What causes "copying %s to %s: %v" in golang/go?

Run `go clean -cache` and rebuild

copying %s to %s: %v

Error message

copying %s to %s: %v

What it means

A wrapper that fires when copying an object file (ofile) into an archive (afile) fails for any reason - including the file-larger-than-stat case (1193), an unexpected EOF where the file shrank (n < size), or a raw I/O error from io.Copy. The placeholders are the source object path and the destination archive path.

Source

Thrown at src/cmd/go/internal/work/gc.go:528

		// about bytes, not runes.
		name := fi.Name()
		if len(name) > 16 {
			name = name[:16]
		} else {
			name += strings.Repeat(" ", 16-len(name))
		}
		size := fi.Size()
		fmt.Fprintf(w, "%s%-12d%-6d%-6d%-8o%-10d`\n",
			name, 0, 0, 0, 0644, size)
		n, err := io.Copy(w, src)
		src.Close()
		if err == nil && n < size {
			err = io.ErrUnexpectedEOF
		} else if err == nil && n > size {
			err = fmt.Errorf("file larger than size reported by stat")
		}
		if err != nil {
			return fmt.Errorf("copying %s to %s: %v", ofile, afile, err)
		}
		if size&1 != 0 {
			w.WriteByte(0)
		}
	}

	if err := w.Flush(); err != nil {
		return err
	}
	return dst.Close()
}

// setextld sets the appropriate linker flags for the specified compiler.
func setextld(ldflags []string, compiler []string) ([]string, error) {
	for _, f := range ldflags {
		if f == "-extld" || strings.HasPrefix(f, "-extld=") {
			// don't override -extld if supplied
			return ldflags, nil

View on GitHub (pinned to b6b368adc5)

Solutions

Run `go clean -cache` and rebuild
Ensure no concurrent builds modify the same object files
Check disk space and filesystem health

Example fix

// before
go build ./...
// after
go clean -cache && go build ./...

Defensive patterns

Strategy: retry

Try / catch

// Archive copy failed (race or I/O): clean and retry
if bytes.Contains(out, []byte("copying")) && bytes.Contains(out, []byte("to")) {
    exec.Command("go", "clean", "-cache").Run()
    out, err = cmd.CombinedOutput()
}

Prevention

Run a single build at a time per work directory
Keep GOCACHE and the build output on reliable local storage
Monitor disk space during large builds

When it happens

Trigger: Fires in the archive builder after io.Copy when err != nil (short read, long read, or copy failure), wrapping the underlying error with source and destination paths.

Common situations: Concurrent modification of object files, a full disk, broken build artifacts, or filesystem errors during the archive step.

Related errors

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/38f12a39b62f0684. Report an issue: GitHub.