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Calldata

Calldata is the input data attached to a call, the same data field introduced in Ethereum Transactions for a top-level transaction, and the equivalent input data passed along an internal message call between contracts. This chapter covers what it actually contains and why it's treated differently from memory.

What calldata actually holds

For an ordinary contract call, calldata is ABI-encoded data: the first 4 bytes identify which function is being called (a function selector, derived from hashing the function's signature. See Selectors), followed by the ABI-encoded arguments to that function. The CALLDATALOAD, CALLDATASIZE, and CALLDATACOPY opcodes let executing code read this data. CALLDATALOAD reads a 32-byte word starting at a given offset, CALLDATACOPY copies a range of calldata bytes into memory for further processing.

Why calldata is read-only and separate from memory

Calldata is explicitly read-only from the executing contract's perspective. There's no CALLDATASTORE opcode, because calldata represents the caller's input, fixed at the moment the call was made, not something the callee should be able to mutate. Keeping it structurally separate from Memory (rather than simply copying it into memory automatically) also has a real gas-efficiency benefit: a contract that only needs to read a few specific fields from a large calldata payload can do so directly via CALLDATALOAD without paying the memory-expansion cost of copying the entire payload into memory first, unless it actually needs to.

Calldata gas cost, and why EIP-4844's blob pricing matters here indirectly

Calldata itself has a real, direct gas cost, historically higher per non-zero byte than per zero byte, specifically because non-zero bytes were judged more likely to represent meaningful, compressible-poorly data worth pricing higher (a detail later revisited by EIP-2028, which reduced the non-zero-byte cost specifically to make data-heavy use cases, including early Layer 2 rollups posting transaction data via calldata, more economical). This is the direct link to Layer 2's later chapters: before EIP-4844 introduced dedicated, separately priced blob space, rollups posting their transaction data back to Ethereum for data availability had no better option than paying ordinary calldata gas costs for that data, a real, significant expense this section's calldata gas cost directly explains the origin of.

Common misconceptions

Calldata is not free just because it isn't "storage" in the persistent-state sense. It has a real, direct gas cost proportional to its size, which is exactly why efficient ABI encoding (packing arguments tightly, avoiding unnecessary padding) matters for gas-conscious contract and dapp design.

Calldata is not the same thing as an event log's data (see Events and Logs). Calldata is the input to a call, consumed during execution; logs are output, emitted during execution and stored separately in the block's receipts (see Ethereum Blocks), for external observers to later query.

Further reading

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