How to use a CString with a custom program?

Question

I am currently writing a frontend for my custom on-chain program. The frontend is supposed to take input from a form and then submit the data to the program. Currently I'm following this tutorial (https://docs.solana.com/developing/clients/javascript-api#interacting-with-custom-programs) to write the frontend code.

The issue I'm running into is in composing my struct for the layout of the transaction. My struct is created thusly:

let starshipStruct = {
    index: 1,
    layout: struct([
      u32('instruction'),
      cstr('container'),
      u16('hours'),
    ]),
  };

When I attempt to allocate a Buffer for this struct, I receive an error that the size cannot be -1. I've determined the issue is with the cstr function which is returning a value of -1. How would one set the size for this function? I'm using the @solana/buffer-layout library for these functions. Thanks in advance!

Answer 1

The size of a string in C is roughly "keep going until you see a 0 byte", which doesn't work when you need to know the exact size. This is why you're seeing an error.

You must pre-allocate the space required, wither either the size of an existing string, or a fixed number of bytes. If you do 50 bytes for your string, you can do something like:

let starshipStruct = {
    index: 1,
    layout: struct([
      u32('instruction'),
      seq(u8(), 50),
      u16('hours'),
    ]),
  };

and then interpret those 50 bytes as a utf-8 string in your program.