在前一篇文章中，讲述了如何进行mysql源程序代码的编译链接，但是没有讲述运行情况，在按照上一篇文章代码下进行编译运行后，发现无法链接数据库文件，显然是在mysql_real_connect（）函数中出现了问题。在mysql的英文手册中找到关于mysql_real_connect()的如下描述：

//函数原型描述 MYSQL *mysql_real_connect(MYSQL *mysql, const char *host, const char *user,
const char *passwd, const char *db, unsigned int port, const char *unix_socket,
unsigned long client_flag)

Description

mysql_real_connect() attempts to establish a connection to a MySQL database engine
running on host. mysql_real_connect() must complete successfully before you can
execute any other API functions that require a valid MYSQL connection handle structure.

The parameters are specified as follows:

*

The first parameter should be the address of an existing MYSQL structure. Before
calling mysql_real_connect() you must call mysql_init() to initialize the MYSQL
structure. You can change a lot of connect options with the mysql_options() call.
See Section 17.2.3.47, “mysql_options()”.
*

The value of host may be either a hostname or an IP address. If host is NULL or the
string "localhost", a connection to the local host is assumed. If the OS supports sockets
(Unix) or named pipes (Windows), they are used instead of TCP/IP to connect to the server.
*

The user parameter contains the user's MySQL login ID. If user is NULL or the empty
string "", the current user is assumed. Under Unix, this is the current login name. Under
Windows ODBC, the current username must be specified explicitly. See Section 18.1.9.2,
“Configuring a MyODBC DSN on Windows”.
*

The passwd parameter contains the password for user. If passwd is NULL, only entries
in the user table for the user that have a blank (empty) password field are checked for a
match. This allows the database administrator to set up the MySQL privilege system in
such a way that users get different privileges depending on whether they have specified
a password.

Note: Do not attempt to encrypt the password before calling mysql_real_connect();
password encryption is handled automatically by the client API.
*

db is the database name. If db is not NULL, the connection sets the default database
to this value.
*

If port is not 0, the value is used as the port number for the TCP/IP connection. Note
that the host parameter determines the type of the connection.
*

If unix_socket is not NULL, the string specifies the socket or named pipe that should
be used. Note that the host parameter determines the type of the connection.
*

The value of client_flag is usually 0, but can be set to a combination of the following
flags to enable certain features:

//上面描述了五个参数的主要取值，MYSQL *为mysql_init函数返回的指针，host为null或;;;// localhost时链接的是本地的计算机，当mysql默认安装在unix（或类unix）系统中，root账户是没// 有密码的，因此用户名使用root，密码为null，当db为空的时候，函数链接到默认数据库，在进行; // mysql安装时会存在默认的test数据库，因此此处可以使用test数据库名称，port端口为0，使用;// unix连接方式，unix_socket为null时，表明不使用socket或管道机制，最后一个参数经常设置为0

Flag Name;Flag Description
CLIENT_COMPRESS;Use compression protocol.
CLIENT_FOUND_ROWS;Return the number of found (matched) rows, not the number of
changed rows.
CLIENT_IGNORE_SPACE;Allow spaces after function names. Makes all functions names
reserved words.
CLIENT_INTERACTIVE;Allow interactive_timeout seconds (instead of wait_timeout
seconds) of inactivity before closing the connection. The client's session wait_timeout
variable is set to the value of the session interactive_timeout variable.
CLIENT_LOCAL_FILES;Enable LOAD DATA LOCAL handling.
CLIENT_MULTI_STATEMENTS;Tell the server that the client may send multiple
statements in a single string (separated by ‘;’). If this flag is not set,
multiple-statement execution is disabled. Added in MySQL 4.1.
CLIENT_MULTI_RESULTS;Tell the server that the client can handle multiple result
sets from multiple-statement executions or stored procedures. This is automatically
set if CLIENT_MULTI_STATEMENTS is set. Added in MySQL 4.1.
CLIENT_NO_SCHEMA;Don't allow the db_name.tbl_name.col_name syntax. This is for
ODBC. It causes the parser to generate an error if you use that syntax, which is useful
for trapping bugs in some ODBC programs.
CLIENT_ODBC;The client is an ODBC client. This changes mysqld to be more
ODBC-friendly.
CLIENT_SSL;Use SSL (encrypted protocol). This option should not be set by
application programs; it is set internally in the client library. Instead, use
mysql_ssl_set() before calling mysql_real_connect().

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。
http://blog.csdn.net/newrain021011/archive/2007/03/05/1521622.aspx