decreaseAllowance

DecreaseAllowance is a function commonly found in ERC-20 token contracts. It allows the owner of tokens to reduce the amount of tokens that a designated spender is authorized to transfer on the owner's behalf.

The function typically accepts two arguments: the spender's address and the amount to subtract from the current allowance. It then reduces the existing allowance by the specified value and updates the allowance accordingly, ensuring the result cannot become negative. In modern Solidity, attempting to subtract more than the current allowance will revert, preventing underflow.

DecreaseAllowance is often provided alongside increaseAllowance as a safer alternative to directly calling approve with a new value. The purpose is to give token holders a way to adjust spending rights in a deliberate, bounded manner and to help mitigate certain race conditions associated with changing approvals.

Practical considerations include that not all tokens implement decreaseAllowance, since it is not required by the ERC-20 standard. The caller must be the token owner, and usage patterns commonly involve setting an allowance to zero before increasing or decreasing it to a new level. Some implementations expose SafeERC20 wrappers that call decreaseAllowance reliably.

In summary, decreaseAllowance helps manage spending permissions by lowering an existing allowance, complementing reduce/modify approval flows and transferFrom interactions in ERC-20 tokens.