本文是我的《FFMPEG Tips》系列的第四篇文章，前面的文章有提到如何提取碼流資訊、如何讀每一幀的資料，這些都是離不開網路操作，例如：使用 ffmpeg 讀取一個碼流，常規的程式碼流程示例如下：

AVFormatContext *ic = avformat_alloc_context();

if (avformat_open_input(&ic, url, NULL, NULL) < 0) {
    return -1;
}

if (avformat_find_stream_info(ic, NULL) < 0) {
    return -1;
}

AVPacket avpkt;
av_init_packet(&avpkt);

while (!abort_request) {
    int ret = av_read_frame(ic, &avpkt);
    if (ret < 0) {
        break;
    }
    // processing
}

av_free_packet(&avpkt);
 

其中，

- avformat_open_input 主要負責連線媒體伺服器，以及讀取碼流的頭資訊

- av_read_frame 主要負責每次讀取一幀資料，包括解協議和解封裝

這兩個函式，都有可能會出現耗時很長或者阻塞的情況，比如：

- 網路很爛或者很不穩定

- 伺服器響應比較慢

- 直播流不存在或者沒有資料

因此，我們需要一箇中斷機制，在等待超時或者退出播放的時候，就可以輕鬆中斷掉這個阻塞過程。

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ffmpeg 提供了一個很簡單的回撥機制，即註冊一個自定義的回撥函式，用於外部中斷阻塞的網路操作，用法如下所示：

static int custom_interrupt_callback(void *arg) {
    if (timeout || abort_request) {
        return 1;
    }
    return 0;
}

AVFormatContext *ic = avformat_alloc_context();
ic->interrupt_callback.callback = custom_interrupt_callback;
ic->interrupt_callback.opaque = custom_arg;

當自定義的回撥函式返回 1，則會產生中斷。因此，我們可以在等待超時或者退出播放器的時候，將 timeout 或者 abort_request 置為 1 來達到中斷當前的網路阻塞過程的目的。

本文的知識點就這麼多，關於如何中斷阻塞的網路執行緒就介紹到這兒了，文章中有不清楚的地方歡迎留言