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28. 7. 7. 54 mysql _real _connect ()

Inhoudsopgave

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
Note

mysql_real_connect() is a synchronous function. Its asynchronous counterpart is mysql_real_connect_nonblocking(), for use by applications that require asynchronous communication with the server. See Section 28.7.12, “C API Asynchronous Interface”.

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 handler structure.

The parameters are specified as follows:

  • For the first parameter, specify the address of an existing MYSQL structure. Before calling mysql_real_connect(), call mysql_init() to initialize the MYSQL structure. You can change a lot of connect options with the mysql_options() call. See Section 28.7.7.50, “mysql_options()”.

  • The value of host may be either a host name or an IP address. The client attempts to connect as follows:

    • If host is NULL or the string "localhost", a connection to the local host is assumed:

      • On Windows, the client connects using a shared-memory connection, if the server has shared-memory connections enabled.

      • On Unix, the client connects using a Unix socket file. The unix_socket parameter or the MYSQL_UNIX_PORT environment variable may be used to specify the socket name.

    • On Windows, if host is ".", or TCP/IP is not enabled and no unix_socket is specified or the host is empty, the client connects using a named pipe, if the server has named-pipe connections enabled. If named-pipe connections are not enabled, an error occurs.

    • Otherwise, TCP/IP is used.

    You can also influence the type of connection to use with the MYSQL_OPT_PROTOCOL or MYSQL_OPT_NAMED_PIPE options to mysql_options(). The type of connection must be supported by 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 user name must be specified explicitly. See the Connector/ODBC section of Chapter 28, Connectors and APIs.

  • 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 enables 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.

  • The user and passwd parameters use whatever character set has been configured for the MYSQL object. By default, this is utf8mb4, but can be changed by calling mysql_options(mysql, MYSQL_SET_CHARSET_NAME, "charset_name") prior to connecting.

  • 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 to use. 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:

    • CAN_HANDLE_EXPIRED_PASSWORDS: The client can handle expired passwords. For more information, see Section 6.2.16, “Server Handling of Expired Passwords”.

    • CLIENT_COMPRESS: Use compression in the client/server protocol.

    • CLIENT_FOUND_ROWS: Return the number of found (matched) rows, not the number of changed rows.

    • CLIENT_IGNORE_SIGPIPE: Prevents the client library from installing a SIGPIPE signal handler. This can be used to avoid conflicts with a handler that the application has already installed.

    • CLIENT_IGNORE_SPACE: Permit spaces after function names. Makes all functions names reserved words.

    • CLIENT_INTERACTIVE: Permit interactive_timeout seconds of inactivity (rather than wait_timeout seconds) 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_RESULTS: Tell the server that the client can handle multiple result sets from multiple-statement executions or stored procedures. This flag is automatically enabled if CLIENT_MULTI_STATEMENTS is enabled. See the note following this table for more information about this flag.

    • CLIENT_MULTI_STATEMENTS: Tell the server that the client may send multiple statements in a single string (separated by ; characters). If this flag is not set, multiple-statement execution is disabled. See the note following this table for more information about this flag.

    • CLIENT_NO_SCHEMA Do not permit 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: Unused.

    • CLIENT_OPTIONAL_RESULTSET_METADATA: This flag makes result set metadata optional. Suppression of metadata transfer can improve performance, particularly for sessions that execute many queries that return few rows each. For details about managing result set metadata transfer, see Section 28.7.27, “C API Optional Result Set Metadata”.

    • CLIENT_SSL: Use SSL (encrypted protocol). Do not set this option within an application program; it is set internally in the client library. Instead, use mysql_options() or mysql_ssl_set() before calling mysql_real_connect().

    • CLIENT_REMEMBER_OPTIONS Remember options specified by calls to mysql_options(). Without this option, if mysql_real_connect() fails, you must repeat the mysql_options() calls before trying to connect again. With this option, the mysql_options() calls need not be repeated.

If your program uses CALL statements to execute stored procedures, the CLIENT_MULTI_RESULTS flag must be enabled. This is because each CALL returns a result to indicate the call status, in addition to any result sets that might be returned by statements executed within the procedure. Because CALL can return multiple results, process them using a loop that calls mysql_next_result() to determine whether there are more results.

CLIENT_MULTI_RESULTS can be enabled when you call mysql_real_connect(), either explicitly by passing the CLIENT_MULTI_RESULTS flag itself, or implicitly by passing CLIENT_MULTI_STATEMENTS (which also enables CLIENT_MULTI_RESULTS). CLIENT_MULTI_RESULTS is enabled by default.

If you enable CLIENT_MULTI_STATEMENTS or CLIENT_MULTI_RESULTS, process the result for every call to mysql_query() or mysql_real_query() by using a loop that calls mysql_next_result() to determine whether there are more results. For an example, see Section 28.7.23, “C API Multiple Statement Execution Support”.

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.

Inhoudsopgave Haut

Return Values

A MYSQL* connection handler 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. 

MYSQL mysql;

mysql_init(&mysql);
mysql_options(&mysql,MYSQL_READ_DEFAULT_GROUP,"your_prog_name");
if (!mysql_real_connect(&mysql,"host","user","passwd","database",0,NULL,0))
{
    fprintf(stderr, "Failed to connect to database: Error: %s\n",
          mysql_error(&mysql));
}

By using mysql_options() the MySQL client library reads the [client] and [your_prog_name] sections in the my.cnf file which ensures that your program works, even if someone has set up MySQL in some nonstandard way.

Upon connection, mysql_real_connect() sets the reconnect flag (part of the MYSQL structure) to a value of 1 in versions of the API older than 5.0.3, or 0 in newer versions. A value of 1 for this flag indicates that if a statement cannot be performed because of a lost connection, to try reconnecting to the server before giving up. You can use the MYSQL_OPT_RECONNECT option to mysql_options() to control reconnection behavior.


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