golang/go · error

malformed object file

Error message

malformed object file

What it means

Defined as errBuildIDMalformed in buildid.go and returned when ReadFile cannot find a recognizable Go object, archive, or build-id structure in a file. The package first tries the `!<arch>` archive header, `__.PKGDEF`, `go object `, and `build id ` markers; if none line up with the expected layout, the file is treated as malformed.

Source

Thrown at src/cmd/internal/buildid/buildid.go:20

// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package buildid

import (
	"bytes"
	"debug/elf"
	"fmt"
	"internal/xcoff"
	"io"
	"io/fs"
	"os"
	"strconv"
	"strings"
)

var (
	errBuildIDMalformed = fmt.Errorf("malformed object file")

	bangArch = []byte("!<arch>")
	pkgdef   = []byte("__.PKGDEF")
	goobject = []byte("go object ")
	buildid  = []byte("build id ")
)

// ReadFile reads the build ID from an archive or executable file.
func ReadFile(name string) (id string, err error) {
	f, err := os.Open(name)
	if err != nil {
		return "", err
	}
	defer f.Close()

	buf := make([]byte, 8)
	if _, err := f.ReadAt(buf, 0); err != nil {
		return "", err

View on GitHub (pinned to b6b368adc5)

Solutions

1. Clean the build cache: `go clean -cache`.
2. Rebuild the affected package from source: `go build -a <pkg>`.
3. Verify the file is actually a Go-produced object (e.g. `file <obj>` and inspect for the `go object` magic).
4. If reproducible, check for disk space issues, concurrent writers, or failing hardware during the build.

// before handing an object to buildid.ReadFile, sanity-check the magic:
f, _ := os.Open(obj)
hdr := make([]byte, 8)
_, _ = io.ReadFull(f, hdr)
_ = f.Close()
if !bytes.HasPrefix(hdr, []byte("!<arch>")) && !bytes.HasPrefix(hdr, []byte("\x7fELF")) {
    return errors.New("not a recognizable Go object/archive")
}

id, err := buildid.ReadFile(obj)
if err != nil {
    if errors.Is(err, buildid.ErrBuildIDMalformed) || strings.Contains(err.Error(), "malformed object file") {
        // rebuild from source
        _ = os.Remove(obj)
    }
    return err
}

When it happens

Trigger: Passing a non-Go object file (random binary, corrupted object, truncated build artifact, foreign-toolchain output) to a buildid function such as ReadFile, ReadFile or the rewrite helpers. Also when a cached object in the build cache is partially written or truncated.

Common situations: Corrupted build cache entries (e.g. after a crash, disk full, or `kill -9` during compile), mixing objects from incompatible Go versions, hand-editing .a files, or pointing the toolchain at a non-Go archive.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• corrupt object file
• read namesize failed: %v
• read descsize failed: %v
• read type failed: %v
• read name failed: %v

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/ac2d78e485d3cf52. Report an issue: GitHub.