How to execute an object file: Part 2
Handling relocations
In the previous post, we learned how to parse an object file and import and execute some functions from it. However, the functions in our toy object file were simple and self-contained: they computed their output solely based on their inputs and didn't have any external code or data dependencies. In this post we will build upon the code from part 1, exploring additional steps needed to handle code with some dependencies.
As an example, we may notice that we can actually rewrite our add10 function using our add5 function:
obj.c:
int add5(int num)
{
return num + 5;
}
int add10(int num)
{
num = add5(num);
return add5(num);
}
Let's recompile the object file and try to use it as a library with our loader program:
$ gcc -c obj.c
$ ./loader
Executing add5...
add5(42) = 47
Executing add10...
add10(42) = 42
Whoa! Something is not right here. add5 still produces the correct result, but add10 does not . Depending on your environment and code composition, you may even see the loader program crashing instead of outputting incorrect results. To understand what happened, let's investigate the machine code generated by the compiler. We Continue reading
