Using testing.Benchmark does not produce any output - go

Question

I'm using testing.Benchmark to manually run a couple benchmarks but the result object is always empty.
Am I missing something here?
Here's an example:
package main
import "testing"
func main() {
result := testing.Benchmark(func(parentB *testing.B) {
parentB.Run("example", func(b *testing.B) {
for n := 0; n < b.N; n++ {
println("ok")
}
})
})
println(result.String())
}
This will print ok a couple times and then 0 0 ns/op but the benchmark clearly did run something.

Answer 1

I think you are doing everything right. Doc of testing.Benchmark() says:
Benchmark benchmarks a single function. Useful for creating custom benchmarks that do not use the "go test" command.
If f calls Run, the result will be an estimate of running all its subbenchmarks that don't call Run in sequence in a single benchmark.
Looking into the implementation (Go 1.7.4):
func Benchmark(f func(b *B)) BenchmarkResult {
b := &B{
common: common{
signal: make(chan bool),
w: discard{},
},
benchFunc: f,
benchTime: *benchTime,
}
if !b.run1() {
return BenchmarkResult{}
}
return b.run()
}
This line:
if !b.run1() {
return BenchmarkResult{}
}
b.run1() is supposed to run your passed function once, and detect if it has sub-benchmarks. Yours has. It returns a bool whether more runs are needed. Inside run1():
if b.hasSub || b.finished {
// ...
return true
}
It properly tells it has sub-benchmark, and Benchmark() –with noble simplicity– just returns an empty BenchmarkResult:
if !b.run1() {
return BenchmarkResult{}
}
I do believe that either this is a bug (or rather "incomplete" feature), or doc is incorrect. I suggest to file an issue here: https://github.com/golang/go/issues

Answer 2

Eddited the answer to clarify:
My guess is that you are using go run to run the test. That will not produce any result. In order to run the code exatly as it is written you need to use
go test -bench=. and I think it should work.
The file must be named test_xxx.go where xxx is whatever you want.
If you restucture you code a litle bit it can be run as a single function benchmark:
package main
import "testing"
func main() {
myTest()
}
func myTest() {
fn := func(b *testing.B) {
for n := 0; n < b.N; n++ {
println("ok")
}
}
result := testing.Benchmark(fn)
println(result.String())
}