Working with memory (malloc, etc.) in smart contracts

Question

I'm trying to compile smart contracts written in Rust. So far so good, but when I tried to add memory management contract deployment choked.

(type (;0;) (func (param i32) (result i32)))
(import "env" "malloc" (func $malloc (type 0)))

This line in webassembly makes contract deployment fail with message

2663336ms thread-0   http_plugin.cpp:406 handle_exception] Exception Details: 10 assert_exception: Assert Exception
!"unresolvable": env.malloc
{"module":"env","export":"malloc"}
thread-0  wasm_interface.hpp:41 resolve

{"mod_name":"env","export_name":"malloc"}
thread-0  wasm_interface.hpp:43 resolve

{"_pending_console_output.str()":""}
thread-0  apply_context.cpp:62 exec_one

Although this method is listed in c function reference https://developers.eos.io/eosio-cpp/reference#memory

Other methods like printi and action_data_size works fine. So my question is how do I work with memory in smart contracts and how do I know which foreign functions are in "env" module and which are not?

Answer 1

Answering my own question:

The problem is that part of C API is external to webassembly (like printui, etc) - so these are imported. However memory management is a thing that is compiled to wasm - so malloc and other stuff will appear in your wasm contract code. That's why you don't need to count on EOS functions on that, but rather use your own memory allocator