mrb_io_s_popen(mrb_state *mrb, mrb_value klass)
{
  mrb_value cmd, io, result;
  mrb_value mode = mrb_str_new_cstr(mrb, "r");
  mrb_value opt  = mrb_hash_new(mrb);

  struct mrb_io *fptr;
  const char *pname;
  int pid, flags, fd, write_fd = -1;
  int pr[2] = { -1, -1 };
  int pw[2] = { -1, -1 };
  int doexec;
  int saved_errno;
  int opt_in, opt_out, opt_err;

  mrb_get_args(mrb, "S|SH", &cmd, &mode, &opt);
  io = mrb_obj_value(mrb_data_object_alloc(mrb, mrb_class_ptr(klass), NULL, &mrb_io_type));

  pname = mrb_string_value_cstr(mrb, &cmd);
  flags = mrb_io_modestr_to_flags(mrb, mrb_string_value_cstr(mrb, &mode));

  doexec = (strcmp("-", pname) != 0);
  opt_in = option_to_fd(mrb, opt, "in");
  opt_out = option_to_fd(mrb, opt, "out");
  opt_err = option_to_fd(mrb, opt, "err");

  if (flags & FMODE_READABLE) {
    if (pipe(pr) == -1) {
      mrb_sys_fail(mrb, "pipe");
    }
    mrb_fd_cloexec(mrb, pr[0]);
    mrb_fd_cloexec(mrb, pr[1]);
  }

  if (flags & FMODE_WRITABLE) {
    if (pipe(pw) == -1) {
      if (pr[0] != -1) close(pr[0]);
      if (pr[1] != -1) close(pr[1]);
      mrb_sys_fail(mrb, "pipe");
    }
    mrb_fd_cloexec(mrb, pw[0]);
    mrb_fd_cloexec(mrb, pw[1]);
  }

  if (!doexec) {
    fflush(stdin);
    fflush(stdout);
    fflush(stderr);
  }

  result = mrb_nil_value();
  switch (pid = fork()) {
    case 0: /* child */
      if (opt_in != -1) {
        dup2(opt_in, 0);
      }
      if (opt_out != -1) {
        dup2(opt_out, 1);
      }
      if (opt_err != -1) {
        dup2(opt_err, 2);
      }
      if (flags & FMODE_READABLE) {
        close(pr[0]);
        if (pr[1] != 1) {
          dup2(pr[1], 1);
          close(pr[1]);
        }
      }
      if (flags & FMODE_WRITABLE) {
        close(pw[1]);
        if (pw[0] != 0) {
          dup2(pw[0], 0);
          close(pw[0]);
        }
      }
      if (doexec) {
        for (fd = 3; fd < NOFILE; fd++) {
          close(fd);
        }
        mrb_proc_exec(pname);
        mrb_raisef(mrb, E_IO_ERROR, "command not found: %S", cmd);
        _exit(127);
      }
      result = mrb_nil_value();
      break;

    default: /* parent */
      if ((flags & FMODE_READABLE) && (flags & FMODE_WRITABLE)) {
        close(pr[1]);
        fd = pr[0];
        close(pw[0]);
        write_fd = pw[1];
      } else if (flags & FMODE_READABLE) {
        close(pr[1]);
        fd = pr[0];
      } else {
        close(pw[0]);
        fd = pw[1];
      }

      mrb_iv_set(mrb, io, mrb_intern_cstr(mrb, "@buf"), mrb_str_new_cstr(mrb, ""));

      fptr = mrb_io_alloc(mrb);
      fptr->fd = fd;
      fptr->fd2 = write_fd;
      fptr->pid = pid;
      fptr->readable = ((flags & FMODE_READABLE) != 0);
      fptr->writable = ((flags & FMODE_WRITABLE) != 0);
      fptr->sync = 0;

      DATA_TYPE(io) = &mrb_io_type;
      DATA_PTR(io)  = fptr;
      result = io;
      break;

    case -1: /* error */
      saved_errno = errno;
      if (flags & FMODE_READABLE) {
        close(pr[0]);
        close(pr[1]);
      }
      if (flags & FMODE_WRITABLE) {
        close(pw[0]);
        close(pw[1]);
      }
      errno = saved_errno;
      mrb_sys_fail(mrb, "pipe_open failed.");
      break;
  }
  return result;
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.