go/src/cmd/compile/internal/importer/exportdata.go

func readGopackHeader(r *bufio.Reader) (name string, size int, err error) { … }

// FindExportData positions the reader r at the beginning of the
// export data section of an underlying GC-created object/archive
// file by reading from it. The reader must be positioned at the
// start of the file before calling this function. The hdr result
// is the string before the export data, either "$$" or "$$B".
//
// If size is non-negative, it's the number of bytes of export data
// still available to read from r.
func FindExportData(r *bufio.Reader) (hdr string, size int, err error) { … }