MySQL C API句柄TEXT字段 [英] MySQL C API Handle TEXT field

问题描述

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

When one uses Prepared Statements in MySQL C API to handle TEXT field result, one has to specify the length of the string for an out binding:

 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是已知的常量，但是如何处理文本字段，其中数据长度可能从小到兆字节不等?

In the given example STRING_SIZE is the known constant, but how to be with TEXT fields where data length can vary from small sizes to megabytes?

有标准的方法吗?

推荐答案

mysql_stmt_fetch 的手册页说:

The manual page for mysql_stmt_fetch says:

在某些情况下，您可能想确定一个列值的长度，然后再使用mysql_stmt_fetch()进行获取. ...为此，您可以使用以下策略:

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:

• 在调用mysql_stmt_fetch()检索单个行之前，将STMT_ATTR_UPDATE_MAX_LENGTH传递给mysql_stmt_attr_set()，然后调用mysql_stmt_store_result()在客户端缓存整个结果.设置STMT_ATTR_UPDATE_MAX_LENGTH属性会使列值的最大长度由mysql_stmt_result_metadata()返回的结果集元数据的max_length成员指示.

• 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().

为所涉及的列调用一个零长度缓冲区，并使用一个指针(可以存储实际长度)调用mysql_stmt_fetch().然后使用mysql_stmt_fetch_column()的实际长度.

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().

您可能还想阅读手册mysql_stmt_bind_result

You might also like to read the manual page for mysql_stmt_bind_result

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