docs: add subscribe demo for c (#12028)

* docs: add subscribe demo for c

* docs: add subscribe demo for c

docs-cn/04-develop/06-subscribe.mdx

@@ -176,7 +176,7 @@ $ taos
 
 ### 准备数据
 
-```sql
+```
 # create database "power"
 taos> create database power;
 # use "power" as the database in following operations
@@ -220,10 +220,10 @@ Query OK, 5 row(s) in set (0.004896s)
   </TabItem>
   <TabItem label="C#" value="csharp">
       <CSharp/>
-  </TabItem>
+  </TabItem> */}
   <TabItem label="C" value="c">
       <CDemo/>
-  </TabItem> */}
+  </TabItem> 
 </Tabs>
 
 ### 运行示例程序
@@ -240,14 +240,14 @@ ts: 1597465200000	current: 11.2	voltage: 220	phase: 1	location: Beijing.Haidian
 
 接着，使用 taos 客户端向表中新增一条数据：
 
-```sql
+```
 # taos
 taos> use power;
-taos> insert into d1001 values("2020-08-15 12:40:00.000", 12.4, 220, 1);
+taos> insert into d1001 values(now, 12.4, 220, 1);
 ```
 
 因为这条数据的电流大于 10A，示例程序会将其消费：
 
 ```
-ts: 1597466400000	current: 12.4	voltage: 220	phase: 1	location: Beijing.Chaoyang	groupid: 2
+ts: 1651146662805	current: 12.4	voltage: 220	phase: 1	location: Beijing.Chaoyang	groupid: 2
 ```

docs-cn/04-develop/_category_.yml

-label: 开发指南
-link:
-  slug: /develop
-  type: generated-index
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+label: 开发指南
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docs-examples/c/subscribe_demo.c

 // compile with:
 // gcc -o subscribe_demo subscribe_demo.c -ltaos
-// writing...
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <taos.h>
+
+int nTotalRows;
+
+/**
+ * @brief callback function of subscription.
+ *
+ * @param tsub
+ * @param res
+ * @param param. the additional parameter passed to  taos_subscribe
+ * @param code. error code
+ */
+void subscribe_callback(TAOS_SUB* tsub, TAOS_RES* res, void* param, int code) {
+  if (code != 0) {
+    printf("error: %d\n", code);
+    exit(EXIT_FAILURE);
+  }
+
+  TAOS_ROW    row = NULL;
+  int         num_fields = taos_num_fields(res);
+  TAOS_FIELD* fields = taos_fetch_fields(res);
+  int         nRows = 0;
+
+  while ((row = taos_fetch_row(res))) {
+    char buf[4096] = {0};
+    taos_print_row(buf, row, fields, num_fields);
+    puts(buf);
+    nRows++;
+  }
+
+  nTotalRows += nRows;
+  printf("%d rows consumed.\n", nRows);
+}
+
+int main() {
+  TAOS* taos = taos_connect("localhost", "root", "taosdata", NULL, 6030);
+  if (taos == NULL) {
+    printf("failed to connect to server\n");
+    exit(EXIT_FAILURE);
+  }
+
+  int         restart = 1;  // if the topic already exists, where to subscribe from the begine.
+  const char* topic = "topic-meter-current-bg-10";
+  const char* sql = "select * from power.meters where current > 10";
+  void*       param = NULL;     // additional parameter.
+  int         interval = 2000;  // consumption interval in microseconds.
+  TAOS_SUB*   tsub = taos_subscribe(taos, restart, topic, sql, subscribe_callback, NULL, interval);
+
+  getchar();  // press Enter to stop
+
+  printf("total rows consumed: %d\n", nTotalRows);
+  int keep = 0;  // weather to keep subscribe process
+  taos_unsubscribe(tsub, keep);
+
+  taos_close(taos);
+  taos_cleanup();
+}
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