library C {
   function a() public returns (uint) {
      return 15;

contract A { 
   function a() public returns (uint) {
      return C.a();

This is the code I am using. I am trying to figure out how library C ends up in contract A's bytecode.

Question 1) I use remix and I compile contract A. In the bytecode, I can see the following __$de3f906ec3d3531c3d498f8b283d7f02e6$__. It doesn't matter if I change library C's function a's name or if I change what it returns. In the bytecode, there's always the same __$de3f906ec3d3531c3d498f8b283d7f02e6$__. Of course, If I use the library's function 2 times, then I will meet that strange string 2 times in the bytecode. What is it ?

Question 2) The answer to the above question could be it's a placeholder, where in the end, it's library's address that ends up in there. If that's correct, then how does remix link library C to contract A ? because both library and contract are in the same file, and when I want to deploy, I only deploy contract A (i don't deploy or link library at all). Any idea ? How do I get the library address in that matter ?

1 Answer 1


__$de3f906ec3d3531c3d498f8b283d7f02e6$__ is a placeholder for library address.

The deployment process of smart contracts look somewhat like this

  • Library is compiled
  • Library is deployed, gets an address
  • Contract using library is compiled
  • Contract is "linked" - all placeholders in the bytecode are replaced with the library address
  • Contract is deployed

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