libraries.rst

.. index:: ! library, callcode, delegatecall

Libraries

Libraries are similar to contracts, but their purpose is that they are deployed only once at a specific address and their code is reused using the DELEGATECALL (CALLCODE until Homestead) feature of the EVM. This means that if library functions are called, their code is executed in the context of the calling contract, i.e. this points to the calling contract, and especially the storage from the calling contract can be accessed. As a library is an isolated piece of source code, it can only access state variables of the calling contract if they are explicitly supplied (it would have no way to name them, otherwise). Library functions can only be called directly (i.e. without the use of DELEGATECALL) if they do not modify the state (i.e. if they are view or pure functions), because libraries are assumed to be stateless. In particular, it is not possible to destroy a library.

Note

Until version 0.4.20, it was possible to destroy libraries by circumventing Solidity's type system. Starting from that version, libraries contain a :ref:`mechanism<call-protection>` that disallows state-modifying functions to be called directly (i.e. without DELEGATECALL).

Libraries can be seen as implicit base contracts of the contracts that use them. They will not be explicitly visible in the inheritance hierarchy, but calls to library functions look just like calls to functions of explicit base contracts (using qualified access like L.f()). Of course, calls to internal functions use the internal calling convention, which means that all internal types can be passed and types :ref:`stored in memory <data-location>` will be passed by reference and not copied. To realize this in the EVM, the code of internal library functions that are called from a contract and all functions called from therein will at compile time be included in the calling contract, and a regular JUMP call will be used instead of a DELEGATECALL.

Note

The inheritance analogy breaks down when it comes to public functions. Calling a public library function with L.f() results in an external call (DELEGATECALL to be precise). In contrast, A.f() is an internal call when A is a base contract of the current contract.

.. index:: using for, set

The following example illustrates how to use libraries (but using a manual method, be sure to check out :ref:`using for <using-for>` for a more advanced example to implement a set).

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.6.0 <0.9.0;


// We define a new struct datatype that will be used to
// hold its data in the calling contract.
struct Data {
    mapping(uint => bool) flags;
}

library Set {
    // Note that the first parameter is of type "storage
    // reference" and thus only its storage address and not
    // its contents is passed as part of the call.  This is a
    // special feature of library functions.  It is idiomatic
    // to call the first parameter `self`, if the function can
    // be seen as a method of that object.
    function insert(Data storage self, uint value)
        public
        returns (bool)
    {
        if (self.flags[value])
            return false; // already there
        self.flags[value] = true;
        return true;
    }

    function remove(Data storage self, uint value)
        public
        returns (bool)
    {
        if (!self.flags[value])
            return false; // not there
        self.flags[value] = false;
        return true;
    }

    function contains(Data storage self, uint value)
        public
        view
        returns (bool)
    {
        return self.flags[value];
    }
}


contract C {
    Data knownValues;

    function register(uint value) public {
        // The library functions can be called without a
        // specific instance of the library, since the
        // "instance" will be the current contract.
        require(Set.insert(knownValues, value));
    }
    // In this contract, we can also directly access knownValues.flags, if we want.
}

Of course, you do not have to follow this way to use libraries: they can also be used without defining struct data types. Functions also work without any storage reference parameters, and they can have multiple storage reference parameters and in any position.

The calls to Set.contains, Set.insert and Set.remove are all compiled as calls (DELEGATECALL) to an external contract/library. If you use libraries, be aware that an actual external function call is performed. msg.sender, msg.value and this will retain their values in this call, though (prior to Homestead, because of the use of CALLCODE, msg.sender and msg.value changed, though).

The following example shows how to use :ref:`types stored in memory <data-location>` and internal functions in libraries in order to implement custom types without the overhead of external function calls:

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

struct bigint {
    uint[] limbs;
}

library BigInt {
    function fromUint(uint x) internal pure returns (bigint memory r) {
        r.limbs = new uint[](1);
        r.limbs[0] = x;
    }

    function add(bigint memory a, bigint memory b) internal pure returns (bigint memory r) {
        r.limbs = new uint[](max(a.limbs.length, b.limbs.length));
        uint carry = 0;
        for (uint i = 0; i < r.limbs.length; ++i) {
            uint limbA = limb(a, i);
            uint limbB = limb(b, i);
            unchecked {
                r.limbs[i] = limbA + limbB + carry;

                if (limbA + limbB < limbA || (limbA + limbB == type(uint).max && carry > 0))
                    carry = 1;
                else
                    carry = 0;
            }
        }
        if (carry > 0) {
            // too bad, we have to add a limb
            uint[] memory newLimbs = new uint[](r.limbs.length + 1);
            uint i;
            for (i = 0; i < r.limbs.length; ++i)
                newLimbs[i] = r.limbs[i];
            newLimbs[i] = carry;
            r.limbs = newLimbs;
        }
    }

    function limb(bigint memory a, uint index) internal pure returns (uint) {
        return index < a.limbs.length ? a.limbs[index] : 0;
    }

    function max(uint a, uint b) private pure returns (uint) {
        return a > b ? a : b;
    }
}

contract C {
    using BigInt for bigint;

    function f() public pure {
        bigint memory x = BigInt.fromUint(7);
        bigint memory y = BigInt.fromUint(type(uint).max);
        bigint memory z = x.add(y);
        assert(z.limb(1) > 0);
    }
}

It is possible to obtain the address of a library by converting the library type to the address type, i.e. using address(LibraryName).

As the compiler does not know the address where the library will be deployed, the compiled hex code will contain placeholders of the form __$30bbc0abd4d6364515865950d3e0d10953$__. The placeholder is a 34 character prefix of the hex encoding of the keccak256 hash of the fully qualified library name, which would be for example libraries/bigint.sol:BigInt if the library was stored in a file called bigint.sol in a libraries/ directory. Such bytecode is incomplete and should not be deployed. Placeholders need to be replaced with actual addresses. You can do that by either passing them to the compiler when the library is being compiled or by using the linker to update an already compiled binary. See :ref:`library-linking` for information on how to use the commandline compiler for linking.

In comparison to contracts, libraries are restricted in the following ways:

• they cannot have state variables
• they cannot inherit nor be inherited
• they cannot receive Ether
• they cannot be destroyed

(These might be lifted at a later point.)

.. index:: ! selector; of a library function

Function Signatures and Selectors in Libraries

While external calls to public or external library functions are possible, the calling convention for such calls is considered to be internal to Solidity and not the same as specified for the regular :ref:`contract ABI<ABI>`. External library functions support more argument types than external contract functions, for example recursive structs and storage pointers. For that reason, the function signatures used to compute the 4-byte selector are computed following an internal naming schema and arguments of types not supported in the contract ABI use an internal encoding.

The following identifiers are used for the types in the signatures:

• Value types, non-storage string and non-storage bytes use the same identifiers as in the contract ABI.
• Non-storage array types follow the same convention as in the contract ABI, i.e. <type>[] for dynamic arrays and <type>[M] for fixed-size arrays of M elements.
• Non-storage structs are referred to by their fully qualified name, i.e. C.S for contract C { struct S { ... } }.
• Storage pointer mappings use mapping(<keyType> => <valueType>) storage where <keyType> and <valueType> are the identifiers for the key and value types of the mapping, respectively.
• Other storage pointer types use the type identifier of their corresponding non-storage type, but append a single space followed by storage to it.

The argument encoding is the same as for the regular contract ABI, except for storage pointers, which are encoded as a uint256 value referring to the storage slot to which they point.

Similarly to the contract ABI, the selector consists of the first four bytes of the Keccak256-hash of the signature. Its value can be obtained from Solidity using the .selector member as follows:

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.5.14 <0.9.0;

library L {
    function f(uint256) external {}
}

contract C {
    function g() public pure returns (bytes4) {
        return L.f.selector;
    }
}

Call Protection For Libraries

As mentioned in the introduction, if a library's code is executed using a CALL instead of a DELEGATECALL or CALLCODE, it will revert unless a view or pure function is called.

The EVM does not provide a direct way for a contract to detect whether it was called using CALL or not, but a contract can use the ADDRESS opcode to find out "where" it is currently running. The generated code compares this address to the address used at construction time to determine the mode of calling.

More specifically, the runtime code of a library always starts with a push instruction, which is a zero of 20 bytes at compilation time. When the deploy code runs, this constant is replaced in memory by the current address and this modified code is stored in the contract. At runtime, this causes the deploy time address to be the first constant to be pushed onto the stack and the dispatcher code compares the current address against this constant for any non-view and non-pure function.

This means that the actual code stored on chain for a library is different from the code reported by the compiler as deployedBytecode.