MySQL C API 处理文本字段

Question

当使用 MySQL C API 中的 Prepared Statements 处理 TEXT 字段结果时，必须为 out 绑定(bind)指定字符串的长度:

 MYSQL_BIND    out_bind;
 char          str_data[STRING_SIZE]; 
 my_bool       is_null;
 my_bool       error;

 ....
 /* STRING COLUMN */
 out_bind.buffer_type = MYSQL_TYPE_STRING;
 out_bind.buffer = str_data;
 out_bind.buffer_length = STRING_SIZE;
 out_bind.is_null= &is_null;
 out_bind.length= &length;
 out_bind.error= &error;

 mysql_stmt_bind_result(statement, out_bind)

在给定的示例中，STRING_SIZE 是已知常量，但是如何处理数据长度从小到兆字节不等的 TEXT 字段？

有这方面的标准方法吗？

Answer 1

manual page for mysql_stmt_fetch说:

In some cases you might want to determine the length of a column value before fetching it with mysql_stmt_fetch(). ... To accomplish this, you can use these strategies:

• Before invoking mysql_stmt_fetch() to retrieve individual rows, pass STMT_ATTR_UPDATE_MAX_LENGTH to mysql_stmt_attr_set(), then invoke mysql_stmt_store_result() to buffer the entire result on the client side. Setting the STMT_ATTR_UPDATE_MAX_LENGTH attribute causes the maximal length of column values to be indicated by the max_length member of the result set metadata returned by mysql_stmt_result_metadata().

• Invoke mysql_stmt_fetch() with a zero-length buffer for the column in question and a pointer in which the real length can be stored. Then use the real length with mysql_stmt_fetch_column().

您可能还想阅读 manual page for mysql_stmt_bind_result