Generate random numbers in the smart contract

Question

I am writing smart contract and stucked on this line :

int p = 0;
p = (rand() % 997);

This is giving error

error: use of undeclared identifier 'rand'

I do search and got some reference links but not getting expected answers. Some old answers reference links are also not valid now. Can somebody please help to generate random numbers in a smart contract?

Answer 1

Not safe solution for safe random number oracle rng should be used.

Table

   struct [[eosio::table]] seed{
      uint64_t key = 1;
      uint32_t value = 1;
      auto primary_key() const { return key; }
   };
   typedef eosio::multi_index<"seed"_n, seed> seed_table;

Action in .ccp

int contractclass::random(const int range)
{
   auto _s = seed_table(_self, get_self().value);
   // Find the existing seed
   auto seed_iterator = _s.begin();

   // Initialize the seed with default value if it is not found
   if (seed_iterator == _s.end())
   {
      seed_iterator = _s.emplace(_self, [&](auto &seed) {});
   }
   auto new_seed_value = (seed_iterator->value + current_time_point().elapsed.count()) % current_time_point().elapsed.count();

   // Store the updated seed value in the table
   _s.modify(seed_iterator, _self, [&](auto &s)
             { s.value = new_seed_value; });
   int random_result = new_seed_value % range;
   return random_result;
}

Usage

int rng = random(100);