Qt通过UDP传图片，实现自定义分包和组包

一.包头结构体

//包头
struct PackageHeader
{
    //包头大小(sizeof(PackageHeader))
    unsigned int uTransPackageHdrSize;
    //当前包头的大小(sizeof(PackageHeader)+当前数据包长度)
    unsigned int uTransPackageSize;
    //数据的总大小
    unsigned int uDataSize;
    //数据被分成包的个数
    unsigned int uDataPackageNum;
    //数据包当前的帧号
    unsigned int uDataPackageCurrIndex;
    //数据包在整个数据中的偏移
    unsigned int uDataPackageOffset;
};

每包数据由包头和包体组成，包头用于标记每包数据的特征，包体是图片根据指定大小分出的每段数据，这里指定大小为const int UDP_MAX_SIZE=1200。

为何要分包可参考：TCP、UDP数据包大小的限制

二.分包与组包

1.分包

int dataLength=buffer.data().size();

unsigned char *dataBuffer=(unsigned char *)buffer.data().data();

int packetNum = 0;
int lastPaketSize = 0;
packetNum = dataLength / UDP_MAX_SIZE;
lastPaketSize = dataLength % UDP_MAX_SIZE;
int currentPacketIndex = 0;
if (lastPaketSize != 0)
{
    packetNum = packetNum + 1;
}

PackageHeader packageHead;

packageHead.uTransPackageHdrSize=sizeof(packageHead);
packageHead.uDataSize = dataLength;
packageHead.uDataPackageNum = packetNum;

unsigned char frameBuffer[1024*1000];
memset(frameBuffer,0,1024*1000);
while (currentPacketIndex < packetNum)
{
    if (currentPacketIndex < (packetNum-1))
    {
        packageHead.uTransPackageSize = sizeof(PackageHeader)+UDP_MAX_SIZE;
        packageHead.uDataPackageCurrIndex = currentPacketIndex+1;
        packageHead.uDataPackageOffset = currentPacketIndex*UDP_MAX_SIZE;
        memcpy(frameBuffer, &packageHead, sizeof(PackageHeader));
        memcpy(frameBuffer+sizeof(PackageHeader), dataBuffer+packageHead.uDataPackageOffset, UDP_MAX_SIZE);

        int length=udpsocketSend->writeDatagram(
                    (const char*)frameBuffer, packageHead.uTransPackageSize,
                    QHostAddress(ui->lineEditIP->text()), ui->lineEditPort->text().toInt());

        if(length!=packageHead.uTransPackageSize)
        {
          qDebug()<<"Failed to send image";
        }

        currentPacketIndex++;
    }
    else
    {
        packageHead.uTransPackageSize = sizeof(PackageHeader)+(dataLength-currentPacketIndex*UDP_MAX_SIZE);
        packageHead.uDataPackageCurrIndex = currentPacketIndex+1;
        packageHead.uDataPackageOffset = currentPacketIndex*UDP_MAX_SIZE;
        memcpy(frameBuffer, &packageHead, sizeof(PackageHeader));
        memcpy(frameBuffer+sizeof(PackageHeader), dataBuffer+packageHead.uDataPackageOffset, dataLength-currentPacketIndex*UDP_MAX_SIZE);
        int length=udpsocketSend->writeDatagram(
                    (const char*)frameBuffer, packageHead.uTransPackageSize,
                    QHostAddress(ui->lineEditIP->text()), ui->lineEditPort->text().toInt());

        if(length!=packageHead.uTransPackageSize)
        {
          qDebug()<<"Failed to send image";
        }

        currentPacketIndex++;
    }
}

先将图片转换为QBuffer，这样就可以获取图片的数据和长度，然后根据指定大小UDP_MAX_SIZE分包。在包头中设置每包数据的特征，然后将包体加到包头后面发送出去。

2.组包

static int num = 1;
static uint size = 0;

PackageHeader *packageHead = (PackageHeader *)datagram.data();
if (packageHead->uDataPackageCurrIndex == num)
{
    num++;
    size += packageHead->uTransPackageSize-packageHead->uTransPackageHdrSize;
    if (size > 1024*1000)
    {
        qDebug()<<"image too big";
        num = 1;
        size = 0;
        return;
    }
    if (packageHead->uDataPackageOffset > 1024*1000)
    {
        qDebug()<<"image too big";
        packageHead->uDataPackageOffset = 0;
        num = 1;
        size = 0;
        return;
    }

    memcpy(imageData.data+packageHead->uDataPackageOffset, datagram.data()+packageHead->uTransPackageHdrSize,
           packageHead->uTransPackageSize-packageHead->uTransPackageHdrSize);
    if ((packageHead->uDataPackageNum == packageHead->uDataPackageCurrIndex)
            && (size == packageHead->uDataSize))
    {
        imageData.length = packageHead->uDataSize;

        QImage image;
        image.loadFromData((uchar *)imageData.data,imageData.length,"JPG");
        QPixmap pixmap=QPixmap::fromImage(image);
        ui->labelImage_2->setPixmap(pixmap);
        recvImageNum++;
        ui->lineEditRevFrame->setText(QString::number(recvImageNum));
        ui->lineEditRevSize->setText(QString::number(imageData.length));
        memset(&imageData,0,sizeof(UdpUnpackData));
        num = 1;
        size = 0;
    }
}
else
{
    num = 1;
    size = 0;
    memset(&imageData,0,sizeof(UdpUnpackData));
}

组包是分包的逆过程，根据包头中“数据的总大小”和“数据被分成包的个数”两个字段可以判断组包是否完整，如果完整就在接收端显示出来。
三.示例

界面包括发送端和接收端，接收端的IP地址是自动获取的本机IP地址，上图将发送端的IP地址设置为与接收端IP地址相同，可实现自发自收。

每秒帧数可实时设置每秒发送图片的张数，发送帧数表示一共发送了多少张，接收帧数表示一共接收了多少张，图片大小表示每张图片多少Byte。

可将示例运行于两台计算机，实现双向收发。

源码链接：见http://blog.csdn.net/caoshangpa/article/details/52681572的评论

http://blog.csdn.net/caoshangpa/article/details/52681572