CGO journey: from using to understanding it José Carlos Chávez @jcchavezs January Golang Meetup - GDG Berlin Golang

2 About me José Carlos Chávez - Software Engineer at Traceable.ai - Open source contributor in Observability projects - Maths student - Loving father @jcchavezs

What is CGO? Cgo lets Go packages call C code. Given a Go source file written with some special features, cgo outputs Go and C files that can be combined into a single Go package. Andrew Gerrand https://go.dev/blog/cgo 3 @jcchavezs Foto de Anne Nygård en Unsplash

There are many libraries written in C or C++. If we are willing to use them there are only a couple of options to go for: 1. Rewrite them in Go 2. Use them directly with CGO Why is CGO useful? 4 @jcchavezs

// source: main.go package main //int sum(int a, int b) { return a+b; } import "C" func main() { println(C.sum(1, 1)) } 5 @jcchavezs How does it look like?

// source: main.go package main // Linker Options: #cgo darwin LDFLAGS: -framework Cocoa -framework OpenGL -framework IOKit -framework CoreVideo // Linux Build Tags #cgo linux,!wayland CFLAGS: -D_GLFW_X11 -D_GLFW_GLX #cgo linux,wayland CFLAGS: -D_GLFW_WAYLAND -D_GLFW_EGL ... 6 @jcchavezs How does it really look like?

7 Looking inside

8 @jcchavezs What happens inside? $ go tool cgo main.go $ ls _obj | awk '{print $NF}' >>_cgo_.o >>_cgo_export.c >>_cgo_export.h >>_cgo_flags >>_cgo_gotypes.go >>_cgo_main.c >>main.cgo1.go >>main.cgo2.c

//main.cgo1.go package main //int sum(int a, int b) { return a+b; } import _ "unsafe" func main() { println(( /*line :11:13*/_Cfunc_sum /*line :11:17*/)(1, 1)) } 9 @jcchavezs What happens inside?

10 @jcchavezs Stack dance

Some people, when confronted with a problem, think “I know, I’ll use cgo.” Now they have two problems. Dave Cheney - cgo is not Go 11 @jcchavezs

- Programming gotchas - Build gotchas - Runtime gotchas - Tooling gotchas Gotchas 12 @jcchavezs

Programming gotchas 13 Manual memory management - Go is a garbage-collected runtime, but C is not. - Copying data aren’t always avoidable and eats up CPU. - Slices can be cumbersome due to its lifecycle. Data conversion - Most of the types are 1-1 mapping but some special cases use unsafe.pointer (e.g. string). - Special types lead to manual memory management. - C type used in one Go package is different from the same C type used in another. @jcchavezs

Programming gotchas 14 Passing a function pointer to C Callbacks’ overhead: Go code may pass a Go pointer to C provided the Go memory to which it points does not contain any Go pointers. See: https://golang.org/cmd/cgo/#hdr-Passing_pointers Cgoroutines != goroutines A blocking cgo call occupies a system thread. Go runtime can’t schedule them like it can a goroutine: Kb vs Mb in allocations. More: http://bit.ly/cgo-bench-test @jcchavezs

Build gotchas 15 Complicated builds - The build process is not anymore self contained: e.g. go get, go build and go test now require preconditions (e.g. ldflags, library location, etc) - Build caching mechanics will impose a structure for C/library code. - Loading libraries is cumbersome. The location of the library can be a pain depending on the nature of the source code: - Go applications - Go libraries Slower build times - The cgo tool is invoked to generate the C to Go and Go to C thunks and stubs. - Your system C compiler has to be invoked for every C file in the package. - The individual compilation units are combined together into a single .o file. - The resulting .o file take a trip through the system linker for fix-ups against shared objects they reference. @jcchavezs

Build gotchas 16 Static builds - The premise “single, static binary” isn’t easily achievable. - Unless static libraries you can’t guarantee 100% consistency between test - build - run. (Lack of) Cross compilation - By default CGO is disabled when cross compiling. You can still enable it passing a C cross-compiler for each GOOS+GOARCH - One extra degree of compatibility is added by the C library you are loading. @jcchavezs

Runtime gotchas 17 Dynamic linking - Usage of different libraries for different architectures (e.g. amd64, arm64). Not even talking about distros. - If a library isn’t installed in the host we need to link the library in runtime using LD_LIBRARY_PATH or a CWD relative location (predefined in the code). - It is possible to link the library after a lookup but that requires a manual loading (see example) Call overhead - CGO calls are expensive compared to a call within Go, although the overhead is negligible (see this benchmark) @jcchavezs

Tooling gotchas 18 Debugging - The portions residing in C aren’t as readily accessed through Go’s tooling. - Race detector, pprof for profiling code, coverage, fuzz testing, and source code analysis tools don’t work across the board @jcchavezs

19 @jcchavezs Summary - CGO is not a union, is an intersection of C and Go worlds. - Using libraries introduce another layer of complexity, they don’t come for free. - CGO was a foundational piece in Go evolution.

20 References ● https://blog.marlin.org/cgo-referencing-c-library-in-go ● https://pkg.go.dev/cmd/cgo#hdr-Passing_pointers ● https://www.programmerall.com/article/2072595424/ ● https://medium.com/mysterium-network/golang-c-interopera bility-caf0ba9f7bf3 ● https://www.cockroachlabs.com/blog/the-cost-and-complexi ty-of-cgo/ ● https://honnef.co/posts/2015/06/statically_compiled_go_p rograms__always__even_with_cgo__using_musl/ @jcchavezs