I can't understand why actions can't return values.

A simple use case is for adding records in multi_index.

I need to return the primary key to link that to my off-chain data.

2 Answers 2


Your smart contract can have as many functions as you like. They don't all have to be actions. Only actions are required to return void, every other function can return whatever you like.

If you need to return the data from within an action to an off chain process, then you can create a new action that is empty, but contains the appropriate information in the arguments. This way you can store the primary key of a new table entry by using a history plugin on nodeos to read the argument of the new action. Example:

void mycontract::createentry(\*arguments here\*)
  uint64_t primary_key(0);
  // Make table entry here and change primary key to match table entry

void mycontract::entryresult(uint64_t primary_key){ }
  • I think the question relates to the notion that the contract is generating a primary key in the action and they would like that returned when the action is called.
    – Jason Bert
    Apr 7, 2019 at 13:04
  • Oh I see. I will update my answer accordingly Apr 8, 2019 at 7:21
  • it's just a strange design choice that I can't wrap my head around. I'm considering going into two directions at the moment: 1.writing primary key with eosio::print and pickcing it up in the console - this is just a nasty hack 2. generating primary key off-chain
    – Katalyo
    Apr 8, 2019 at 19:12

In your example, your best bet is to generate the primary key off chain and send it as part of the transaction to the action. It has the benefit of giving you more flexibility of the uniqueness of it (e.g. a GUID) and will consume less resources on chain. The contract can still give you the robust check that any primary key passed in is unique.

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