docs: add async query example for c

docs-examples/c/async_query_example.c

-// compiple with:
+// compile with:
 // gcc -o async_query_example async_query_example.c -ltaos
 
-// writing...
\ No newline at end of file
+#include <inttypes.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <taos.h>
+
+typedef int16_t VarDataLenT;
+
+#define TSDB_NCHAR_SIZE sizeof(int32_t)
+#define VARSTR_HEADER_SIZE sizeof(VarDataLenT)
+
+#define GET_FLOAT_VAL(x) (*(float *)(x))
+#define GET_DOUBLE_VAL(x) (*(double *)(x))
+
+#define varDataLen(v) ((VarDataLenT *)(v))[0]
+
+int printRow(char *str, TAOS_ROW row, TAOS_FIELD *fields, int numFields) {
+  int  len = 0;
+  char split = ' ';
+
+  for (int i = 0; i < numFields; ++i) {
+    if (i > 0) {
+      str[len++] = split;
+    }
+
+    if (row[i] == NULL) {
+      len += sprintf(str + len, "%s", "NULL");
+      continue;
+    }
+
+    switch (fields[i].type) {
+      case TSDB_DATA_TYPE_TINYINT:
+        len += sprintf(str + len, "%d", *((int8_t *)row[i]));
+        break;
+
+      case TSDB_DATA_TYPE_UTINYINT:
+        len += sprintf(str + len, "%u", *((uint8_t *)row[i]));
+        break;
+
+      case TSDB_DATA_TYPE_SMALLINT:
+        len += sprintf(str + len, "%d", *((int16_t *)row[i]));
+        break;
+
+      case TSDB_DATA_TYPE_USMALLINT:
+        len += sprintf(str + len, "%u", *((uint16_t *)row[i]));
+        break;
+
+      case TSDB_DATA_TYPE_INT:
+        len += sprintf(str + len, "%d", *((int32_t *)row[i]));
+        break;
+
+      case TSDB_DATA_TYPE_UINT:
+        len += sprintf(str + len, "%u", *((uint32_t *)row[i]));
+        break;
+
+      case TSDB_DATA_TYPE_BIGINT:
+        len += sprintf(str + len, "%" PRId64, *((int64_t *)row[i]));
+        break;
+
+      case TSDB_DATA_TYPE_UBIGINT:
+        len += sprintf(str + len, "%" PRIu64, *((uint64_t *)row[i]));
+        break;
+
+      case TSDB_DATA_TYPE_FLOAT: {
+        float fv = 0;
+        fv = GET_FLOAT_VAL(row[i]);
+        len += sprintf(str + len, "%f", fv);
+      } break;
+
+      case TSDB_DATA_TYPE_DOUBLE: {
+        double dv = 0;
+        dv = GET_DOUBLE_VAL(row[i]);
+        len += sprintf(str + len, "%lf", dv);
+      } break;
+
+      case TSDB_DATA_TYPE_BINARY:
+      case TSDB_DATA_TYPE_NCHAR: {
+        int32_t charLen = varDataLen((char *)row[i] - VARSTR_HEADER_SIZE);
+        memcpy(str + len, row[i], charLen);
+        len += charLen;
+      } break;
+
+      case TSDB_DATA_TYPE_TIMESTAMP:
+        len += sprintf(str + len, "%" PRId64, *((int64_t *)row[i]));
+        break;
+
+      case TSDB_DATA_TYPE_BOOL:
+        len += sprintf(str + len, "%d", *((int8_t *)row[i]));
+      default:
+        break;
+    }
+  }
+
+  return len;
+}
+
+void printHeader(TAOS_RES *res) {
+  int         numFields = taos_num_fields(res);
+  TAOS_FIELD *fields = taos_fetch_fields(res);
+  char        header[256] = {0};
+  int         len = 0;
+  for (int i = 0; i < numFields; ++i) {
+    len += sprintf(header + len, "%s ", fields[i].name);
+  }
+  puts(header);
+}
+
+// ANCHOR: demo
+
+/**
+ * @brief call back function of taos_fetch_row_a
+ *
+ * @param param : the third parameter you passed to taos_fetch_row_a
+ * @param res : pointer of TAOS_RES
+ * @param numOfRow : number of rows fetched in this batch. will be 0 if there is no more data.
+ * @return void*
+ */
+void *fetch_row_callback(void *param, TAOS_RES *res, int numOfRow) {
+  printf("numOfRow = %d \n", numOfRow);
+  int         numFields = taos_num_fields(res);
+  TAOS_FIELD *fields = taos_fetch_fields(res);
+  TAOS       *_taos = (TAOS *)param;
+  if (numOfRow > 0) {
+    for (int i = 0; i < numOfRow; ++i) {
+      TAOS_ROW row = taos_fetch_row(res);
+      char     temp[256] = {0};
+      printRow(temp, row, fields, numFields);
+      puts(temp);
+    }
+    taos_fetch_rows_a(res, fetch_row_callback, _taos);
+  } else {
+    taos_free_result(res);
+    taos_close(_taos);
+    taos_cleanup();
+  }
+}
+
+/**
+ * @brief callback function of taos_query_a
+ *
+ * @param param: the fourth parameter you passed to taos_query_a
+ * @param res : the result set
+ * @param code : status code
+ * @return void*
+ */
+void *select_callback(void *param, TAOS_RES *res, int code) {
+  printf("query callback ...\n");
+  TAOS *_taos = (TAOS *)param;
+  if (code == 0 && res) {
+    printHeader(res);
+    taos_fetch_rows_a(res, fetch_row_callback, _taos);
+  } else {
+    printf("failed to exeuce taos_query. error: %s\n", taos_errstr(res));
+    taos_free_result(res);
+    taos_close(_taos);
+    taos_cleanup();
+    exit(EXIT_FAILURE);
+  }
+}
+
+int main() {
+  TAOS *taos = taos_connect("localhost", "root", "taosdata", "power", 6030);
+  if (taos == NULL) {
+    puts("failed to connect to server");
+    exit(EXIT_FAILURE);
+  }
+  // param one is the connection returned by taos_connect.
+  // param two is the SQL to execute.
+  // param three is the callback function.
+  // param four can be any pointer. It will be passed to your callback function as the first parameter. we use taos
+  // here, because we want to close it after getting data.
+  taos_query_a(taos, "SELECT * FROM meters", select_callback, taos);
+  getchar();
+}
+
+// output:
+// query callback ...
+// ts current voltage phase location groupid
+// numOfRow = 8
+// 1538548685000 10.300000 219 0.310000 beijing.chaoyang 2
+// 1538548695000 12.600000 218 0.330000 beijing.chaoyang 2
+// 1538548696800 12.300000 221 0.310000 beijing.chaoyang 2
+// 1538548696650 10.300000 218 0.250000 beijing.chaoyang 3
+// 1538548685500 11.800000 221 0.280000 beijing.haidian 2
+// 1538548696600 13.400000 223 0.290000 beijing.haidian 2
+// 1538548685000 10.800000 223 0.290000 beijing.haidian 3
+// 1538548686500 11.500000 221 0.350000 beijing.haidian 3
+// numOfRow = 0
+// ANCHOR_END: demo
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