WriteBytes does not return how many bytes were not written

Hello,
At the moment we encountered that writeBytes does not return how many bytes were written. For a CTS / RTS flow, this is absolutly necessary. Is there a reason why this is not needed / or how we use this library correctly with hardware CTS / RTS without losing data in the middle?

Thanks a lot!

Best

Daniel

Hi,

I don't believe this is possible currently. To support this without breaking backwards compatibility, a new function (e.g. write(bytes[])) would have to be written and the old boolean functions would have to be update to check the return length.

This would require Windows and Unix C++ code to be updated to return jint and the current functions would be wrapped to still return a boolean.

Disclaimer, this library is mostly in maintenance mode, so any features would have to be authored by the vested/interested parties.

Relevant functions:

Unix:

jssc/src/main/cpp/_nix_based/jssc.cpp

Lines 528 to 535 in 6886b44

    
           JNIEXPORT jboolean JNICALL Java_jssc_SerialNativeInterface_writeBytes 
        
             (JNIEnv *env, jobject, jlong portHandle, jbyteArray buffer){ 
        
               jbyte* jBuffer = env->GetByteArrayElements(buffer, JNI_FALSE); 
        
               jint bufferSize = env->GetArrayLength(buffer); 
        
               jint result = write(portHandle, jBuffer, (size_t)bufferSize); 
        
               env->ReleaseByteArrayElements(buffer, jBuffer, 0); 
        
               return result == bufferSize ? JNI_TRUE : JNI_FALSE; 
        
           }

Windows:

jssc/src/main/cpp/windows/jssc.cpp

Lines 234 to 257 in 6886b44

    
           JNIEXPORT jboolean JNICALL Java_jssc_SerialNativeInterface_writeBytes 
        
             (JNIEnv *env, jobject, jlong portHandle, jbyteArray buffer){ 
        
               HANDLE hComm = (HANDLE)portHandle; 
        
               DWORD lpNumberOfBytesTransferred; 
        
               DWORD lpNumberOfBytesWritten; 
        
               OVERLAPPED *overlapped = new OVERLAPPED(); 
        
               jboolean returnValue = JNI_FALSE; 
        
               jbyte* jBuffer = env->GetByteArrayElements(buffer, JNI_FALSE); 
        
               overlapped->hEvent = CreateEventA(NULL, true, false, NULL); 
        
               if(WriteFile(hComm, jBuffer, (DWORD)env->GetArrayLength(buffer), &lpNumberOfBytesWritten, overlapped)){ 
        
                   returnValue = JNI_TRUE; 
        
               } 
        
               else if(GetLastError() == ERROR_IO_PENDING){ 
        
                   if(WaitForSingleObject(overlapped->hEvent, INFINITE) == WAIT_OBJECT_0){ 
        
                       if(GetOverlappedResult(hComm, overlapped, &lpNumberOfBytesTransferred, false)){ 
        
                           returnValue = JNI_TRUE; 
        
                       } 
        
                   } 
        
               } 
        
               env->ReleaseByteArrayElements(buffer, jBuffer, 0); 
        
               CloseHandle(overlapped->hEvent); 
        
               delete overlapped; 
        
               return returnValue; 
        
           }

@tresf

Hi @tresf,

Thanks for your update! I’m happy to implement the changes, still I would first want to know if there was a reason behind it not to implement it like this from the beginning?
So is this at all possible with the C stack being used? Or do I have to try?
Thanks!
Best
Daniel

@hiddenalpha

I would first want to know if there was a reason behind it not to implement it like this from the beginning?

Hi, this project is a fork of the original from https://github.com/scream3r/java-simple-serial-connector, so the history behind this decision likely lies there.

So is this at all possible with the C stack being used? Or do I have to try?

I think the Unix code makes it relatively easy, but I'm not sure about the win32 API. Perhaps the answer lies in GetOverlappedResult returning a zero? I'm not very familiar with hardware flow control and only partially familiar with the C++ portions of the codebase. We do have some other volunteers here as well as a Slack channel if interested. @hiddenalpha has been very helpful with the C++ portions as well.

Looks related to issue #96

Make 'write()' return number of bytes written

Make writeBytes() return how many bytes actually got written

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WriteBytes does not return how many bytes were not written #129

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	JNIEXPORT jboolean JNICALL Java_jssc_SerialNativeInterface_writeBytes
	(JNIEnv *env, jobject, jlong portHandle, jbyteArray buffer){
	jbyte* jBuffer = env->GetByteArrayElements(buffer, JNI_FALSE);
	jint bufferSize = env->GetArrayLength(buffer);
	jint result = write(portHandle, jBuffer, (size_t)bufferSize);
	env->ReleaseByteArrayElements(buffer, jBuffer, 0);
	return result == bufferSize ? JNI_TRUE : JNI_FALSE;
	}

	JNIEXPORT jboolean JNICALL Java_jssc_SerialNativeInterface_writeBytes
	(JNIEnv *env, jobject, jlong portHandle, jbyteArray buffer){
	HANDLE hComm = (HANDLE)portHandle;
	DWORD lpNumberOfBytesTransferred;
	DWORD lpNumberOfBytesWritten;
	OVERLAPPED *overlapped = new OVERLAPPED();
	jboolean returnValue = JNI_FALSE;
	jbyte* jBuffer = env->GetByteArrayElements(buffer, JNI_FALSE);
	overlapped->hEvent = CreateEventA(NULL, true, false, NULL);
	if(WriteFile(hComm, jBuffer, (DWORD)env->GetArrayLength(buffer), &lpNumberOfBytesWritten, overlapped)){
	returnValue = JNI_TRUE;
	}
	else if(GetLastError() == ERROR_IO_PENDING){
	if(WaitForSingleObject(overlapped->hEvent, INFINITE) == WAIT_OBJECT_0){
	if(GetOverlappedResult(hComm, overlapped, &lpNumberOfBytesTransferred, false)){
	returnValue = JNI_TRUE;
	}
	}
	}
	env->ReleaseByteArrayElements(buffer, jBuffer, 0);
	CloseHandle(overlapped->hEvent);
	delete overlapped;
	return returnValue;
	}

WriteBytes does not return how many bytes were not written #129

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tresf commented on Sep 21, 2022

danielgruber commented on Sep 21, 2022

tresf commented on Sep 21, 2022

hiddenalpha commented on Sep 22, 2022

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