如何使用 JNA 传递结构数组

Question

我参考了另一篇 stackoverflow 文章“如何将指针映射到 JNA 中的结构数组”，提出以下代码来枚举 Windows 服务依赖项。

结构和函数声明：

static class SERVICE_STATUS extends Structure {
    public int dwServiceType;
    public int dwCurrentState;
    public int dwControlsAccepted;
    public int dwWin32ExitCode;
    public int dwServiceSpecificExitCode;
    public int dwCheckPoint;
    public int dwWaitHint;
    public SERVICE_STATUS(){}
}

static class ENUM_SERVICE_STATUS extends Structure {
    public ENUM_SERVICE_STATUS(){ }

    public WString lpServiceName;
    public WString lpDisplayName;
    SERVICE_STATUS serviceStatus;
}

boolean EnumDependentServicesW(Pointer hService, int serviceState, ENUM_SERVICE_STATUS serviceStatuses, int size, IntByReference bytesNeeded, IntByReference servicesReturned);

如果只有一个服务依赖，下面的代码可以工作：

IntByReference bytesNeeded = new IntByReference();
IntByReference numberOfServices = new IntByReference();
Advapi32.ENUM_SERVICE_STATUS serviceStatus = new Advapi32.ENUM_SERVICE_STATUS();
Advapi32.ENUM_SERVICE_STATUS[] serviceStatuses = (Advapi32.ENUM_SERVICE_STATUS[]) serviceStatus.toArray(1);

if (!advapi32.EnumDependentServicesW(serviceHandle, Advapi32.SERVICE_ACTIVE, null, 0, bytesNeeded, numberOfServices)) {
    if (advapi32.EnumDependentServicesW (serviceHandle, Advapi32.SERVICE_ACTIVE, serviceStatuses[0], bytesNeeded.getValue(), bytesNeeded, numberOfServices)) {
        for(int i = numberOfServices.getValue() - 1; i >= 0; i--){
            logger.debug("Service Name: " + serviceStatuses[i].lpServiceName.toString());
    }
}

如果有 2 个服务依赖项，我会在 logger.debug 调用中获得 lpServiceName 的 NullPointerException：

IntByReference bytesNeeded = new IntByReference();
IntByReference numberOfServices = new IntByReference();
Advapi32.ENUM_SERVICE_STATUS serviceStatus = new Advapi32.ENUM_SERVICE_STATUS();
Advapi32.ENUM_SERVICE_STATUS[] serviceStatuses = (Advapi32.ENUM_SERVICE_STATUS[]) serviceStatus.toArray(2);

if (!advapi32.EnumDependentServicesW(serviceHandle, Advapi32.SERVICE_ACTIVE, null, 0, bytesNeeded, numberOfServices)) { 
    if (advapi32.EnumDependentServicesW (serviceHandle, Advapi32.SERVICE_ACTIVE, serviceStatuses[0], bytesNeeded.getValue(), bytesNeeded, numberOfServices)) {
        for(int i = numberOfServices.getValue() - 1; i >= 0; i--){
            logger.debug("Service Name: " + serviceStatuses[i].lpServiceName.toString());
    }
}

如预期的那样，上面代码的 numberOfServices 值为 2。我试图传递结构数组而不是指针，因为我希望 JNA 进行内存同步。我应该如何传递/使用结构数组？

Answer 1

根据docs为EnumDependentServices，

lpServices [out, optional]

指向数组的指针 接收名称和服务的 ENUM_SERVICE_STATUS 结构 数据库中每个依赖服务的状态信息。 缓冲区必须足够大以容纳结构以及字符串 他们的成员指向的地方。

您几乎忽略了bytesNeeded 报告的所需缓冲区大小。您应该使用bytesNeeded 值来创建所需大小的Memory 实例，然后使用该Memory 实例来创建新的ENUM_SERVICE_STATUS 实例，而不是独立于所需缓冲区大小来创建结构。