Qt實現環形緩衝區的兩種方法
阿新 • • 發佈：2019-01-04
一個環形buffer，在尾部追加資料，從頭部讀取資料，適合用作IO的緩衝區。
一.使用QList和QByteArray
這個方法參考的是Qt原始碼中的QRingBuffer類，這個類不是Qt API的一部分，所以Qt助手裡是查不到的，它的存在只是為了服務其他的原始碼。
QRingBuffer的原始檔在D:\Qt\Qt5.7.0\5.7\Src\qtbase\src\corelib\tools目錄中，由qringbuffer_p.h和qringbuffer.cpp實現。
QRingBuffer實現的環形緩衝區大概如下圖所示。
qringbuffer.h
qringbuffer.cpp#ifndef QRINGBUFFER_P_H #define QRINGBUFFER_P_H #include <QByteArray> #include <QList> #ifndef QRINGBUFFER_CHUNKSIZE #define QRINGBUFFER_CHUNKSIZE 4096 #endif enum { //1G-1位元組 MaxAllocSize = (1 << (std::numeric_limits<int>::digits - 1)) - 1 }; enum { //1G-1-16位元組 MaxByteArraySize = MaxAllocSize - sizeof(QtPrivate::remove_pointer<QByteArray::DataPtr>::type) }; class QRingBuffer { public: //預設分配QRINGBUFFER_CHUNKSIZE大小的buffer QRingBuffer(int growth = QRINGBUFFER_CHUNKSIZE) : head(0), tail(0), tailBuffer(0), basicBlockSize(growth), bufferSize(0) { } ~QRingBuffer(){} //獲取環形緩衝區指定位置的指標 //length，輸出這個指定位置到緩衝區結尾的長度 char *readPointerAtPosition(qint64 pos, qint64 &length); //申請空間：從尾開始，返回新空間的指標 char *reserve(qint64 bytes); //申請空間：從頭開始，返回新空間的指標 char *reserveFront(qint64 bytes); //縮短空間 void truncate(qint64 pos) { if (pos < bufferSize) chop(bufferSize - pos); } //判斷buffers資料是否為空 bool isEmpty() { return bufferSize == 0; } //從頭讀取一個字元，並轉換為int返回 int getChar() { if (isEmpty()) return -1; char c = *readPointer(); free(1); return int(uchar(c)); } //在緩衝區尾部新增字元 void putChar(char c) { char *ptr = reserve(1); *ptr = c; } //在緩衝區頭部新增字元 void ungetChar(char c) { if (head > 0) { --head; buffers.first()[head] = c; ++bufferSize; } else { char *ptr = reserveFront(1); *ptr = c; } } //清空緩衝區 void clear(); //讀取maxLength長度資料到data中，如果buffers中的資料少於maxLength，則讀取所有資料， //返回讀取資料的長度 qint64 read(char *data, qint64 maxLength); //讀取buffers中的第一個buffer QByteArray read(); //從指定位置pos拷貝maxLength長度的資料到data中 //返回實際擷取的資料長度 qint64 peek(char *data, qint64 maxLength, qint64 pos = 0); //擴充套件最後一個buffer void append(const char *data, qint64 size); //在最後新增一個新buffer void append(const QByteArray &qba); //從頭釋放lenght長度空間，一般需要配合reserve使用 qint64 skip(qint64 length) { qint64 bytesToSkip = qMin(length, bufferSize); free(bytesToSkip); return bytesToSkip; } //從尾釋放length長度空間，一般需要配合reserve使用 void chop(qint64 length); //讀取一行,包括該行的結束標誌'\n' qint64 readLine(char *data, qint64 maxLength); bool canReadLine() { return indexOf('\n', bufferSize) >= 0; } private: //獲取下一個資料塊的大小 //如果只剩一個buffer，返回最後一個buffer所含資料的大小；否則返回第一個buffer所含資料的大小。 qint64 nextDataBlockSize() { return (tailBuffer == 0 ? tail : buffers.first().size()) - head; } //獲取緩衝區第一個有效資料的指標 char *readPointer() { return bufferSize == 0 ? Q_NULLPTR : (buffers.first().data() + head); } qint64 indexOf(char c, qint64 maxLength, qint64 pos = 0); //釋放空間 void free(qint64 bytes); private: QList<QByteArray> buffers; //標識第一個buffer資料起始位置和最後一個buffer資料的結尾位置 int head, tail; //大小為buffers.size()-1，如果為0，說明只剩一個buffer int tailBuffer; //初始分配空間的大小 int basicBlockSize; //buffers資料總大小 qint64 bufferSize; }; #endif // QRINGBUFFER_P_H
main.cpp#include "qringbuffer.h" #include <string.h> char *QRingBuffer::readPointerAtPosition(qint64 pos, qint64 &length) { if (pos >= 0) { pos += head; for (int i = 0; i < buffers.size(); ++i) { length = (i == tailBuffer ? tail : buffers[i].size()); if (length > pos) { length -= pos; return buffers[i].data() + pos; } pos -= length; } } length = 0; return 0; } void QRingBuffer::free(qint64 bytes) { Q_ASSERT(bytes <= bufferSize); while (bytes > 0) { const qint64 blockSize = buffers.first().size() - head; if (tailBuffer == 0 || blockSize > bytes) { if (bufferSize <= bytes) { if (buffers.first().size() <= basicBlockSize) { bufferSize = 0; head = tail = 0; } else { clear(); } } else { Q_ASSERT(bytes < MaxByteArraySize); head += int(bytes); bufferSize -= bytes; } return; } bufferSize -= blockSize; bytes -= blockSize; buffers.removeFirst(); --tailBuffer; head = 0; } } char *QRingBuffer::reserve(qint64 bytes) { if (bytes <= 0 || bytes >= MaxByteArraySize) return 0; if (buffers.isEmpty()) { buffers.append(QByteArray()); buffers.first().resize(qMax(basicBlockSize, int(bytes))); } else { const qint64 newSize = bytes + tail; //如果超過最後一個buffer所含資料的大小，則最後一個buffer需要從新分配 if (newSize > buffers.last().size()) { //滿足以下條件時，將最後一個buffer的容積縮小到其當前所含資料的大小， //然後新開闢一個buffer，並將該buffer資料的結尾位置tail設定為0 if (newSize > buffers.last().capacity() && (tail >= basicBlockSize || newSize >= MaxByteArraySize)) { buffers.last().resize(tail); buffers.append(QByteArray()); ++tailBuffer; tail = 0; } //將最後一個buffer進行擴容 buffers.last().resize(qMax(basicBlockSize, tail + int(bytes))); } } char *writePtr = buffers.last().data() + tail; bufferSize += bytes; Q_ASSERT(bytes < MaxByteArraySize); tail += int(bytes); return writePtr; } char *QRingBuffer::reserveFront(qint64 bytes) { if (bytes <= 0 || bytes >= MaxByteArraySize) return 0; if (head < bytes) { if (buffers.isEmpty()) { buffers.append(QByteArray()); } else { buffers.first().remove(0, head); if (tailBuffer == 0) tail -= head; } head = qMax(basicBlockSize, int(bytes)); if (bufferSize == 0) { tail = head; } else { buffers.prepend(QByteArray()); ++tailBuffer; } buffers.first().resize(head); } head -= int(bytes); bufferSize += bytes; return buffers.first().data() + head; } void QRingBuffer::chop(qint64 length) { Q_ASSERT(length <= bufferSize); while (length > 0) { if (tailBuffer == 0 || tail > length) { if (bufferSize <= length) { if (buffers.first().size() <= basicBlockSize) { bufferSize = 0; head = tail = 0; } else { clear(); } } else { Q_ASSERT(length < MaxByteArraySize); tail -= int(length); bufferSize -= length; } return; } bufferSize -= tail; length -= tail; buffers.removeLast(); --tailBuffer; tail = buffers.last().size(); } } void QRingBuffer::clear() { if (buffers.isEmpty()) return; buffers.erase(buffers.begin() + 1, buffers.end()); buffers.first().clear(); head = tail = 0; tailBuffer = 0; bufferSize = 0; } qint64 QRingBuffer::indexOf(char c, qint64 maxLength, qint64 pos) { if (maxLength <= 0 || pos < 0) return -1; qint64 index = -(pos + head); for (int i = 0; i < buffers.size(); ++i) { qint64 nextBlockIndex = qMin(index + (i == tailBuffer ? tail : buffers[i].size()), maxLength); if (nextBlockIndex > 0) { const char *ptr = buffers[i].data(); if (index < 0) { ptr -= index; index = 0; } const char *findPtr = reinterpret_cast<const char *>(memchr(ptr, c, nextBlockIndex - index)); if (findPtr) return qint64(findPtr - ptr) + index + pos; if (nextBlockIndex == maxLength) return -1; } index = nextBlockIndex; } return -1; } qint64 QRingBuffer::read(char *data, qint64 maxLength) { const qint64 bytesToRead = qMin(bufferSize, maxLength); qint64 readSoFar = 0; while (readSoFar < bytesToRead) { const qint64 bytesToReadFromThisBlock = qMin(bytesToRead - readSoFar, nextDataBlockSize()); if (data) memcpy(data + readSoFar, readPointer(), bytesToReadFromThisBlock); readSoFar += bytesToReadFromThisBlock; free(bytesToReadFromThisBlock); } return readSoFar; } QByteArray QRingBuffer::read() { if (bufferSize == 0) return QByteArray(); QByteArray qba(buffers.takeFirst()); //避免調整大小時不必要的記憶體分配，使QByteArray更高效 qba.reserve(0); if (tailBuffer == 0) { qba.resize(tail); tail = 0; } else { --tailBuffer; } qba.remove(0, head); head = 0; bufferSize -= qba.size(); return qba; } qint64 QRingBuffer::peek(char *data, qint64 maxLength, qint64 pos) { qint64 readSoFar = 0; if (pos >= 0) { pos += head; for (int i = 0; readSoFar < maxLength && i < buffers.size(); ++i) { qint64 blockLength = (i == tailBuffer ? tail : buffers[i].size()); if (pos < blockLength) { blockLength = qMin(blockLength - pos, maxLength - readSoFar); memcpy(data + readSoFar, buffers[i].data() + pos, blockLength); readSoFar += blockLength; pos = 0; } else { pos -= blockLength; } } } return readSoFar; } void QRingBuffer::append(const char *data, qint64 size) { char *writePointer = reserve(size); if (size == 1) *writePointer = *data; else if (size) ::memcpy(writePointer, data, size); } void QRingBuffer::append(const QByteArray &qba) { if (tail == 0) { if (buffers.isEmpty()) buffers.append(qba); else buffers.last() = qba; } else { buffers.last().resize(tail); buffers.append(qba); ++tailBuffer; } tail = qba.size(); bufferSize += tail; } qint64 QRingBuffer::readLine(char *data, qint64 maxLength) { if (!data || --maxLength <= 0) return -1; qint64 i = indexOf('\n', maxLength); i = read(data, i >= 0 ? (i+1) : maxLength); data[i] = '\0'; return i; }
測試結果#include <qringbuffer.h> #include <QDebug> int main() { //測試環形緩衝區的寫入和讀取+++++++++++++++++++++++++++++++ qDebug()<<QStringLiteral("測試環形緩衝區的寫入和讀取+++++++++++++++++++++++++++++++"); //方法1 QRingBuffer ringBuffer; ringBuffer.append("CSDN ",5); ringBuffer.append("blog ",5); QByteArray qba("http://blog.csdn.net/caoshangpa"); ringBuffer.append(qba); QByteArray head=ringBuffer.read(); QByteArray tail=ringBuffer.read(); qDebug()<<head<<tail; //方法2 ringBuffer.clear(); ringBuffer.append("CSDN ",5); ringBuffer.append("blog ",5); ringBuffer.append(qba); char str[100]={'\0'}; ringBuffer.read(str,100); qDebug()<<str; //測試在緩衝區頭和尾新增字元+++++++++++++++++++++++++++++++ qDebug()<<QStringLiteral("測試在緩衝區頭和尾新增字元+++++++++++++++++++++++++++++++"); ringBuffer.clear(); ringBuffer.append("CSDN ",5); ringBuffer.append("blog ",5); ringBuffer.append(qba); //頭 ringBuffer.ungetChar('{'); //尾 ringBuffer.putChar('}'); memset(str,0,100); ringBuffer.read(str,100); qDebug()<<str; //測試讀取一行+++++++++++++++++++++++++++++++++++++++++++ qDebug()<<QStringLiteral("測試讀取一行+++++++++++++++++++++++++++++++++++++++++++"); ringBuffer.clear(); ringBuffer.append("CSDN ",5); ringBuffer.append("blog\n",5); ringBuffer.append(qba); memset(str,0,100); if(ringBuffer.canReadLine()) ringBuffer.readLine(str,100); qDebug()<<str; //測試拷貝資料+++++++++++++++++++++++++++++++++++++++++++ qDebug()<<QStringLiteral("測試拷貝資料+++++++++++++++++++++++++++++++++++++++++++"); ringBuffer.clear(); ringBuffer.append("CSDN ",5); ringBuffer.append("blog ",5); ringBuffer.append(qba); memset(str,0,100); ringBuffer.peek(str,10), qDebug()<<str; //證明peek只是拷貝資料 memset(str,0,100); ringBuffer.read(str,100); qDebug()<<str; //測試釋放指定長度緩衝區+++++++++++++++++++++++++++++++++++++++++++ qDebug()<<QStringLiteral("測試釋放指定長度緩衝區+++++++++++++++++++++++++++++++++++++++++++"); ringBuffer.clear(); ringBuffer.append("CSDN ",5); ringBuffer.append("blog ",5); ringBuffer.append(qba); memset(str,0,100); ringBuffer.skip(10), ringBuffer.read(str,100); qDebug()<<str; //測試申請指定長度緩衝區+++++++++++++++++++++++++++++++++++++++++++ qDebug()<<QStringLiteral("測試申請指定長度緩衝區+++++++++++++++++++++++++++++++++++++++++++"); ringBuffer.clear(); ringBuffer.reserve(100); qint64 lenToTail=0; char * pos=ringBuffer.readPointerAtPosition(10,lenToTail); char * test="CSDN blog http://blog.csdn.net/caoshangpa"; memcpy(pos,test,strlen(test)); ringBuffer.skip(10); ringBuffer.chop(100-10-strlen(test)); qDebug()<<ringBuffer.read(); qDebug()<<lenToTail; system("pause"); }
二.使用QSemaphore
用定時器將當前時間儲存到QStringList物件中，然後通過現場去QStringList物件中取出並列印，通過兩個QSemaphore物件進行同步，形成環形緩衝區。
thread.h
#ifndef THREAD_H
#define THREAD_H
#include <QThread>
#include <QSemaphore>
#include <QStringList>
class Thread : public QThread
{
Q_OBJECT
public:
Thread();
~Thread();
protected:
void run();
private slots:
void slotTimeout();
};
#endif // THREAD_H#include "thread.h"
#include <QDebug>
#include <QTimer>
#include <QTime>
//1024是這個list的緩衝區大小，當然可以隨意設定；80的意思是初始空閒的緩衝區大小是1024
QSemaphore freeBytes(1024);
//初始被使用的緩衝區大小是0
QSemaphore usedBytes(0);
QStringList timeList;
Thread::Thread()
{
QTimer *timer=new QTimer(this);
connect(timer,SIGNAL(timeout()),this,SLOT(slotTimeout()));
timer->start(50);
}
Thread::~Thread()
{
}
void Thread::run()
{
//獲取並列印list中的時間
while(true)
{
usedBytes.acquire();
qDebug()<<timeList.takeFirst();
freeBytes.release();
}
}
//通過定時器給list存入當前時間
void Thread::slotTimeout()
{
freeBytes.acquire();//先申請一個位置，相當於空閒的緩衝區變為1023
timeList.append(QTime::currentTime().toString("now : hh:mm:ss:zzz"));
usedBytes.release();//給使用的大小+1
}
#include <QCoreApplication>
#include "thread.h"
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
Thread *thread=new Thread;
thread->start();
return a.exec();
}
