c++ - ReadFile() 读取的字节数少于其参数中指定的值

Question

我有这段代码用于从串行端口读取字节，但对于客户端设备，消息 "No enough information" 显示，他说串行端口中有足够的信息，但此消息 被展示。

int SuccessfulRead(unsigned char * s , DWORD bytesRead){

    if( bytesRead == 2 ){
        //...
        return 1;
    }

    return 0;
}

//Reading

int i=1 , breakWhile=0 ;
while(!breakWhile)
{
DWORD dwRead;
BOOL fWaitingOnRead = FALSE; //overlapped
OVERLAPPED osReader = {0};
unsigned char lpBuf[2];

osReader.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
if (osReader.hEvent == NULL){
MessageBox( NULL , L"Unable to create event" , L"Error" , MB_OK);
breakWhile=1;
}
if (!fWaitingOnRead) {
   // Issue read operation.
   if (!ReadFile(hPort2, lpBuf, 2, &dwRead,&osReader)) {
       if (GetLastError() != ERROR_IO_PENDING){     
           MessageBox(NULL , L"Error in communication" , L"ERROR" , MB_OK);
           break;
       }
       else
           fWaitingOnRead = TRUE;
   }
   else {
       // read completed immediately
       int res=SuccessfulRead(lpBuf,dwRead);
       if( !res){
           MessageBox(NULL , L"No enough information" , L"" , MB_OK);
           break;
       }
   }
}



#define READ_TIMEOUT   2000      // milliseconds
DWORD dwRes=0;
if (fWaitingOnRead) {
dwRes = WaitForSingleObject(osReader.hEvent,READ_TIMEOUT);
switch(dwRes)
{
    // Read completed.
    case WAIT_OBJECT_0:{
        if (!GetOverlappedResult(hPort2, &osReader, &dwRead,FALSE))
            MessageBox(NULL , L"Error in GetOverlappedResult" , L"Error" , MB_OK);

        else{
            // Read completed successfully.
            int res=SuccessfulRead(lpBuf,dwRead);
            if(!res){//less than two bytes read that is one byte readed
            MessageBox(NULL , L"No enough information" , L"" , MB_OK);
            breakWhile=1;//exit loop
            }
        //Reset flag so that another opertion can be issued.
        fWaitingOnRead = FALSE;
                     }
        break;
    case WAIT_TIMEOUT:
        if( i==1){//failed in first try
            MessageBox(NULL , L"There is no data please try again", L"" , MB_OK);
        }


        breakWhile=1;//exit loop

        break;
                       }
}
                  }
CloseHandle(osReader.hEvent);
i++;
}

每次必须读取 2 个字节，因为 ReadFile(hPort2, lpBuf, 2, &dwRead,&osReader) ，但是第一次读取的是1个字节，所以说明串口中的数据不超过1个字节。 但似乎code vision读取该字节，为什么当超过 2 个字节时该程序只读取 1 个字节？当在ReadFile()时要读取的字节数为 2。

为什么它不起作用以及如何让它起作用？

是否与timeout有关值？

谢谢

Answer 1

这里:

        int res=SuccessfulRead(lpBuf,dwRead);
        if(!res){//less than two bytes read that is one byte readed

注释表明您期望 >= 两个字节，但 SuccessfulRead() 仅在读取恰好两个字节时返回非零:

if( bytesRead == 2 ){ // EXACTLY two bytes!!!
    //...
    return 1;
}

您需要:

if( bytesRead <= 2 ){ // AT LEAST two bytes
...

确保评论正确。