The calldatacopy operation is a low-level EVM instruction in Solidity that allows direct copying of data from the transaction’s call data (the input data sent with the call) into memory. This operation is often used in inline assembly to efficiently access specific parts of calldata without needing high-level Solidity functions. calldatacopy is especially useful in contracts that need to handle raw calldata.
What calldatacopy Does
Operation: calldatacopy(destOffset, calldataOffset, length)
Parameters:
destOffset: The offset in memory where the copied data should start.
calldataOffset: The starting point in calldata to copy from.
length: The number of bytes to copy.
When executed, calldatacopy copies length bytes of calldata, starting at calldataOffset, and places it in memory at the specified destOffset.
This operation is helpful for handling dynamic calldata, accessing specific function arguments, or optimizing gas usage by avoiding unnecessary high-level data handling.
Here’s an example of a function that uses calldatacopy to copy part of the calldata to memory. In this example, let’s assume we are creating a contract function that copies a specific range of calldata bytes into memory for processing:
What calldatacopy Does
Operation: calldatacopy(destOffset, calldataOffset, length)
Parameters:
destOffset: The offset in memory where the copied data should start.
calldataOffset: The starting point in calldata to copy from.
length: The number of bytes to copy.
When executed, calldatacopy copies length bytes of calldata, starting at calldataOffset, and places it in memory at the specified destOffset.
This operation is helpful for handling dynamic calldata, accessing specific function arguments, or optimizing gas usage by avoiding unnecessary high-level data handling.
Here’s an example of a function that uses calldatacopy to copy part of the calldata to memory. In this example, let’s assume we are creating a contract function that copies a specific range of calldata bytes into memory for processing:
Explanation of the Example
Memory Allocation:
The function allocates a new bytes array copiedData to store the copied calldata.
When creating a dynamic bytes array in Solidity, the first 32 bytes in memory represent the array’s length. Therefore, the data itself starts 32 bytes after the start of the array’s memory location.
Setting the Destination Offset:
Using inline assembly, we determine destOffset, the exact memory location where calldatacopy will start copying data. This location is calculated as add(copiedData, 32), which skips the first 32 bytes reserved for the array length and points to the start of the actual data section.
Copying Data Using calldatacopy:
The calldatacopy operation copies length bytes from calldataOffset in the calldata to destOffset in memory. This effectively stores the specified portion of calldata in copiedData.
Returning the Copied Data:
Once the data is copied into copiedData, the function returns this array, which now contains the specified section of calldata.
If you want to only copy certain parts of the calldata, you can change the calldataOffset input and the desired length.
