How do you find a matching IDL instruction using the Anchor instruction discriminator from instruction data?

Question

How are the first 8 bytes (discriminator) on instruction data determined for an anchor instruciton? If I have the idl what's the best way to use instruction data to find which instruction in the IDL it is?

I'm seeing random bytes like 51, 179, 232, 242

Answer 1

You have to get the first 8 bytes of the sha256 hash of the global:[instruction_name].

From the anchor source

// Namespace for calculating instruction sighash signatures for any instruction
// not affecting program state.
pub const SIGHASH_GLOBAL_NAMESPACE: &str = "global";

// We don't technically use sighash, because the input arguments aren't given.
// Rust doesn't have method overloading so no need to use the arguments.
// However, we do namespace methods in the preeimage so that we can use
// different traits with the same method name.
pub fn sighash(namespace: &str, name: &str) -> [u8; 8] {
    let preimage = format!("{}:{}", namespace, name);

    let mut sighash = [0u8; 8];
    sighash.copy_from_slice(&crate::hash::hash(preimage.as_bytes()).to_bytes()[..8]);
    sighash
}

The same logic applies to the 8 byte discriminator for accounts, you take the first 8 bytes of the sha256 hash of account:[AccountStructName]