Table of Contents:
Overview
Building a binary for coverage profiling
Running a coverage-instrumented binary
Working with coverage data files
Frequently Asked Questions
Resources
Glossary
Beginning in Go 1.20, Go supports collection of coverage profiles from applications and from integration tests, larger and more complex tests for Go programs.
Go provides easy-to-use support for collecting coverage profiles at the level of package unit tests via the “go test -coverprofile=... <pkg_target>” command. Starting with Go 1.20, users can now collect coverage profiles for larger integration tests: more heavy-weight, complex tests that perform multiple runs of a given application binary.
For unit tests, collecting a coverage profile and generating a report requires two steps: a go test -coverprofile=... run, followed by an invocation of go tool cover {-func,-html} to generate a report.
For integration tests, three steps are needed: a build step, a run step (which may involve multiple invocations of the binary from the build step), and finally a reporting step, as described below.
To build an application for collecting coverage profiles, pass the -cover flag when invoking go build on your application binary target. See the section below for a sample go build -cover invocation. The resulting binary can then be run using an environment variable setting to capture coverage profiles (see the next section on running).
During a given “go build -cover” invocation, the Go command will select packages in the main module for coverage profiling; other packages that feed into the build (dependencies listed in go.mod, or packages that are part of the Go standard library) will not be included by default.
For example, here is a toy program containing a main package, a local main-module package greetings and a set of packages imported from outside the module, including (among others) rsc.io/quote and fmt (link to full program).
$ cat go.mod
module mydomain.com
go 1.20
require rsc.io/quote v1.5.2
require (
golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c // indirect
rsc.io/sampler v1.3.0 // indirect
)
$ cat myprogram.go
package main
import (
"fmt"
"mydomain.com/greetings"
"rsc.io/quote"
)
func main() {
fmt.Printf("I say %q and %q\n", quote.Hello(), greetings.Goodbye())
}
$ cat greetings/greetings.go
package greetings
func Goodbye() string {
return "see ya"
}
$ go build -cover -o myprogram.exe .
$
If you build this program with the “-cover” command line flag and run it, exactly two packages will be included in the profile: main and mydomain.com/greetings; the other dependent packages will be excluded.
Users who want to have more control over which packages are included for coverage can build with the “-coverpkg” flag. Example:
$ go build -cover -o myprogramMorePkgs.exe -coverpkg=io,mydomain.com,rsc.io/quote . $
In the build above, the main package from mydomain.com as well as the rsc.io/quote and io packages are selected for profiling; since mydomain.com/greetings isn't specifically listed, it will be excluded from the profile, even though it resides in the main module.
Binaries built with “-cover” write out profile data files at the end of their execution to a directory specified via the environment variable GOCOVERDIR. Example:
$ go build -cover -o myprogram.exe myprogram.go $ mkdir somedata $ GOCOVERDIR=somedata ./myprogram.exe I say "Hello, world." and "see ya" $ ls somedata covcounters.c6de772f99010ef5925877a7b05db4cc.2424989.1670252383678349347 covmeta.c6de772f99010ef5925877a7b05db4cc $
Note the two files that were written to the directory somedata: these (binary) files contain the coverage results. See the following section on reporting for more on how to produce human-readable results from these data files.
If the GOCOVERDIR environment variable is not set, a coverage-instrumented binary will still execute correctly, but will issue a warning. Example:
$ ./myprogram.exe warning: GOCOVERDIR not set, no coverage data emitted I say "Hello, world." and "see ya" $
Integration tests can in many cases involve multiple program runs; when the program is built with “-cover”, each run will produce a new data file. Example
$ mkdir somedata2 $ GOCOVERDIR=somedata2 ./myprogram.exe // first run I say "Hello, world." and "see ya" $ GOCOVERDIR=somedata2 ./myprogram.exe -flag // second run I say "Hello, world." and "see ya" $ ls somedata2 covcounters.890814fca98ac3a4d41b9bd2a7ec9f7f.2456041.1670259309405583534 covcounters.890814fca98ac3a4d41b9bd2a7ec9f7f.2456047.1670259309410891043 covmeta.890814fca98ac3a4d41b9bd2a7ec9f7f $
Coverage data output files come in two flavors: meta-data files (containing the items that are invariant from run to run, such as source file names and function names), and counter data files (which record the parts of the program that executed).
In the example above, the first run produced two files (counter and meta), whereas the second run generated only a counter data file: since meta-data doesn't change from run to run, it only needs to be written once.
Go 1.20 introduces a new tool, ‘covdata’, that can be used to read and manipulate coverage data files from a GOCOVERDIR directory.
Go's covdata tool runs in a variety of modes. The general form of a covdata tool invocation takes the form
$ go tool covdata <mode> -i=<dir1,dir2,...> ...flags...
where the “-i” flag provides a list of directories to read, where each directories is derived from an execution of a coverage-instrumented binary (via GOCOVERDIR).
This section discusses how to use “go tool covdata” to produce human-readable reports from coverage data files.
To report a “percent statements covered” metric for each instrumented package, use the command “go tool covdata percent -i=<directory>”. Using the example from the running section above:
$ ls somedata covcounters.c6de772f99010ef5925877a7b05db4cc.2424989.1670252383678349347 covmeta.c6de772f99010ef5925877a7b05db4cc $ go tool covdata percent -i=somedata main coverage: 100.0% of statements mydomain.com/greetings coverage: 100.0% of statements $
The “statements covered” percentages here correspond directly to those reported by go test -cover.
You can convert binary coverage data files into the legacy textual format generated by “go test -coverprofile=<outfile>” using the covdata textfmt selector. The resulting text file can then be used with “go tool cover -func” or “go tool cover -html” to create additional reports. Example:
$ ls somedata covcounters.c6de772f99010ef5925877a7b05db4cc.2424989.1670252383678349347 covmeta.c6de772f99010ef5925877a7b05db4cc $ go tool covdata textfmt -i=somedata -o profile.txt $ cat profile.txt mode: set mydomain.com/myprogram.go:10.13,12.2 1 1 mydomain.com/greetings/greetings.go:3.23,5.2 1 1 $ go tool cover -func=profile.txt mydomain.com/greetings/greetings.go:3: Goodbye 100.0% mydomain.com/myprogram.go:10: main 100.0% total: (statements) 100.0% $
The merge subcommand of “go tool covdata” can be used to merge together profiles from multiple data directories.
For example, consider a program that runs on both macOS and on Windows. The author of this program might want to combine coverage profiles from separate runs on each operating system into a single profile corpus, so as to produce a cross-platform coverage summary. For example:
$ ls windows_datadir covcounters.f3833f80c91d8229544b25a855285890.1025623.1667481441036838252 covcounters.f3833f80c91d8229544b25a855285890.1025628.1667481441042785007 covmeta.f3833f80c91d8229544b25a855285890 $ ls macos_datadir covcounters.b245ad845b5068d116a4e25033b429fb.1025358.1667481440551734165 covcounters.b245ad845b5068d116a4e25033b429fb.1025364.1667481440557770197 covmeta.b245ad845b5068d116a4e25033b429fb $ ls macos_datadir $ mkdir merged $ go tool covdata merge -i=windows_datadir,macos_datadir -o merged $
The merge operation above will combine the data from the specified input directories and write a new set of merged data files to the directory “merged”.
Most “go tool covdata” commands support a “-pkg” flag to perform package selection as part of the operation; the argument to “-pkg” takes the same form as that used by the Go command's “-coverpkg” flag. Example:
$ ls somedata covcounters.c6de772f99010ef5925877a7b05db4cc.2424989.1670252383678349347 covmeta.c6de772f99010ef5925877a7b05db4cc $ go tool covdata percent -i=somedata -pkg=mydomain.com/greetings mydomain.com/greetings coverage: 100.0% of statements $ go tool covdata percent -i=somedata -pkg=nonexistentpackage $
The “-pkg” flag can be used to select the specific subset of packages of interest for a given report.
go.mod filego build -cover in GOPATH/GO111MODULE=off mode?-coverpkg=main select my main package for profiling?go.mod fileBy default, go build -cover will instrument all main module packages for coverage, but will not instrument imports outside the main module (e.g. standard library packages or imports listed in go.mod). One way to request instrumentation for all non-stdlib dependencies is to feed the output of go list into -coverpkg. Here is an example, again using the example program cited above:
$ go list -f '{{"{{if not .Standard}}{{.ImportPath}}{{end}}"}}' -deps . | paste -sd "," > pkgs.txt
$ go build -o myprogram.exe -coverpkg=`cat pkgs.txt` .
$ mkdir somedata
$ GOCOVERDIR=somedata ./myprogram.exe
$ go tool covdata percent -i=somedata
golang.org/x/text/internal/tag coverage: 78.4% of statements
golang.org/x/text/language coverage: 35.5% of statements
mydomain.com coverage: 100.0% of statements
mydomain.com/greetings coverage: 100.0% of statements
rsc.io/quote coverage: 25.0% of statements
rsc.io/sampler coverage: 86.7% of statements
$
go build -cover in GO111MODULE=off mode?Yes, go build -cover does work with GO111MODULE=off. When building a program in GO111MODULE=off mode, only the package specifically named as the target on the command line will be instrumented for profiling. Use the -coverpkg flag to include additional packages in the profile.
Programs built with go build -cover will only write out complete profile data at the end of execution if the program invokes os.Exit() or returns normally from main.main. If a program terminates in an unrecovered panic, or if the program hits a fatal exception (such as a segmentation violation, divide by zero, etc), profile data from statements executed during the run will be lost.
-coverpkg=main select my main package for profiling?The -coverpkg flag accepts a list of import paths, not a list of package names. If you want to select your main package for coverage instrumention, please identify it by import path, not by name. Example (using this example program):
$ go list -m mydomain.com $ go build -coverpkg=main -o oops.exe . warning: no packages being built depend on matches for pattern main $ go build -coverpkg=mydomain.com -o myprogram.exe . $ mkdir somedata $ GOCOVERDIR=somedata ./myprogram.exe I say "Hello, world." and "see ya" $ go tool covdata percent -i=somedata mydomain.com coverage: 100.0% of statements $
cmd/go package docs describe the build and test flags associated with coverage. unit test: Tests within a *_test.go file associated with a specific Go package, utilizing Go's testing package.
integration test: A more comprehensive, heavier weight test for a given application or binary. Integration tests typically involve building a program or set of programs, then performing a series of runs of the programs using multiple inputs and scenarios, under control of a test harness that may or may not be based on Go's testing package.