golang/go · error

number of output files (%d) not equal to number of input fil

Error message

number of output files (%d) not equal to number of input files (%d)

What it means

Thrown by 'go tool cover' parseFlags in -pkgcfg mode when the count of output file paths read from -outfilelist does not equal the count of input .go files (flag.Args()). The tool pairs inputs to outputs one-to-one to write instrumented copies, so a mismatch would misroute outputs. The first outfilelist entry is reserved for the cover-vars file, so the remaining count is compared.

Source

Thrown at src/cmd/cover/cover.go:187

		}

		if flag.NArg() == 0 {
			return fmt.Errorf("missing source file(s)")
		} else {
			if *pkgcfg != "" {
				if *output != "" {
					return fmt.Errorf("please use '-outfilelist' flag instead of '-o'")
				}
				var err error
				if outputfiles, err = readOutFileList(*outfilelist); err != nil {
					return err
				}
				covervarsoutfile = outputfiles[0]
				outputfiles = outputfiles[1:]
				numInputs := len(flag.Args())
				numOutputs := len(outputfiles)
				if numOutputs != numInputs {
					return fmt.Errorf("number of output files (%d) not equal to number of input files (%d)", numOutputs, numInputs)
				}
				if err := readPackageConfig(*pkgcfg); err != nil {
					return err
				}
				return nil
			} else {
				if *outfilelist != "" {
					return fmt.Errorf("'-outfilelist' flag applicable only when -pkgcfg used")
				}
			}
			if flag.NArg() == 1 {
				return nil
			}
		}
	} else if flag.NArg() == 0 {
		return nil
	}
	return fmt.Errorf("too many arguments")

View on GitHub (pinned to b6b368adc5)

Solutions

1. Regenerate -outfilelist so it has exactly (number of input files + 1) lines: the cover-vars file first, then one output per input
2. Let the go command drive -pkgcfg/-outfilelist so the lists stay in sync
3. Add a build-step assertion that len(outputfiles)-1 == len(inputs) before invoking cover

Example fix

# inputs: a.go b.go c.go (3)
# before (outfilelist has 3 lines)
go tool cover -mode=set -pkgcfg=p.cfg -outfilelist=list.txt a.go b.go c.go
# after (outfilelist has 4 lines: covervars.go + 3 outputs)
covvars.go\na.cover.go\nb.cover.go\nc.cover.go

# Verify outfilelist length == inputs + 1 (cover-vars file first)
mapfile -t outs < "$OUTFILELIST"
inputs=( "$@" )
if [ $(( ${#outs[@]} - 1 )) -ne ${#inputs[@]} ]; then
  echo "outfilelist must have inputs+1 lines" >&2; exit 2
fi

When it happens

Trigger: Passing 3 input files but an outfilelist with 3 entries (only 2 outputs after the reserved cover-vars entry), or any N-inputs vs (M-1)-outputs mismatch.

Common situations: Forgetting that the first outfilelist line is the cover-vars file, not an instrumented output. Generating the outfilelist with a script that drops or duplicates a line. Editing the input list without regenerating outfilelist.

Related errors

• please use '-outfilelist' flag instead of '-o'
• '-outfilelist' flag applicable only when -pkgcfg used
• too many options
• -var: %q is not a valid identifier
• unknown -mode %v

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/e2a7abfd79af6f07. Report an issue: GitHub.