While not all (if any) libraries available for StarkNet's contract have been audited, it is important to make use of battle tested and trustworthy libraries. StarkNet has a pretty tight community, so when in doubt, ask on Discord, on one of the many telegram groups or [Newton](https://www.newton.wtf/), and other devs will be able to guide you on choosing the best libraries. 

As usual, the most reputable and useful collection of libraries is OpenZeppelin's [cairo-contract](https://github.com/OpenZeppelin/cairo-contracts). For any other need, have a look at [Cairo Goldmine](https://github.com/beautyisourbusiness/cairo-goldmine).

As the ecosystem matures, we will surely see more and more libraries being audited and this will allow a more standardised way of writing smart contracts. In the meantime, when in doubt, always ask your fellow developers/security engineers.

ctrlc03

What libraries are secure to use in the Cairo language? How to know which library is audited? 

Newton

What libraries are secure to use in Cairo?

Can anyone share some articles explaining SNARK/STARK concepts and rollups (Optimism)?

_This question was originally posted on [StarkNet Discord](https://www.newton.so/view?tags=discord)_

Can anyone share some articles explaining SNARK/STARK concepts and rollups (Optimism)?

>Hello! This is day 5 of the 17 days of the Cairo Challenge. And I have no idea how to solve the [playground](https://www.cairo-lang.org/playground/) exercise “Revoked references”. Can you run these 6 steps and write down what you see?

```
// The scope of some types of references in Cairo (e.g., return values of
// functions and temporary variables) is somewhat restricted.
// For example, a call to another function may revoke those references.
//
// 1. Try to run the following code. You should get an error that `x` was revoked
//    (this is because of the second call to `foo`).
// 2. In simple cases, like this one, the compiler can solve this automatically
//    if the "alloc_locals" keyword is used within the function's scope.
//    Add "alloc_locals;" to the beginning of main(), and make sure the code works.
// 3. To understand what happens behind the scenes, replace "alloc_locals;" with
//    "ap += SIZEOF_LOCALS;" - this command allocates memory cells for local variables
//    (you'll need them below) but doesn't enable the mechanism that
//    automatically resolves revoked references.
//    Trying to run the code now should result in the original error.
// 4. Solve the problem manually by copying the value of `x` to a local variable
//    (e.g., "local x2 = x;"). Local variables are not revoked when functions
//    are called.
//    You can read more about local variables [here](https://www.cairo-lang.org/docs/how_cairo_works/consts.html#local-vars).
// 5. You can even use the same name: "local x = x;".
//    This is called "reference rebinding", where the meaning of `x` changes:
//      Before this line, `x` refers to the return value.
//      After that, `x` refers to the local variable.
// 6. A shorter syntax is "let (local x) = foo(10);" which achieves the same goal.
//    Try it!

%builtins output

from starkware.cairo.common.serialize import serialize_word

// Returns a^3 for a != 0 and 1 otherwise.
func foo(a) -> (res: felt) {
    if (a == 0) {
        return (res=1);
    } else {
        return (res=a * a * a);
    }
}

// Outputs the value 10^3 + 5^3.
func main{output_ptr: felt*}() {
    let (x) = foo(10);
    let (y) = foo(5);
    serialize_word(x + y);
    return ();
}
```

---

> Answers to this question are a part of the ✨ 17 days of Cairo Lang with Playground & Newton. ✨
> 

*Vote for your favorite answer - the best answer will win a $10 award. A new day – a new reward! During the next 17 days, our goal is to attract more developers to the Cairo language and to systematize the knowledge of Cairo lang. [Read rules](https://www.notion.so/17-days-of-Cairo-Lang-with-Newton-550dfcda6a2d449ca468be293c776578)*



Cairo Lang / StarkNet: What are Revoked references? What is alloc_locals?

How do I use keccak functions in Cairo? I am getting the error `Unknown identifier 'bitwise_ptr'` 


ㅤ
_This question was originally posted on [Triality](https://triality.org/question/662)_

Cairo error: Unknown identifier 'bitwise_ptr' when using keccak

Are you able to nest mappings in Cairo like you can in Solidity?

e.g. in Solidity you can do
```solidity
mapping(address => mapping(uint256 => bool));
```
is this doable in Cairo? I want to do something similar to

```cairo

func mappingA(value: felt) -> func mappingB(value: felt)  -> (value: felt){
}
```

mapping(x, y) -> (z) is not gonna work, here is some context:

I'm working through the sample ERC721_Metadata.cairo and ERC721_Metadata_Base contract shown here https://github.com/starknet-edu/starknet-erc721/tree/main/contracts/token/ERC721

It looks like from what I can tell the tokenUri is set for each domain, and even though calling the @view function does take in a tokenId, calling to set the tokenUri does not and it's not clear to me how that mapping could exist otherwise. So I wanted to modify the contract to have a token_id -> token_uri mapping. Because of the way those mappings exist though, you need to provide both the token_uri_len: felt and token_uri: felt*  when writing them. This means it can only be done once, otherwise you'd need a specific @storage_var for every token that's minted, just to hold it's tokenuri. I was hoping to do something like 

```cairo
// Represents a tokenUri
struct TokenUri {
    // The length of the token bytes
    token_uri_len: felt,
    // The pointer to the beginning of the token uri bytes
    token_uri: felt*,
}

@storage_var
func token_uris(token_id: felt) -> (token_uri: TokenUri) {
}
```


But because of the token_url: felt*  pointer in that struct, it can't be used as a storage variable return type. It gives an error when I try to compile showing
```cairo
starknet_contracts/ERC721_Metadata.cairo:29:36: The return type of storage variables must be a felts-only type (cannot contain pointers).
func token_uris(token_id: felt) -> (token_uri: TokenUri) {
```

_This question was originally posted on [StarkNet Discord](https://www.newton.so/view?tags=discord)_

Are you able to nest mappings in Cairo like you can in Solidity?

Hey guys, has anyone had `get_caller_address` problems in devnet ? I feel like it's returning address 0 when it shouldn't so I made this small contract to test it

```%lang starknet

from starkware.cairo.common.cairo_builtins import HashBuiltin

from starkware.starknet.common.syscalls import (get_caller_address)


func caller_addr() -> (res: felt) {
}



func fn_to_be_called{syscall_ptr: felt*, pedersen_ptr: HashBuiltin*, range_check_ptr: felt}() {
    let (caller) = get_caller_address();
    caller_addr.write(caller);    

    return ();
}


func get_caller_addr{syscall_ptr: felt*, pedersen_ptr: HashBuiltin*, range_check_ptr: felt}() -> (caller_addr: felt) {
    let (caller) = caller_addr.read();

    return (caller_addr=caller);
}```

and indeed `get_caller_addr` return address 0 after I called `fn_to_be_called` in the devnet. This is causing some problems in more complex contracts of course. Is it just me ?



_This question was originally posted on [StarkNet Discord](https://www.newton.so/view?tags=discord)_

Has anyone had `get_caller_address` problems in devnet ?

There are many people who are pretty new to Cairo Language, so this thread is about some good resources, to begin with. What projects, educational courses, Github Repos etc. about StarkNet can you recommend?

---
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What resources (Github repos, projects) do you follow about Cairo Language? Why?

I'm looking for a StarkNet guide. I'm an experienced CTO and I want to know more about it, check out, and learn more. Also want to get hyped. What can you recommend to read to learn more about StarkNet?

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