# Units and Globally Available Variables

## Units and Globally Available Variables

### Ether Units

A literal number can take a suffix of `wei`, `finney`, `szabo` or `ether` to specify a subdenomination of Ether, where Ether numbers without a postfix are assumed to be Wei.

```
assert(1 wei == 1);
assert(1 szabo == 1e12);
assert(1 finney == 1e15);
assert(1 ether == 1e18);
```

The only effect of the subdenomination suffix is a multiplication by a power of ten.

### Time Units

Suffixes like `seconds`, `minutes`, `hours`, `days` and `weeks` after literal numbers can be used to specify units of time where seconds are the base unit and units are considered naively in the following way:

> * `1 == 1 seconds`
> * `1 minutes == 60 seconds`
> * `1 hours == 60 minutes`
> * `1 days == 24 hours`
> * `1 weeks == 7 days`

Take care if you perform calendar calculations using these units, because not every year equals 365 days and not even every day has 24 hours because of [leap seconds](https://en.wikipedia.org/wiki/Leap_second). Due to the fact that leap seconds cannot be predicted, an exact calendar library has to be updated by an external oracle.

{% hint style="info" %}
The suffix `years` has been removed in version 0.5.0 due to the reasons above.

These suffixes cannot be applied to variables. For example, if you want to interpret a function parameter in days, you can in the following way:
{% endhint %}

```
function f(uint start, uint daysAfter) public {
    if (now >= start + daysAfter * 1 days) {
      // ...
    }
}
```

### Special Variables and Functions

There are special variables and functions which always exist in the global namespace and are mainly used to provide information about the blockchain or are general-use utility functions.

#### Block and Transaction Properties

* `blockhash(uint blockNumber) returns (bytes32)`: hash of the given block - only works for 256 most recent, excluding current, blocks
* `block.coinbase` (`address payable`): current block miner’s address
* `block.difficulty` (`uint`): current block difficulty
* `block.gaslimit` (`uint`): current block gaslimit
* `block.number` (`uint`): current block number
* `block.timestamp` (`uint`): current block timestamp as seconds since unix epoch
* `gasleft() returns (uint256)`: remaining gas
* `msg.data` (`bytes calldata`): complete calldata
* `msg.sender` (`address payable`): sender of the message (current call)
* `msg.sig` (`bytes4`): first four bytes of the calldata (i.e. function identifier)
* `msg.value` (`uint`): number of wei sent with the message
* `now` (`uint`): current block timestamp (alias for `block.timestamp`)
* `tx.gasprice` (`uint`): gas price of the transaction
* `tx.origin` (`address payable`): sender of the transaction (full call chain)

{% hint style="info" %}
The values of all members of `msg`, including `msg.sender` and `msg.value` can change for every **external** function call. This includes calls to library functions.
{% endhint %}

{% hint style="info" %}
Do not rely on `block.timestamp`, `now` and `blockhash` as a source of randomness, unless you know what you are doing.

Both the timestamp and the block hash can be influenced by miners to some degree. Bad actors in the mining community can for example run a casino payout function on a chosen hash and just retry a different hash if they did not receive any money.

The current block timestamp must be strictly larger than the timestamp of the last block, but the only guarantee is that it will be somewhere between the timestamps of two consecutive blocks in the canonical chain.
{% endhint %}

{% hint style="info" %}
The block hashes are not available for all blocks for scalability reasons. You can only access the hashes of the most recent 256 blocks, all other values will be zero.
{% endhint %}

{% hint style="info" %}
The function `blockhash` was previously known as `block.blockhash`. It was deprecated in version 0.4.22 and removed in version 0.5.0.
{% endhint %}

{% hint style="info" %}
The function `gasleft` was previously known as `msg.gas`. It was deprecated in version 0.4.21 and removed in version 0.5.0.
{% endhint %}

#### ABI Encoding and Decoding Functions

* `abi.decode(bytes memory encodedData, (...)) returns (...)`: ABI-decodes the given data, while the types are given in parentheses as second argument. Example: `(uint a, uint[2] memory b, bytes memory c) = abi.decode(data, (uint, uint[2], bytes))`
* `abi.encode(...) returns (bytes memory)`: ABI-encodes the given arguments
* `abi.encodePacked(...) returns (bytes memory)`: Performs [packed encoding](https://psolidity.readthedocs.io/en/latest/abi-spec.html#abi-packed-mode) of the given arguments
* `abi.encodeWithSelector(bytes4 selector, ...) returns (bytes memory)`: ABI-encodes the given arguments starting from the second and prepends the given four-byte selector
* `abi.encodeWithSignature(string memory signature, ...) returns (bytes memory)`: Equivalent to `` abi.encodeWithSelector(bytes4(keccak256(bytes(signature))), ...)` ``

{% hint style="info" %}
These encoding functions can be used to craft data for external function calls without actually calling an external function. Furthermore, `keccak256(abi.encodePacked(a, b))` is a way to compute the hash of structured data (although be aware that it is possible to craft a “hash collision” using different function parameter types).
{% endhint %}

See the documentation about the [ABI](https://psolidity.readthedocs.io/en/latest/abi-spec.html#abi) and the [tightly packed encoding](https://psolidity.readthedocs.io/en/latest/abi-spec.html#abi-packed-mode) for details about the encoding.

#### Error Handling

See the dedicated section on [assert and require](https://psolidity.readthedocs.io/en/latest/control-structures.html#assert-and-require) for more details on error handling and when to use which function.`assert(bool condition)`:causes an invalid opcode and thus state change reversion if the condition is not met - to be used for internal errors.`require(bool condition)`:reverts if the condition is not met - to be used for errors in inputs or external components.`require(bool condition, string memory message)`:reverts if the condition is not met - to be used for errors in inputs or external components. Also provides an error message.`revert()`:abort execution and revert state changes`revert(string memory reason)`:abort execution and revert state changes, providing an explanatory string

#### Mathematical and Cryptographic Functions

`addmod(uint x, uint y, uint k) returns (uint)`:compute `(x + y) % k` where the addition is performed with arbitrary precision and does not wrap around at `2**256`. Assert that `k != 0` starting from version 0.5.0.`mulmod(uint x, uint y, uint k) returns (uint)`:compute `(x * y) % k` where the multiplication is performed with arbitrary precision and does not wrap around at `2**256`. Assert that `k != 0` starting from version 0.5.0.`keccak256(bytes memory) returns (bytes32)`:compute the Keccak-256 hash of the input`sha256(bytes memory) returns (bytes32)`:compute the SHA-256 hash of the input`ripemd160(bytes memory) returns (bytes20)`:compute RIPEMD-160 hash of the input`ecrecover(bytes32 hash, uint8 v, bytes32 r, bytes32 s) returns (address)`:recover the address associated with the public key from elliptic curve signature or return zero on error ([example usage](https://ethereum.stackexchange.com/q/1777/222))

{% hint style="info" %}
Function `ecrecover` returns an `address`, and not an `address payable`. See [address payable](https://psolidity.readthedocs.io/en/latest/types.html#address) for conversion, in case you need to transfer funds to the recovered address.

It might be that you run into Out-of-Gas for `sha256`, `ripemd160` or `ecrecover` on a *private blockchain*. The reason for this is that those are implemented as so-called precompiled contracts and these contracts only really exist after they received the first message (although their contract code is hardcoded). Messages to non-existing contracts are more expensive and thus the execution runs into an Out-of-Gas error. A workaround for this problem is to first send e.g. 1 Wei to each of the contracts before you use them in your actual contracts. This is not an issue on the official or test net.
{% endhint %}

{% hint style="info" %}
There used to be an alias for `keccak256` called `sha3`, which was removed in version 0.5.0.
{% endhint %}

#### Members of Address Types

`.balance` (`uint256`):balance of the [Address](https://psolidity.readthedocs.io/en/latest/types.html#address) in Wei `payable>.transfer(uint256 amount)`:send given amount of Wei to [Address](https://psolidity.readthedocs.io/en/latest/types.html#address), reverts on failure, forwards 2300 gas stipend, not adjustable `payable>.send(uint256 amount) returns (bool)`:send given amount of Wei to [Address](https://psolidity.readthedocs.io/en/latest/types.html#address), returns `false` on failure, forwards 2300 gas stipend, not adjustable`.call(bytes memory) returns (bool, bytes memory)`:issue low-level `CALL` with the given payload, returns success condition and return data, forwards all available gas, adjustable`.delegatecall(bytes memory) returns (bool, bytes memory)`:issue low-level `DELEGATECALL` with the given payload, returns success condition and return data, forwards all available gas, adjustable`.staticcall(bytes memory) returns (bool, bytes memory)`:issue low-level `STATICCALL` with the given payload, returns success condition and return data, forwards all available gas, adjustable

For more information, see the section on [Address](https://psolidity.readthedocs.io/en/latest/types.html#address).

{% hint style="warning" %}
There are some dangers in using `send`: The transfer fails if the call stack depth is at 1024 (this can always be forced by the caller) and it also fails if the recipient runs out of gas. So in order to make safe Ether transfers, always check the return value of `send`, use `transfer` or even better: Use a pattern where the recipient withdraws the money.
{% endhint %}

{% hint style="info" %}
Prior to version 0.5.0, Solidity allowed address members to be accessed by a contract instance, for example `this.balance`. This is now forbidden and an explicit conversion to address must be done: `address(this).balance`.
{% endhint %}

{% hint style="info" %}
If state variables are accessed via a low-level delegatecall, the storage layout of the two contracts must align in order for the called contract to correctly access the storage variables of the calling contract by name. This is of course not the case if storage pointers are passed as function arguments as in the case for the high-level libraries.
{% endhint %}

{% hint style="info" %}
Prior to version 0.5.0, `.call`, `.delegatecall` and `.staticcall` only returned the success condition and not the return data.
{% endhint %}

{% hint style="info" %}
Prior to version 0.5.0, there was a member called `callcode` with similar but slightly different semantics than `delegatecall`.
{% endhint %}