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