What is the first 8 bytes of the instruction's data ?

[pooriagg]

In anchor programs, first 8 bytes of the account's data is the discriminator that fine but what is the first 8 bytes of the instruction's data when interaction with the anchor program's instruction ?! how we can compute that ?

[buffalojoec]

It's also an 8-byte discriminator!

It's calculated like this:

anchor/lang/syn/src/codegen/program/dispatch.rs

Lines 20 to 26 in 6cf2004

	instruction::#ix_name_camel::DISCRIMINATOR => {
	__private::__global::#ix_method_name(
	program_id,
	accounts,
	ix_data,
	)
	}

So in other words, it's the first 8 bytes of the string "global:{ix_name}" where ix_name is the name of your instruction function.

[pooriagg]

thx for response, but my instruction's (method) name is "update" and the first 8 bytes of the data that calls that is [219, 200, 88, 176, 158, 63, 253, 127] can you just show a real example ?!

[buffalojoec]

let ix_name = "update";
let preimage = format!("global:{ix_name}");
let hash = solana_program::hash::hash(preimage.as_bytes()); // SHA256
let discriminator = &hash.to_bytes()[..8];
println!("discriminator: {:?}", discriminator);

[pooriagg]

thx a lot