
balances[msg.sender] -= amount;
balances[to] += amount;
emit Transfer(msg.sender, to, amount); // Compiler generates LOG2
}
Event Filtering
Off-chain services use topics for fast filtering without parsing all event data:
// Listen for Transfer events from specific address
const logs = await getLogs({
address: tokenAddress,
topics: [
keccak256("Transfer(address,indexed address,indexed uint256)"),
null, // Match any 'from'
"0xaddressToFilterFor" // Match specific 'to'
]
});
Multiple Events
A transaction can emit multiple logs, which are returned in order:
event Approval(address indexed owner, address indexed spender, uint256 value);
function approve(address spender, uint256 amount) public {
allowances[msg.sender][spender] = amount;
emit Approval(msg.sender, spender, amount); // LOG2
return true;
}
Implementation Notes
Topic Encoding
Topics are 256-bit values. For dynamic types (strings, arrays), the keccak256 hash is used:
event StringLog(string indexed data);
// Topic is keccak256(data), not the string itself
event DynamicLog(uint256[] indexed arr);
// Topic is keccak256(abi.encode(arr)), not individual values
Data vs Topics
- Topics (0-4): Indexed parameters, optimized for efficient filtering
- Data: Non-indexed parameters, stored but not indexed
event Transfer(
address indexed from, // Topic 1
address indexed to, // Topic 2
uint256 value // Data (non-indexed)
);
// Generates: LOG2 with topics=[from, to] and data=abi.encode(value)
References
