mysql中的mysql_real_connect连接参数设置

For some parameters, it is possible to have the value taken from an option file rather
than from an explicit value in the mysql_real_connect() call. To do this, call
mysql_options() with the MYSQL_READ_DEFAULT_FILE or MYSQL_READ_DEFAULT_GROUP option
before calling mysql_real_connect(). Then, in the mysql_real_connect() call, specify
the “no-value” value for each parameter to be read from an option file:

    *

      For host, specify a value of NULL or the empty string ("").
    *

      For user, specify a value of NULL or the empty string.
    *

      For passwd, specify a value of NULL. (For the password, a value of the empty string in
 the mysql_real_connect() call cannot be overridden in an option file, because the empty
string indicates explicitly that the MySQL account must have an empty password.)
    *

      For db, specify a value of NULL or the empty string.
    *

      For port, specify a value of 0.
    *

      For unix_socket, specify a value of NULL.

If no value is found in an option file for a parameter, its default value is used as
indicated in the descriptions given earlier in this section.

Return Values

A MYSQL* connection handle if the connection was successful, NULL if the connection
was unsuccessful. For a successful connection, the return value is the same as the value
of the first parameter.
// 返回值：当连接成功时，返回MYSQL连接句柄，失败，返回NULL。当成功时，返回值与第一个参数值是// 相同的。

Errors

    *

      CR_CONN_HOST_ERROR

      Failed to connect to the MySQL server.
    *

      CR_CONNECTION_ERROR

      Failed to connect to the local MySQL server.
    *

      CR_IPSOCK_ERROR

      Failed to create an IP socket.
    *

      CR_OUT_OF_MEMORY

      Out of memory.
    *

      CR_SOCKET_CREATE_ERROR

      Failed to create a Unix socket.
    *

      CR_UNKNOWN_HOST

      Failed to find the IP address for the hostname.
    *

      CR_VERSION_ERROR

      A protocol mismatch resulted from attempting to connect to a server with a client
library that uses a different protocol version. This can happen if you use a very old
client library to connect to a new server that wasn't started with the --old-protocol
option.
    *

      CR_NAMEDPIPEOPEN_ERROR

      Failed to create a named pipe on Windows.
    *

      CR_NAMEDPIPEWAIT_ERROR

      Failed to wait for a named pipe on Windows.
    *

      CR_NAMEDPIPESETSTATE_ERROR

      Failed to get a pipe handler on Windows.
    *

      CR_SERVER_LOST

      If connect_timeout > 0 and it took longer than connect_timeout seconds to connect to
the server or if the server died while executing the init-command.

    因此mysql_real_connect()函数调用为：
       mysql_real_connect(mysql,"localhost","root",NULL,"test",0,NULL,0);
判断是否出错，出错调用mysql_error()函数显示出错信息，或使用mysql_errno()函数获取出错代号。

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