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In this track: Dynamic ABI and nested layouts
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ABI coder version behavior

Compiler generation and decoder checks are separate from the basic layout.

LET’S PICTURE IT

Two readers can follow the same form layout while performing different validation checks.

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Follow a dynamic string from head to tailStep 1 / 4
Follow a dynamic string from head to tailArgument headbyte 0 · offset 32hex word ends in 20String tailstarts at byte 32length + contentPositions are measured from argument byte zero, excluding any selectorOffset destinationargument byte 32read a length wordLength = 5hello is five UTF-8 bytescontent starts at byte 64A length word and an offset word have different jobsContent word · argument byte 6468 65 6c 6c 6f5 payload bytes27 padding bytesBlocks are schematic; their drawn widths do not represent the exact byte ratioHeadoffset: 32argument byte 0Tail lengthlength: 5argument byte 32Tail contenthello + paddingargument byte 6432 + 32 + 32 = 96 bytesWith a four-byte selector, total calldata length is 100 bytes

This example encodes one string parameter with value hello. A dynamic argument uses an offset in its head, even when it is the only argument.

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Know the compiler context

Solidity 0.8.0 made ABI coder v2 the default. Older code and explicit pragmas may use different coder behavior, so record the compiler version when reproducing an example.

Understand the added support

Coder v2 supports nested arrays and structs more broadly than the older coder and performs additional checks in some cases. It does not add a version marker to ordinary calldata.

Do not infer version from bytes alone

A simple uint256 call can look identical across coder versions. The byte sequence does not automatically reveal which compiler, source pragma or client library created it.

Test the actual boundary

For compatibility work, compile the relevant source under the intended version and test supported schemas and malformed inputs. A successful simple example does not establish every nested case.

Trace a local example

types: (uint256,string)[]
values: [[["1","one"]]]
no function selector prefix

word 0 · argument offset 0 bytes
0x0000000000000000000000000000000000000000000000000000000000000020

word 1 · argument offset 32 bytes
0x0000000000000000000000000000000000000000000000000000000000000001

word 2 · argument offset 64 bytes
0x0000000000000000000000000000000000000000000000000000000000000020

word 3 · argument offset 96 bytes
0x0000000000000000000000000000000000000000000000000000000000000001

word 4 · argument offset 128 bytes
0x0000000000000000000000000000000000000000000000000000000000000040

word 5 · argument offset 160 bytes
0x0000000000000000000000000000000000000000000000000000000000000003

word 6 · argument offset 192 bytes
0x6f6e650000000000000000000000000000000000000000000000000000000000

A nested dynamic schema illustrates why actual coder support matters.

TURN UNDERSTANDING INTO PRACTICEOpen the ABI encoder →
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Does ordinary calldata carry an ABI coder version byte?

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