What causes "file larger than size reported by stat" in golang/go?

Ensure no concurrent process modifies object files during a go build

file larger than size reported by stat

Error message

file larger than size reported by stat

What it means

While assembling an ar-format archive of object files, the go command writes each file preceded by a header whose size field comes from a prior stat. After io.Copy it checks the copied byte count against that size; if more bytes were read than stat reported (n > size), the file grew between stat and read - a race where the object file was modified concurrently.

Source

Thrown at src/cmd/go/internal/work/gc.go:525

			return err
		}
		// Note: Not using %-16.16s format because we care
		// 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 {

View on GitHub (pinned to b6b368adc5)

Solutions

Ensure no concurrent process modifies object files during a go build
Re-run the build - transient races often do not recur
Move the build to a local filesystem if using NFS/FUSE

Example fix

// before (generator races with go build)
go generate & go build
// after (sequential)
go generate && go build

Defensive patterns

Strategy: retry

Try / catch

// A transient race: retry the build once
if bytes.Contains(out, []byte("file larger than size reported by stat")) {
    out, err = cmd.CombinedOutput()
}

Prevention

Never run generators or concurrent builds that write the same object files
Build on a local filesystem, not NFS/FUSE
Sequence `go generate` before `go build` rather than in parallel

When it happens

Trigger: Fires in the archive-assembly code of gc.go when io.Copy returns n > size, where size = fi.Size() from the earlier os.Stat on the object file.

Common situations: A code generator or concurrent go build writing the object file mid-archive, a FUSE filesystem with inconsistent stat/read, or a network filesystem with caching lag.

Related errors

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/db734a7df22e01ac. Report an issue: GitHub.