1. You should add a URL when you you create a post on HN. You can indent code two spaces on HN, eg:

  Stack s;
  stack_init(&s);
  dict_init();
  exec(&s, "10 20 + ."); // Prints "30"
  exec(&s, "1 2 3 4 .s"); // Shows stack contents

3. I'm not an expert in C but I thought header files shouldn't have code in them. The code should be in a .c file

4. Maybe move the code from USAGE into its own .c file.

  #include "stacklib.h"
  
  int main() {
    Stack s;
    stack_init(&s);
    dict_init();
    exec(&s, "10 20 + .");
    printf("\n");
    return 0;
  }

technically, "header only libraries" can be exceptions to C code not being in header files. See STB as an example https://github.com/nothings/stb. The advantage theoretically is that you can #include this library and use its code and types from just one file, its a decent model IMHO, but it can be jarring to someone unfamiliar with header only libraries.