如何使用JNA传递结构数组

Question

I referenced another stackoverflow article "How do I map a pointer to an array of structures in JNA" to come up with the following code to enumerate Windows service dependencies.

Structure and function declarations:

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);

If there is only one service dependency, the following code works:

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());
    }
}

If there are 2 service dependencies, I get a NullPointerException for lpServiceName in the logger.debug call:

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());
    }
}

The numberOfServices value for the code above is 2, as expected. I'm trying to pass the structure array instead of a pointer because I want JNA to do the memory synching. How should I be passing/using the array of structures?

Answer 1

According to the docs for EnumDependentServices ,

lpServices [out, optional]

A pointer to an array of ENUM_SERVICE_STATUS structures that receives the name and service status information for each dependent service in the database. The buffer must be large enough to hold the structures, plus the strings to which their members point.

You're pretty much ignoring the required buffer size as reported by bytesNeeded . You should use the bytesNeeded value to create a Memory instance of the requisite size, then use that Memory instance to create a new ENUM_SERVICE_STATUS instance, instead of creating the structure independently of the required buffer size.