SDL SDL_CondWait及SDL_CondSignal使用

頭文件
#include "SDL.h"
#include "SDL_thread.h"

int SDL_CondWait(SDL_cond *cond, SDL_mutex *mut);//等待條件變量
SDL_CondWait：釋放傳遞進來的鎖，然后當條件變量滿足的時候，等待其他的線程調用SDL_CondSignal或者SDL_CondBroadcast
改變條件變量cond,然后重新獲取鎖，然后返回。在調用該函數之前必須先調用SDL_LockMutex獲取
鎖。當信號被觸發的時候就返回0，錯誤的時候返回-1

簡而言之，就是釋放鎖，讓其他的線程有機會獲取到鎖，滿足條件的時候，其他的線程會調用SDL_CondSignal或者SDL_CondBroadcast
釋放鎖，并且讓當前的線程繼續執行下去

int SDL_CondSignal(SDL_cond *cond);//喚醒正在等待條件變量發生改變的線程

代碼

typedef struct PacketQueue {
AVPacketList *first_pkt, *last_pkt;
int nb_packets;??????? //為包的總數
int size;??? //為所有包的大小
SDL_mutex *mutex; //互斥鎖
SDL_cond *cond; //條件變量
} PacketQueue;
我們自己創建的用于構建Packet隊列的數據結構


AVPacketList
A simple linked list for packets.
AVPacket pkt
AVPacketList * next

void packet_queue_init(PacketQueue *q)
{

??? memset(q,0,sizeof(PacketQueue));
??? q->mutex = SDL_CreateMutex();
??? q->cond=SDL_CreateCond();
}

對PacketQueue數據結構進行初始化

用于給PacketQueue數據結構中填入包的函數
int??? packet_queue_put(PacketQueue??? *q,AVPacket??? *pkt)
{
??? AVPacketList??? *pkt1;
??? if(av_dup_packet(pkt)<0)
??? {
??????? return??? -1;??? //檢查是否為NULL，為NULL則自己填充，否則返回-1
??? }
??? pkt1??? =??? av_malloc(sizeof(AVPacketList));//給AVPacketList分配空間
? ?

??? if (!pkt1)
??????? return -1;
??? pkt1->pkt = *pkt;
??? pkt1->next = NULL;
??? SDL_LockMutex(q->mutex); //對PacketQueue進行操作，先鎖定互斥變量
??? if (!q->last_pkt)
??????? q->first_pkt = pkt1;
??? else
??????? q->last_pkt->next = pkt1;
??? q->last_pkt = pkt1;
??? q->nb_packets++;
??? q->size += pkt1->pkt.size;
??? SDL_CondSignal(q->cond); //發送條件信號，方便等待數據的地方喚醒
??? SDL_UnlockMutex(q->mutex); //解鎖
??? return 0;
}

接收數據
static int packet_queue_get(PacketQueue *q, AVPacket *pkt, int block)
{
??? AVPacketList??? *pkt1;
??? int??? ret;
??? SDL_LockMutex(q->mutex);//鎖定mutex
??? for(;;)
??? {
??????? if(quit)
??????? {
??????????? ret??? =-1;
??????????? break;
??????? }
??????? pkt1=q->first_pkt;
??????? if (pkt1) {
??????????? q->first_pkt = pkt1->next;
??????????? if (!q->first_pkt)
??????????????? q->last_pkt = NULL;
??????????? q->nb_packets--;
??????????? q->size -= pkt1->pkt.size;
??????????? *pkt = pkt1->pkt;
??????????? av_free(pkt1);
??????????? ret = 1;
??????????? break;
??????? }

//如果block設置為false,說明不阻塞等待數據包，直接返回
??????? else if (!block) {
??????????? ret = 0;
??????????? break;
??????? }
??????? else {

//等待數據包進入隊列，然后才返回
??????????? SDL_CondWait(q->cond, q->mutex);
??????? }
??? }
??? SDL_UnlockMutex(q->mutex);
??? return ret;
}
SDL_CondWait先檢測是否滿足條件，若不滿足，解鎖mutex，wait，
直至被SDL_CondSignal()函數或者SDL_CondBroadcast()函數通知，
則鎖定mutex并返回

參考
http://blog.sina.com.cn/s/blog_4ae178ba01010fns.html