Go object files

C developers will be familiar with compiling C source files (.c) into object files (.o) before linking them into their final form. Did you know you can do the same thing with the Go compiler as well? This project is a quick demonstration of how to turn main.go into main.o!

The files

This project contains the following Go sources:

• main.go: a simple Go program for summing two integers
• sum.go: the function SumInt used by main.go

The program

Of course it is possible to build this project with go build:

go build -o sum

and then use the resulting binary executable to sum two integers:

$ ./sum 2 3
5

Object files

However, what does go build actually do? Well, one way to figure this out is by using go build -x to print the commands that go build executes. One such step is to produce object files for the Go sources:

go tool compile sum.go

The above command will produce the object file sum.o:

$ ls *.o
sum.o

But what happens when we try to create the object file for main.go?

$ go tool compile main.go
main.go:37:14: undefined: SumInt

In fact, this is the same error we would receive if we tried to use go run to run the program with just main.go:

$ go run main.go 2 3
# command-line-arguments
./main.go:37:14: undefined: SumInt

In order to create the object file or compile the program, Go has to create a dependency graph for all the files that depend on one another. The symbol used by main.go, SumInt, is in the file sum.go. If we were to compile main.o using both main.go and sum.go, then it would work.

First let's clean up the previous object file we created:

rm -f sum.o

Now let's compile main.go into an object file:

go tool compile main.go sum.go

Verify the result:

$ ls *.o
main.o

And main.o is the only object file created.

Conclusion

This project does not dig into a fraction of what is possible with go tool compile or look at go tool link at all. However, most developers will never need to manually execute these commands. That's the beauty of go build. Still, beauty is not only skin-deep, it's what is on the inside that counts, and understanding how go build works is a great goal to have!