c++ gstreamer使用

1，gstream是个啥？

GStreamer 是用来构建流媒体应用的开源多媒体框架，实际上就是可以用来解码mp4的一个东东。

2，编译安装

我的开发模块的ubuntu18.04系统自带gstream，并且交由pkg-config管理，灰常方便。就不用编译安装等一通操作了。

gstreamer的各种目录：

pkg-config --cflags --libs gstreamer-1.0

#返回
-pthread -I/usr/include/gstreamer-1.0 -I/usr/include/glib-2.0 -I/usr/lib/aarch64-linux-gnu/glib-2.0/include -lgstreamer-1.0 -lgobject-2.0 -lglib-2.0

gstreamer的pc文件的目录：

/usr/lib/aarch64-linux-gnu/pkgconfig/gstreamer-1.0.pc

编译命令灰常简单啦，我猜应当是这样滴：

g++ test.cpp -o test `pkg-config --cflags --libs gstreamer-1.0`

 3，gstreamer支持命令行操作

Gstreamer自带了gst-inspect-1.0和gst-launch-1.0命令行工具

#输入命令：
gst-launch-1.0 --help


Usage:
  gst-launch-1.0 [OPTION…] PIPELINE-DESCRIPTION

Help Options:
  -h, --help                        Show help options
  --help-all                        Show all help options
  --help-gst                        Show GStreamer Options

Application Options:
  -t, --tags                        Output tags (also known as metadata)
  -c, --toc                         Output TOC (chapters and editions)
  -v, --verbose                     Output status information and property notifications
  -q, --quiet                       Do not print any progress information
  -m, --messages                    Output messages
  -X, --exclude=PROPERTY-NAME       Do not output status information for the specified property if verbose output is enabled (can be used multiple times)
  -f, --no-fault                    Do not install a fault handler
  -e, --eos-on-shutdown             Force EOS on sources before shutting the pipeline down
  --version                         Print version information and exit

#输入命令：
~/temp/gsm$ gst-inspect-1.0 --help


Usage:
  gst-inspect-1.0 [OPTION…] [ELEMENT-NAME | PLUGIN-NAME]

Help Options:
  -h, --help                           Show help options
  --help-all                           Show all help options
  --help-gst                           Show GStreamer Options

Application Options:
  -a, --print-all                      Print all elements
  -b, --print-blacklist                Print list of blacklisted files
  --print-plugin-auto-install-info     Print a machine-parsable list of features the specified plugin or all plugins provide.
                                       Useful in connection with external automatic plugin installation mechanisms
  --plugin                             List the plugin contents
  -t, --types                          A slashes ('/') separated list of types of elements (also known as klass) to list. (unordered)
  --exists                             Check if the specified element or plugin exists
  --atleast-version                    When checking if an element or plugin exists, also check that its version is at least the version specified
  -u, --uri-handlers                   Print supported URI schemes, with the elements that implement them
  --version                            Print version information and exit

调用命令行参数播放mp3

gst-launch-1.0 playbin uri=file:///d:/gstreamer/music.mp3 

调用命令行播放mp4或者流媒体 

gst-launch-1.0 playbin uri=file:///d:/gstreamer/36.mp4
#file后面是，冒号，绝对路径
gst-launch-1.0 playbin uri=file:///rtsp://xxx:xxxxx@44.44.44.44/h264/ch1/main/av_stream

4,gstreamer配置windows环境：

这个就比较厉害了，并不需要源码编译哦，两个exe直接执行一下就可以了。

详细教程请参考：感谢作者系列。

只需要注意两件事：

1，第一次运行应该会提示你，缺少libstreamer-1.0-0.dll,在哪里呢？我的动态库在D:gstreamer1.0x86_64in下，为了保险，我干脆把该文件夹下所有的dll和exe都拷贝到了运行文件夹下。

2，需要配置一个环境变量，GST_PLUGIN_PATH，内容写到D:gstreamer1.0x86_64

5，开发

参考官网教程

1），基本教程1

 如下是开发文档上的，目前跑起来除了能打印finish以外，啥也不能做，主要是知道几个api就可以了：

gst_init（）初始化GStreamer 。

gst_parse_launch（）从文本描述快速构建管道 。

playbin创建自动播放管道。

gst_element_set_state（）通知GStreamer开始播放 。

gst_element_get_bus（）和gst_bus_timed_pop_filtered（）来释放资源

#include <iostream>
#include <gst/gst.h>
#include <glib.h>

int main(int argc, char *argv[]) {
    GstElement *pipeline;
    GstBus *bus;
    GstMessage *msg;

    /* Initialize GStreamer */
    gst_init(&argc, &argv);
    //初始化gstream，这是使用项目的前提，

    /* Build the pipeline */
    pipeline =gst_parse_launch("playbin uri=file:///D:/gstream/1.mp4",NULL);
    //gst_parse_launch使用系统预设的管道来处理流媒体。如果我没理解错的话，gst_parse_launch创建的是一个由playbin单元素组成的管道
    //当然你也可以不使用系统预设，那么你就需要手动组装了。

    /* Start playing */
    gst_element_set_state(pipeline, GST_STATE_PLAYING);
    //将我们的元素设置为playing状态才能开始播放

    /* Wait until error or EOS */
    bus = gst_element_get_bus(pipeline);
    //Returns the bus of the element？？不太能懂，大约就是获取一个可以操作的pipeline对象吧
    msg =gst_bus_timed_pop_filtered(bus, GST_CLOCK_TIME_NONE,GST_MESSAGE_ERROR );  //| GST_MESSAGE_EOS
    //遇到错误或者播放完毕以后gst_bus_timed_pop_filtered（）会返回一条消息
    /* Free resources */
    if (msg != NULL) {
        gst_message_unref(msg);
        //需要使用gst_message_unref（）将msg释放,此函数专门清除gst_bus_timed_pop_filtered
        gst_object_unref(bus);
        gst_element_set_state(pipeline, GST_STATE_NULL);
        gst_object_unref(pipeline);
    }
    printf("finish");
    return 0;
}

2）创建元件并且链接起来

#include <gst/gst.h>

int main(int argc, char *argv[]) {
    GstElement *pipeline, *source, *sink;
    GstBus *bus;
    GstMessage *msg;
    GstStateChangeReturn ret;

    /* Initialize GStreamer */
    gst_init(&argc, &argv);

    /* Create the elements */
    source = gst_element_factory_make("videotestsrc", "source");
    sink = gst_element_factory_make("autovideosink", "sink");
    //创建元件，参数：元件的类型，元件名称
    //videotestsrc是一个源元素（它产生数据），它创建一个测试视频模式。
    //autovideosink是一个接收器元素（它消耗数据），它在窗口上显示它接收到的图像。

    /* Create the empty pipeline */
    pipeline = gst_pipeline_new("test-pipeline");
    //创建管道，管道是一种特殊类型的bin，（估计就是箱柜）

    if (!pipeline || !source || !sink) {
        g_printerr("Not all elements could be created.
");
        return -1;
    }

    /* Build the pipeline */
    gst_bin_add_many(GST_BIN(pipeline), source, sink, NULL);
    //向管道中添加元件，以null结尾，添加单个和可以，函数是：gst_bin_add()
    if (gst_element_link(source, sink) != TRUE) {
        g_printerr("Elements could not be linked.
");
        gst_object_unref(pipeline);
        return -1;
    }

    /* Modify the source's properties */
    g_object_set(source, "pattern", 0, NULL);
    //修改元件的属性

    /* Start playing */
    ret = gst_element_set_state(pipeline, GST_STATE_PLAYING);
    if (ret == GST_STATE_CHANGE_FAILURE) {
        g_printerr("Unable to set the pipeline to the playing state.
");
        gst_object_unref(pipeline);
        return -1;
    }
    //设置管道开始工作
    //调用gst_element_set_state()，并且检查其返回值是否有错误。

    /* Wait until error or EOS */
    bus = gst_element_get_bus(pipeline);
    //获取pipeline的总线
    msg = gst_bus_timed_pop_filtered(bus, GST_CLOCK_TIME_NONE, GST_MESSAGE_ERROR );
    //gst_bus_timed_pop_filtered()等待执行结束并返回GstMessage

    /* Parse message：
    GstMessage是一种非常通用的结构，
    通过使用 GST_MESSAGE_TYPE()宏可以获得其中的消息
    */
    if (msg != NULL) {
        GError *err;
        gchar *debug_info;

        switch (GST_MESSAGE_TYPE(msg)) {
        case GST_MESSAGE_ERROR:
            gst_message_parse_error(msg, &err, &debug_info);
            g_printerr("Error received from element %s: %s
", GST_OBJECT_NAME(msg->src), err->message);
            g_printerr("Debugging information: %s
", debug_info ? debug_info : "none");
            g_clear_error(&err);
            g_free(debug_info);
            break;
        case GST_MESSAGE_EOS:
            g_print("End-Of-Stream reached.
");
            break;
        default:
            /* We should not reach here because we only asked for ERRORs and EOS */
            g_printerr("Unexpected message received.
");
            break;
        }
        gst_message_unref(msg);
    }

    /* Free resources */
    gst_object_unref(bus);
    gst_element_set_state(pipeline, GST_STATE_NULL);
    gst_object_unref(pipeline);
    return 0;
}

重点：

如何使用创建元素 gst_element_factory_make()

如何使用创建空管道 gst_pipeline_new()

如何使用以下方法向管道添加元素 gst_bin_add_many()

如何将元素彼此链接在一起 gst_element_link()

这个程序跑起来就是出现一个花屏的窗口，让人惊讶的是，它耗费很少的内存和cpu，反正肉眼没看出来，耗费比较多的inter显卡，大约8%的样子。

3），添加衬垫，添加回调，手动链接衬垫，

网络原因，这个代码编译没问题，但是跑不起来

#include <gst/gst.h>

/* Structure to contain all our information, so we can pass it to callbacks */
typedef struct _CustomData {
    GstElement *pipeline;
    GstElement *source;
    GstElement *convert;
    GstElement *resample;
    GstElement *sink;
} CustomData;
//先建立一个结构，里面放了一个pipeline指针和四个元件指针

/* Handler for the pad-added signal */
static void pad_added_handler(GstElement *src, GstPad *pad, CustomData *data);
//声明一个函数，叫添加衬垫的函数pad_added_handler。
int main(int argc, char *argv[]) {
    CustomData data;
    GstBus *bus;
    GstMessage *msg;
    GstStateChangeReturn ret;
    gboolean terminate = FALSE;

    /* Initialize GStreamer */
    gst_init(&argc, &argv);
    //同样需要先初始化

    /* Create the elements */
    data.source = gst_element_factory_make("uridecodebin", "source");
    data.convert = gst_element_factory_make("audioconvert", "convert");
    data.resample = gst_element_factory_make("audioresample", "resample");
    data.sink = gst_element_factory_make("autoaudiosink", "sink");

    /* Create the empty pipeline */
    data.pipeline = gst_pipeline_new("test-pipeline");
    //先把data里的信息创建出来，创建了一个pipeline和四个元件

    if (!data.pipeline || !data.source || !data.convert || !data.resample || !data.sink) {
        g_printerr("Not all elements could be created.
");
        return -1;
    }

    /* Build the pipeline. Note that we are NOT linking the source at this
    * point. We will do it later. */
    gst_bin_add_many(GST_BIN(data.pipeline), data.source, data.convert, data.resample, data.sink, NULL);
    if (!gst_element_link_many(data.convert, data.resample, data.sink, NULL)) {
        g_printerr("Elements could not be linked.
");
        gst_object_unref(data.pipeline);
        return -1;
    }

    /* Set the URI to play */
    g_object_set(data.source, "uri", "https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm", NULL);
    //大约是将source元件的衬垫链接到某个网址上

    /* Connect to the pad-added signal */
    g_signal_connect(data.source, "pad-added", G_CALLBACK(pad_added_handler), &data);
    //GSignals是GStreamer中的关键点。它们使您可以在发生事情时（通过回调）得到通知，所以我们为source元件添加了一个回调
    //这个回调好像没有传递参数啊喂，好吧，官方是真么说的：src是GstElement触发信号的。在此示例中，它只能是uridecodebin。newpad是刚刚添加到src元素中的，我理解为哦我们为source添加回调这件事就是增加了一个衬垫，data是当作指针来传递信号的

    /* Start playing */
    ret = gst_element_set_state(data.pipeline, GST_STATE_PLAYING);
    if (ret == GST_STATE_CHANGE_FAILURE) {
        g_printerr("Unable to set the pipeline to the playing state.
");
        gst_object_unref(data.pipeline);
        return -1;
    }

    /* Listen to the bus */
    bus = gst_element_get_bus(data.pipeline);
    do {
        msg = gst_bus_timed_pop_filtered(bus, GST_CLOCK_TIME_NONE, GST_MESSAGE_ANY);
        //等待执行结束并且返回
        //顺带说一句，以前的老语法是GST_MESSAGE_STATE_CHANGED | GST_MESSAGE_ERROR | GST_MESSAGE_EOS这样的，所以下文中的case用的是这几个错误信息，但是现在这个语法不被支持了。嗯嗯
        /* Parse message */
        if (msg != NULL) {
            GError *err;
            gchar *debug_info;

            switch (GST_MESSAGE_TYPE(msg)) {
            case GST_MESSAGE_ERROR:
                gst_message_parse_error(msg, &err, &debug_info);
                g_printerr("Error received from element %s: %s
", GST_OBJECT_NAME(msg->src), err->message);
                g_printerr("Debugging information: %s
", debug_info ? debug_info : "none");
                g_clear_error(&err);
                g_free(debug_info);
                terminate = TRUE;
                break;
            case GST_MESSAGE_EOS:
                g_print("End-Of-Stream reached.
");
                terminate = TRUE;
                break;
            case GST_MESSAGE_STATE_CHANGED:
                /* We are only interested in state-changed messages from the pipeline */
                if (GST_MESSAGE_SRC(msg) == GST_OBJECT(data.pipeline)) {
                    GstState old_state, new_state, pending_state;
                    gst_message_parse_state_changed(msg, &old_state, &new_state, &pending_state);
                    g_print("Pipeline state changed from %s to %s:
",
                        gst_element_state_get_name(old_state), gst_element_state_get_name(new_state));
                }
                break;
            default:
                /* We should not reach here */
                g_printerr("Unexpected message received.
");
                break;
            }
            gst_message_unref(msg);
        }
    } while (!terminate);
    //只要不中止，就一直监视执行结束的状态

    /* Free resources */
    gst_object_unref(bus);
    gst_element_set_state(data.pipeline, GST_STATE_NULL);
    gst_object_unref(data.pipeline);
    return 0;
}

/* This function will be called by the pad-added signal */
static void pad_added_handler(GstElement *src, GstPad *new_pad, CustomData *data) {
    GstPad *sink_pad = gst_element_get_static_pad(data->convert, "sink");
    //pipeline的链接顺序是：source-convert-resample-sink，我们为source添加了回调，然后此处在回调内部获取了convert的对应的衬垫
    GstPadLinkReturn ret;
    GstCaps *new_pad_caps = NULL;
    GstStructure *new_pad_struct = NULL;
    const gchar *new_pad_type = NULL;

    g_print("Received new pad '%s' from '%s':
", GST_PAD_NAME(new_pad), GST_ELEMENT_NAME(src));

    /* If our converter is already linked, we have nothing to do here */
    if (gst_pad_is_linked(sink_pad)) {
        g_print("We are already linked. Ignoring.
");
        goto exit;
    }
    //此处应该是检查新为source添加的衬垫是不是已经链接到了convert衬垫

    /* Check the new pad's type */
    new_pad_caps = gst_pad_get_current_caps(new_pad);
    new_pad_struct = gst_caps_get_structure(new_pad_caps, 0);
    new_pad_type = gst_structure_get_name(new_pad_struct);
    if (!g_str_has_prefix(new_pad_type, "audio/x-raw")) {
        g_print("It has type '%s' which is not raw audio. Ignoring.
", new_pad_type);
        goto exit;
    }
    //检查这个衬垫当前输出的数据类型，经过一番解析，如果发现里面没有"audio/x-raw"，那说明这不是解码音频的

    /* Attempt the link */
    ret = gst_pad_link(new_pad, sink_pad);
    if (GST_PAD_LINK_FAILED(ret)) {
        g_print("Type is '%s' but link failed.
", new_pad_type);
    }
    else {
        g_print("Link succeeded (type '%s').
", new_pad_type);
    }
    //如果两个衬垫没链接，那就人为地链接起来

exit:
    //这个语法就厉害了，首先定义了一个exit标号，如果前文中goto exit;那转到的就将会是此处
    /* Unreference the new pad's caps, if we got them */
    if (new_pad_caps != NULL)
        gst_caps_unref(new_pad_caps);

    /* Unreference the sink pad */
    gst_object_unref(sink_pad);
}

6，开发2

参考自己下载的pdf教程

1），打印gstreamer的版本信息：

#include <iostream>
#include <gst/gst.h>
#include <glib.h>

//#include <gst/gst.h>
int main(int argc,char *argv[])
{
    const gchar *nano_str;
    guint major, minor, micro, nano;
    gst_init(&argc, &argv);
    gst_version(&major, &minor, &micro, &nano);
    if (nano == 1)
        nano_str = "(CVS)";
    else if (nano == 2)
        nano_str = "(Prerelease)";
    else
        nano_str = "";
    printf("This program is linked against GStreamer %d.%d.%d %s
",major, minor, micro, nano_str);
    return 0;
}

2），gstreamer封装的argparse

运行命令：xxx.exe --help

#include <iostream>
#include <gst/gst.h>
#include <glib.h>

#include <gst/gst.h>
int main(int argc,char *argv[])
{
    gboolean silent = FALSE;
    gchar *savefile = NULL;
    GOptionContext *ctx;
    GError *err = NULL;
    GOptionEntry entries[] = {
        { "silent", 's', 0, G_OPTION_ARG_NONE, &silent,"do not output status information", NULL },
        { "output", 'o', 0, G_OPTION_ARG_STRING, &savefile,"save xml representation of pipeline to FILE and exit", "FILE" },
        { NULL }
    };
    ctx = g_option_context_new("- Your application");
    g_option_context_add_main_entries(ctx, entries, NULL);
    g_option_context_add_group(ctx, gst_init_get_option_group());
    if (!g_option_context_parse(ctx, &argc, &argv, &err)) {
        g_print("Failed to initialize: %s
", err->message);
        g_error_free(err);
        return 1;
    }
    printf("Run me with --help to see the Application options appended.
");
    return 0;
}

3),创建gst元件对象

gst_element_factory_make

#include <iostream>
#include <gst/gst.h>
#include <glib.h>


int main(int argc, char *argv[])
{
    GstElement *element;
    gchar *name;
    /* init GStreamer */
    gst_init(&argc, &argv);
    /* create element */
    element = gst_element_factory_make("fakesrc", "source");
    //创建一个jst元件
    if (!element) {
        g_print("Failed to create element of type 'fakesrc'
");
        return -1;
    }
    else {
        g_print("gstelement ok!
");
    }
    element = gst_element_factory_make("fakesrc", "source");
    /* get name */
    g_object_get(G_OBJECT(element), "name", &name, NULL);
    //g_object_get获取gobject对象的名字属性
    g_print("The name of the element is '%s'.
", name);
    g_free(name);
    gst_object_unref(GST_OBJECT(element));
    //释放jst元件，必须手动释放
    return 0;
}

元件的四种状态：

GST_STATE_NULL: 默认状态。该状态将会回收所有被该元件占用的资源。
GST_STATE_READY: 准备状态。元件会得到所有所需的全局资源，这些全局资源将被通过该元 件的数据流所使用。例如打开设备、分配缓存等。但在这种状态下，数据流仍未开始被处 理，所 以数据流的位置信息应该自动置 0。如果数据流先前被打开过，它应该被关闭，并且其位置信 息、特性信息应该被重新置为初始状态。
GST_STATE_PAUSED: 在这种状态下，元件已经对流开始了处理，但此刻暂停了处理。因此该 状态下元件可以修改流的位置信息，读取或者处理流数据，以及一旦状态变为 PLAYING，流可 以重放数据流。这种情况下，时钟是禁止运行的。总之， PAUSED 状态除了不能运行时钟外， 其它与 PLAYING 状态一模一样。处于 PAUSED 状态的元件会很快变换到 PLAYING 状态。举 例来说，视频或音频输出元件会等待数据的到来，并将它们压入队列。一旦状态改变，元件就会 处理接收到的数据。同样，视频接收元件能够播放数据的第 一帧。(因为这并不会影响时钟)。自 动加载器（Autopluggers）可以对已经加载进管道的插件进行这种状态转换。其它更多的像 codecs 或者 filters 这种元件不需要在这个状态上做任何事情。
GST_STATE_PLAYING: PLAYING 状态除了当前运行时钟外，其它与 PAUSED 状态一模一 样。你可以通过函数 gst_element_set_state()来改变一个元件的状态。你如果显式地改变一个元件 的状态，GStreamer 可能会 使它在内部经过一些中间状态。例如你将一个元件从 NULL 状态设 置为 PLAYING 状态，GStreamer 在其内部会使得元件经历过 READY 以及 PAUSED 状态。 当处于 GST_STATE_PLAYING 状态，管道会自动处理数据。它们不需要任何形式的迭代。

 

4），查看插件

#include <iostream>
#include <gst/gst.h>
#include <glib.h>

int main(int argc,char *argv[])
{
    GstElementFactory *factory;
    //声明插件，插件是GstElementFactory

    /* init GStreamer */
    gst_init(&argc, &argv);
    /* get factory */
    factory = gst_element_factory_find("audiotestsrc");
    //寻找系统里是否有这个插件
    if (!factory) {
        g_print("You don't have the 'audiotestsrc' element installed!
");
        return -1;
    }
    /* display information */
    g_print("The '%s' element is a member of the category %s.
"
        "Description: %s
",
        gst_plugin_feature_get_name(GST_PLUGIN_FEATURE(factory)),
        gst_element_factory_get_klass(factory),
        gst_element_factory_get_description(factory));
    //打印出插件的信息
    return 0;
}

这个功能就像是命令行里的如下命令：

gst-inspect-1.0   audiotestsrc
#gst-inspect-1.0 加插件名

 5），链接元件

gst_bin_add_many

gst_element_link

#include <iostream>
#include <gst/gst.h>
#include <glib.h>

#include <gst/gst.h>
int
main(int argc,
    char *argv[])
{
    GstElement *pipeline;
    GstElement *source, *filter, *sink;
    //声明一个源元件，过滤元件和接收元件
    /* init */
    gst_init(&argc, &argv);
    /* create pipeline */
    pipeline = gst_pipeline_new("my-pipeline");
    /* create elements */
    source = gst_element_factory_make("fakesrc", "source");
    filter = gst_element_factory_make("identity", "filter");
    sink = gst_element_factory_make("fakesink", "sink");
    //选择3个插件创建3个不同的元件
    /* must add elements to pipeline before linking them */
    gst_bin_add_many(GST_BIN(pipeline), source, filter, sink, NULL);
    /* link */
    if (!gst_element_link_many(source, filter, sink, NULL)) {
        g_warning("Failed to link elements!");
    }
    return 0;
}

6），箱柜（箱柜本身是一个元件，但是它内部还可以是一串链接起来的元件）

gst_pipeline_new

#include <iostream>
#include <gst/gst.h>
#include <glib.h>

#include <gst/gst.h>
int
main(int argc,
    char *argv[])
{
    GstElement *bin, *pipeline, *source, *sink;
    /* init */
    gst_init(&argc, &argv);
    /* create */
    pipeline = gst_pipeline_new("my_pipeline");
    bin = gst_pipeline_new("my_bin");
    //创建箱柜：gst_pipeline_new和gst_bin_new

    source = gst_element_factory_make("fakesrc", "source");
    sink = gst_element_factory_make("fakesink", "sink");
    /* set up pipeline */
    gst_bin_add_many(GST_BIN(bin), source, sink, NULL);
    gst_bin_add(GST_BIN(pipeline), bin);
    //添加元件到箱柜
    gst_bin_remove(GST_BIN(bin),sink);
    //从箱柜中移除元件，移除的元件自动被销毁，

    gst_element_link(source, sink);
    //链接元件，因为sink元件被我移除了，所以可能实际上运行不起来

    gst_object_unref(GST_OBJECT(source));
    gst_object_unref(GST_OBJECT(sink));
    return 0;
}

7），bus总线

获取bus总线：gst_pipeline_get_bus
在总线上添加一个回调函数（官方语言叫watch）：gst_bus_add_watch

#include <gst/gst.h>
static GMainLoop *loop;
static gboolean
my_bus_callback(GstBus *bus,GstMessage *message,gpointer data)
{
    g_print("Got %s message
", GST_MESSAGE_TYPE_NAME(message));
    switch (GST_MESSAGE_TYPE(message)) {
    case GST_MESSAGE_ERROR: {
        GError *err;
        gchar *debug;
        gst_message_parse_error(message, &err, &debug);
        g_print("Error: %s
", err->message);
        g_error_free(err);
        g_free(debug);
        g_main_loop_quit(loop);
        break;
    }
    case GST_MESSAGE_EOS:
        /* end-of-stream */
        g_main_loop_quit(loop);
        break;
    default:
        /* unhandled message */
        g_print("something happend!
");
        break;
    }
    /* we want to be notified again the next time there is a message
    * on the bus, so returning TRUE (FALSE means we want to stop watching
    * for messages on the bus and our callback should not be called again)
    */
    return TRUE;
}
gint main(gint argc,gchar *argv[])
{
    GstElement *pipeline;
    GstBus *bus;
    /* init */
    gst_init(&argc, &argv);
    /* create pipeline, add handler */
    pipeline = gst_pipeline_new("my_pipeline");
    /* adds a watch for new message on our pipeline's message bus to
    * the default GLib main context, which is the main context that our
    * GLib main loop is attached to below
    */
    bus = gst_pipeline_get_bus(GST_PIPELINE(pipeline));
    //首先获取总线
    gst_bus_add_watch(bus, my_bus_callback, NULL);
    //然后添加一个消息处理器：设置消息处理器到管道的总线上gst_bus_add_watch ()
    gst_object_unref(bus);
    
    /* create a mainloop that runs/iterates the default GLib main context
    * (context NULL), in other words: makes the context check if anything
    * it watches for has happened. When a message has been posted on the
    * bus, the default main context will automatically call our
    * my_bus_callback() function to notify us of that message.
    * The main loop will be run until someone calls g_main_loop_quit()
    */
    loop = g_main_loop_new(NULL, FALSE);
    g_main_loop_run(loop);
    /* clean up */
    gst_element_set_state(pipeline, GST_STATE_NULL);
    /*gst_element_unref(pipeline);
    gst_main_loop_unref(loop);*/
    return 0;
}

 8），消息类型