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SQLConnect Function

Conformance

Version Introduced: ODBC 1.0 Standards Compliance: ISO 92

Summary

SQLConnect establishes connections to a driver and a data source. The connection handle references storage of all information about the connection to the data source, including status, transaction state, and error information.


SQLRETURN SQLConnect(
     SQLHDBC        ConnectionHandle,
     SQLCHAR *      ServerName,
     SQLSMALLINT    NameLength1,
     SQLCHAR *      UserName,
     SQLSMALLINT    NameLength2,
     SQLCHAR *      Authentication,
     SQLSMALLINT    NameLength3);
ConnectionHandle

[Input] Connection handle.

ServerName

[Input] Data source name. The data might be located on the same computer as the program, or on another computer somewhere on a network. For information about how an application chooses a data source, see Choosing a Data Source or Driver.

NameLength1

[Input] Length of *ServerName in characters.

UserName

[Input] User identifier.

NameLength2

[Input] Length of *UserName in characters.

Authentication

[Input] Authentication string (typically the password).

NameLength3

[Input] Length of *Authentication in characters.

SQL_SUCCESS, SQL_SUCCESS_WITH_INFO, SQL_ERROR, SQL_INVALID_HANDLE, or SQL_STILL_EXECUTING.

When SQLConnect returns SQL_ERROR or SQL_SUCCESS_WITH_INFO, an associated SQLSTATE value can be obtained by calling SQLGetDiagRec with a HandleType of SQL_HANDLE_DBC and a Handle of ConnectionHandle. The following table lists the SQLSTATE values typically returned by SQLConnect and explains each one in the context of this function; the notation "(DM)" precedes the descriptions of SQLSTATEs returned by the Driver Manager. The return code associated with each SQLSTATE value is SQL_ERROR, unless noted otherwise.

SQLSTATE

Error

Description

01000

General warning

Driver-specific informational message. (Function returns SQL_SUCCESS_WITH_INFO.)

01S02

Option value changed

The driver did not support the specified value of the ValuePtr argument in SQLSetConnectAttr and substituted a similar value. (Function returns SQL_SUCCESS_WITH_INFO.)

08001

Client unable to establish connection

The driver was unable to establish a connection with the data source.

08002

Connection name in use

(DM) The specified ConnectionHandle had already been used to establish a connection with a data source, and the connection was still open or the user was browsing for a connection.

08004

Server rejected the connection

The data source rejected the establishment of the connection for implementation-defined reasons.

08S01

Communication link failure

The communication link between the driver and the data source to which the driver was trying to connect failed before the function completed processing.

28000

Invalid authorization specification

The value specified for the argument UserName or the value specified for the argument Authentication violated restrictions defined by the data source.

HY000

General error

An error occurred for which there was no specific SQLSTATE and for which no implementation-specific SQLSTATE was defined. The error message returned by SQLGetDiagRec in the *MessageText buffer describes the error and its cause.

HY001

Memory allocation error

(DM) The Driver Manager was unable to allocate memory that is required to support execution or completion of the function.

HY008

Operation canceled

Asynchronous processing was enabled for the ConnectionHandle. The SQLConnect function was called, and before it completed execution, SQLCancelHandle Function was called on the ConnectionHandle, and then the SQLConnect function was called again on the ConnectionHandle.

Or, the SQLConnect function was called, and before it completed execution, SQLCancelHandle was called on the ConnectionHandle from a different thread in a multithread application.

HY010

Function sequence error

(DM) An asynchronously executing function (not this one) was called for the ConnectionHandle and was still executing when this function was called.

HY013

Memory management error

The function call could not be processed because the underlying memory objects could not be accessed, possibly because of low memory conditions.

HY090

Invalid string or buffer length

(DM) The value specified for argument NameLength1, NameLength2, or NameLength3 was less than 0 but not equal to SQL_NTS.

(DM) The value specified for argument NameLength1 exceeded the maximum length for a data source name.

HYT00

Timeout expired

The query timeout period expired before the connection to the data source completed. The timeout period is set through SQLSetConnectAttr, SQL_ATTR_LOGIN_TIMEOUT.

HY114

Driver does not support connection level asynchronous function execution

(DM) The application enabled the asynchronous operation on the connection handle before making the connection. However, the driver does not support asynchronous operations on the connection handle.

HYT01

Connection timeout expired

The connection timeout period expired before the data source responded to the request. The connection timeout period is set through SQLSetConnectAttr, SQL_ATTR_CONNECTION_TIMEOUT.

IM001

Driver does not support this function

(DM) The driver specified by the data source name does not support the function.

IM002

Data source not found and no default driver specified

(DM) The data source name specified in the argument ServerName was not found in the system information, nor was there a default driver specification.

IM003

Specified driver could not be connected to

(DM) The driver listed in the data source specification in system information was not found or could not be connected to for some other reason.

IM004

Driver's SQLAllocHandle on SQL_HANDLE_ENV failed

(DM) During SQLConnect, the Driver Manager called the driver's SQLAllocHandle function with a HandleType of SQL_HANDLE_ENV and the driver returned an error.

IM005

Driver's SQLAllocHandle on SQL_HANDLE_DBC failed

(DM) During SQLConnect, the Driver Manager called the driver's SQLAllocHandle function with a HandleType of SQL_HANDLE_DBC and the driver returned an error.

IM006

Driver's SQLSetConnectAttr failed

During SQLConnect, the Driver Manager called the driver's SQLSetConnectAttr function and the driver returned an error. (Function returns SQL_SUCCESS_WITH_INFO.)

IM009

Unable to connect to translation DLL

The driver was unable to connect to the translation DLL that was specified for the data source.

IM010

Data source name too long

(DM) *ServerName was longer than SQL_MAX_DSN_LENGTH characters.

IM014

The specified DSN contains an architecture mismatch between the Driver and Application

(DM) 32-bit application uses a DSN connecting to a 64-bit driver; or vice versa.

IM015

Driver's SQLConnect on SQL_HANDLE_DBC_INFO_HANDLE failed

If a driver returns SQL_ERROR, the Driver Manager will return SQL_ERROR to the application and the connection will fail.

For more information about SQL_HANDLE_DBC_INFO_TOKEN, see Developing Connection-Pool Awareness in an ODBC Driver.

IM017

Polling is disabled in asynchronous notification mode

Whenever the notification model is used, polling is disabled.

IM018

SQLCompleteAsync has not been called to complete the previous asynchronous operation on this handle.

If the previous function call on the handle returns SQL_STILL_EXECUTING and if notification mode is enabled, SQLCompleteAsync must be called on the handle to do post-processing and complete the operation.

S1118

Driver does not support asynchronous notification

When the driver does not support asynchronous notification, you cannot set SQL_ATTR_ASYNC_DBC_EVENT or SQL_ATTR_ASYNC_DBC_RETCODE_PTR.

For information about why an application uses SQLConnect, see Connecting with SQLConnect.

The Driver Manager does not connect to a driver until the application calls a function (SQLConnect, SQLDriverConnect, or SQLBrowseConnect) to connect to the driver. Until that point, the Driver Manager works with its own handles and manages connection information. When the application calls a connection function, the Driver Manager checks whether a driver is currently connected to for the specified ConnectionHandle:

  • If a driver is not connected to, the Driver Manager connects to the driver and calls SQLAllocHandle with a HandleType of SQL_HANDLE_ENV, SQLAllocHandle with a HandleType of SQL_HANDLE_DBC, SQLSetConnectAttr (if the application specified any connection attributes), and the connection function in the driver. The Driver Manager returns SQLSTATE IM006 (Driver's SQLSetConnectOption failed) and SQL_SUCCESS_WITH_INFO for the connection function if the driver returned an error for SQLSetConnectAttr. For more information, see Connecting to a Data Source or Driver.

  • If the specified driver is already connected to on the ConnectionHandle, the Driver Manager calls only the connection function in the driver. In this case, the driver must make sure that all connection attributes for the ConnectionHandle maintain their current settings.

  • If a different driver is connected to, the Driver Manager calls SQLFreeHandle with a HandleType of SQL_HANDLE_DBC, and then, if no other driver is connected to in that environment, it calls SQLFreeHandle with a HandleType of SQL_HANDLE_ENV in the connected driver and then disconnects that driver. It then performs the same operations as when a driver is not connected to.

The driver then allocates handles and initializes itself.

When the application calls SQLDisconnect, the Driver Manager calls SQLDisconnect in the driver. However, it does not disconnect the driver. This keeps the driver in memory for applications that repeatedly connect to and disconnect from a data source. When the application calls SQLFreeHandle with a HandleType of SQL_HANDLE_DBC, the Driver Manager calls SQLFreeHandle with a HandleType of SQL_HANDLE_DBC and then SQLFreeHandle with a HandleType of SQL_HANDLE_ENV in the driver, and then disconnects the driver.

An ODBC application can establish more than one connection.

The contents of *ServerName affect how the Driver Manager and a driver work together to establish a connection to a data source.

  • If *ServerName contains a valid data source name, the Driver Manager locates the corresponding data source specification in the system information and connects to the associated driver. The Driver Manager passes each SQLConnect argument to the driver.

  • If the data source name cannot be found or ServerName is a null pointer, the Driver Manager locates the default data source specification and connects to the associated driver. The Driver Manager passes to the driver the UserName and Authentication arguments unmodified, and "DEFAULT" for the ServerName argument.

  • If the ServerName argument is "DEFAULT", the Driver Manager locates the default data source specification and connects to the associated driver. The Driver Manager passes each SQLConnect argument to the driver.

  • If the data source name cannot be found or ServerName is a null pointer, and the default data source specification does not exist, the Driver Manager returns SQL_ERROR with SQLSTATE IM002 (Data source name not found and no default driver specified).

After it is connected to by the Driver Manager, a driver can locate its corresponding data source specification in the system information and use driver-specific information from the specification to complete its set of required connection information.

If a default translation library is specified in the system information for the data source, the driver connects to it. A different translation library can be connected to by calling SQLSetConnectAttr with the SQL_ATTR_TRANSLATE_LIB attribute. A translation option can be specified by calling SQLSetConnectAttr with the SQL_ATTR_TRANSLATE_OPTION attribute.

If a driver supports SQLConnect, the driver keyword section of the system information for the driver must contain the ConnectFunctions keyword with the first character set to "Y."

Connection Pooling

Connection pooling allows an application to reuse a connection that has already been created. When connection pooling is enabled and SQLConnect is called, the Driver Manager tries to make the connection using a connection that is part of a pool of connections in an environment that has been designated for connection pooling. This environment is a shared environment that is used by all applications that use the connections in the pool.

For more information on connection pooling, see Connection Pooling.

Connection pooling is enabled before the environment is allocated by calling SQLSetEnvAttr to set SQL_ATTR_CONNECTION_POOLING to SQL_CP_ONE_PER_DRIVER (which specifies a maximum of one pool per driver) or SQL_CP_ONE_PER_HENV (which specifies a maximum of one pool per environment). SQLSetEnvAttr in this case is called with EnvironmentHandle set to null, which makes the attribute a process-level attribute. If SQL_ATTR_CONNECTION_POOLING is set to SQL_CP_OFF, connection pooling is disabled.

After connection pooling has been enabled, SQLAllocHandle with a HandleType of SQL_HANDLE_ENV is called to allocate an environment. The environment allocated by this call is a shared environment because connection pooling has been enabled. However, the environment that will be used is not determined until SQLAllocHandle with a HandleType of SQL_HANDLE_DBC is called.

SQLAllocHandle with a HandleType of SQL_HANDLE_DBC is called to allocate a connection. The Driver Manager tries to find an existing shared environment that matches the environment attributes set by the application. If no such environment exists, one is created as an implicit shared environment. If a matching shared environment is found, the environment handle is returned to the application and its reference count is incremented.

However, the connection that will be used is not determined until SQLConnect is called. At that point, the Driver Manager tries to find an existing connection in the connection pool that matches the criteria requested by the application. These criteria include the connection options requested in the call to SQLConnect (the values of the ServerName, UserName, and Authentication keywords) and any connection attributes set since SQLAllocHandle with a HandleType of SQL_HANDLE_DBC was called. The Driver Manager checks these criteria against the corresponding connection keywords and attributes in connections in the pool. If a match is found, the connection in the pool is used. If no match is found, a new connection is created.

If the SQL_ATTR_CP_MATCH environment attribute is set to SQL_CP_STRICT_MATCH, the match must be exact for a connection in the pool to be used. If the SQL_ATTR_CP_MATCH environment attribute is set to SQL_CP_RELAXED_MATCH, the connection options in the call to SQLConnect must match but not all the connection attributes must match.

The following rules are applied when a connection attribute, as set by the application before SQLConnect is called, does not match the connection attribute of the connection in the pool:

  • If the connection attribute must be set before the connection is made:

    If SQL_ATTR_CP_MATCH is SQL_CP_STRICT_MATCH, SQL_ATTR_PACKET_SIZE in the pooled connection must be identical to the attribute set by the application. If SQL_CP_RELAXED_MATCH, the values of SQL_ATTR_PACKET_SIZE can be different.

    The value of SQL_ATTR_LOGIN_VALUE does not affect the match.

  • If the connection attribute can be set either before or after the connection is made:

    If the connection attribute has not been set by the application but has been set on the connection in the pool, and there is a default, the connection attribute in the pooled connection is set back to the default and a match is declared. If there is no default, the pooled connection is not considered a match.

    If the connection attribute has been set by the application but has not been set on the connection in the pool, the connection attribute on the pool is changed to that set by the application and a match is declared.

    If the connection attribute has been set by the application, and has also been set on the connection in the pool but the values are different, the value of the application's connection attribute is used and a match is declared.

  • If the values of driver-specific connection attributes are not identical and SQL_ATTR_CP_MATCH is set to SQL_CP_STRICT_MATCH, the connection in the pool is not used.

When the application calls SQLDisconnect to disconnect, the connection is returned to the connection pool and is available for reuse.

Optimizing Connection Pooling Performance

When distributed transactions are involved, it is possible to optimize connection pooling performance by using SQL_DTC_TRANSITION_COST, which is a SQLUINTEGER bitmask. The transitions referred to are the transitions of the connection attribute SQL_ATTR_ENLIST_IN_DTC going from value 0 to nonzero, and vice versa. This is a connection going from not enlisted in a distributed transaction to enlisted in a distributed transaction, and vice versa. Depending on how the driver has implemented enlistment (setting connection attribute SQL_ATTR_ENLIST_IN_DTC), these transitions may be expensive and should therefore be avoided for best performance.

The value returned by the driver contains any combination of the following bits:

  • SQL_DTC_ENLIST_EXPENSIVE, when set, implies the zero to nonzero transition is significantly more expensive than a transition from nonzero to another nonzero value (enlisting a previously enlisted connection in its next transaction).

  • SQL_DTC_UNENLIST_EXPENSIVE, when set, implies the nonzero to zero transition is significantly more expensive than using a connection whose SQL_ATTR_ENLIST_IN_DTC attribute is already set to zero.

There is a performance versus connection usage tradeoff. If a driver indicates that one or more of these transitions are expensive, the driver manager's connection pooler responds to this by keeping more connections in the pool. Some of the connections in the pool are preferred for nontransactional use, and some are preferred for transactional use. However, if the driver indicates that these transitions are not expensive, fewer connections can be used, perhaps alternating between nontransactional and transactional use.

Drivers that do not support SQL_ATTR_ENLIST_IN_DTC do not need to support SQL_DTC_TRANSITION_COST. For drivers that support SQL_ATTR_ENLIST_IN_DTC but not SQL_DTC_TRANSITION_COST, it is assumed that the transitions are not expensive, as if the driver returned 0 (no bits set) for this value.

Although SQL_DTC_TRANSITION_COST was introduced in ODBC 3.5, an ODBC 2.x driver can also support it because the driver manager will query this information regardless of the driver version.

Code Example

In the following example, an application allocates environment and connection handles. It then connects to the SalesOrders data source with the user ID JohnS and the password Sesame and processes data. When it has finished processing data, it disconnects from the data source and frees the handles.

// SQLConnect_ref.cpp
// compile with: odbc32.lib
#include <windows.h>
#include <sqlext.h>

int main() {
   SQLHENV henv;
   SQLHDBC hdbc;
   SQLHSTMT hstmt;
   SQLRETURN retcode;

   SQLCHAR * OutConnStr = (SQLCHAR * )malloc(255);
   SQLSMALLINT * OutConnStrLen = (SQLSMALLINT *)malloc(255);

   // Allocate environment handle
   retcode = SQLAllocHandle(SQL_HANDLE_ENV, SQL_NULL_HANDLE, &henv);

   // Set the ODBC version environment attribute
   if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
      retcode = SQLSetEnvAttr(henv, SQL_ATTR_ODBC_VERSION, (void*)SQL_OV_ODBC3, 0); 

      // Allocate connection handle
      if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
         retcode = SQLAllocHandle(SQL_HANDLE_DBC, henv, &hdbc); 

         // Set login timeout to 5 seconds
         if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
            SQLSetConnectAttr(hdbc, SQL_LOGIN_TIMEOUT, (SQLPOINTER)5, 0);

            // Connect to data source
            retcode = SQLConnect(hdbc, (SQLCHAR*) "NorthWind", SQL_NTS, (SQLCHAR*) NULL, 0, NULL, 0);

            // Allocate statement handle
            if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
               retcode = SQLAllocHandle(SQL_HANDLE_STMT, hdbc, &hstmt); 

               // Process data
               if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
                  SQLFreeHandle(SQL_HANDLE_STMT, hstmt);
               }

               SQLDisconnect(hdbc);
            }

            SQLFreeHandle(SQL_HANDLE_DBC, hdbc);
         }
      }
      SQLFreeHandle(SQL_HANDLE_ENV, henv);
   }
}

Related Functions

For information about

See

Allocating a handle

SQLAllocHandle Function

Discovering and enumerating values required to connect to a data source

SQLBrowseConnect Function

Disconnecting from a data source

SQLDisconnect Function

Connecting to a data source using a connection string or dialog box

SQLDriverConnect Function

Returning the setting of a connection attribute

SQLGetConnectAttr Function

Setting a connection attribute

SQLSetConnectAttr Function

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