Solidity decimal values - solidity

Question

I need put a minimum value of user can pay, and this value must be e.g 0.5 BNB.
But I don't know how I can do this.
uint comissionFee = 5; // 5 will represent 0,5
uint256 minimunPay = comissionFee*100/1000;
But return 0
pragma solidity ^0.8.4;
function payFees() public payable {
require(msg.value >= 0.5); // <-- not compatible with uint256
(bool success,) = owner.call{value: msg.value}("");
require(success, "Failed to send money");
}

Answer 1

Firstly, there are no double numbers in solidity lang.
All calculations are done with whole numbers. To solve this issue, all tokens on Ethereum/Binance Chain consider some big decimal number as a threshold of double and ceil parts.
In ERC20 tokens this number is calculated based on the decimal field of the contract: 10^decimal.
For the native token ETH/BNB, there is a fixed decimal equal to 18.
Your code could be fixed this way:
pragma solidity ^0.8.4;
function payFees() public payable {
require(msg.value >= 0.5 ether);
(bool success,) = owner.call{value: msg.value}("");
require(success, "Failed to send money");
}
OR
pragma solidity ^0.8.4;
function payFees() public payable {
require(msg.value >= 5 * 10**17);
(bool success,) = owner.call{value: msg.value}("");
require(success, "Failed to send money");
}

Answer 2

In solidity msg.value is not represented as Ether but as Wei a smaller denomination of ether you can think of it as ether with 18 decimals. Example:
1 ether is 1000000000000000000 Wei. Note that this applies for most chains to my knowledge BSC is also based on an 18 decimal system so 0.5BNB would be 500000000000000000 Wei this way you can check decimal values like this:
pragma solidity ^0.8.4;
function payFees() public payable {
require(msg.value >= 500000000000000000);
(bool success,) = owner.call{value: msg.value}("");
require(success, "Failed to send money");
}
Or:
pragma solidity ^0.8.4;
function payFees() public payable {
require(msg.value >= 0.5 ether);
(bool success,) = owner.call{value: msg.value}("");
require(success, "Failed to send money");
}
You can also take a look at this for a better demonstration of how this works.

Answer 3

Solidity works with integers. Why are there no decimal numbers in Solidity?
Uniswap created FixedPoint. you can use this function:
function fraction(uint256 numerator, uint256 denominator) internal pure returns (uq112x112 memory) {
require(denominator > 0, 'FixedPoint::fraction: division by zero');
if (numerator == 0) return FixedPoint.uq112x112(0);
if (numerator <= uint144(-1)) {
uint256 result = (numerator << RESOLUTION) / denominator;
require(result <= uint224(-1), 'FixedPoint::fraction: overflow');
return uq112x112(uint224(result));
} else {
uint256 result = FullMath.mulDiv(numerator, Q112, denominator);
require(result <= uint224(-1), 'FixedPoint::fraction: overflow');
return uq112x112(uint224(result));
}
}
in your solidity code:
using FixedPoint for *;
FixedPoint.fraction(numeratorValue,denominatorValue)