FreeTDS库函数dbbind参数详解
Binds a regular result column (a column of results from a SELECT statement's select list) to a program variable.
Syntax
RETCODE dbbind (
PDBPROCESS dbproc ,
INT column ,
INT vartype ,
DBINT varlen ,
LPBYTE varaddr );
Arguments
dbproc
Is the DBPROCESS structure that is the handle for a particular workstation or Microsoft® SQL Server™ 2000 process. It contains all the information that DB-Library uses to manage communications and data between the workstation and SQL Server.
column
Is the column number of the row data to be copied to a program variable. The first column is number 1.
vartype
Is a description of the binding's data type. It corresponds to the data type of the program variable that receives the copy of the data from the DBPROCESS. The dbbind function supports a wide range of type conversions, so vartype can be different from the type returned by the SQL query. For instance, a SQLMONEY result can be bound to a DBFLT8 program variable, using FLT8BIND, and the appropriate data conversion happens automatically. For more information about a list of the data conversions provided by DB-Library, see dbwillconvert . For more information about a list of the type definitions used by DB-Library, see DB-Library for C Data Types .
The following table lists legal vartypes recognized by dbbind and the program variable and SQL Server type tokens that each refers to.
vartype | varaddr | SQL Server data type of column |
|---|---|---|
| CHARBIND | DBCHAR | SQLCHAR, SQLVARCHAR, or SQLTEXT |
| STRINGBIND | DBCHAR | SQLCHAR, SQLVARCHAR, or SQLTEXT |
| NTBSTRINGBIND | DBCHAR | SQLCHAR, SQLVARCHAR, or SQLTEXT |
| VARYCHARBIND | DBVARYCHAR | SQLCHAR, SQLVARCHAR, or SQLTEXT |
| BINARYBIND | DBBINARY | SQLBINARY, SQLVARBINARY, or SQLIMAGE |
| VARYBINBIND | DBVARYBIN | SQLBINARY, SQLVARBINARY, or SQLIMAGE |
| TINYBIND | DBTINYINT | SQLINT1 or SQLINTN |
| SMALLBIND | DBSMALLINT | SQLINT2 or SQLINTN |
| INTBIND | DBINT | SQLINT4 or SQLINTN |
| FLT4BIND | DBFLT4 | SQLFLT4 or SQLFLTN |
| FLT8BIND | DBFLT8 | SQLFLT8 or SQLFLTN |
| BITBIND | DBBIT | SQLBIT |
| SMALLMONEYBIND | DBMONEY4 | SQLMONEY4 or SQLMONEYN |
| MONEYBIND | DBMONEY | SQLMONEY or SQLMONEYN |
| DECIMALBIND | DBDECIMAL | SQLDECIMAL |
| NUMERICBIND | DBNUMERIC | SQLNUMERIC |
| SRCDECIMALBIND | DBDECIMAL | SQLDECIMAL |
| SRCNUMERICBIND | DBNUMERIC | SQLNUMERIC |
| SMALLDATETIBIND | DBDATETIM4 | SQLDATETIM4 or SQLDATETIMN |
| DATETIMEBIND | DBDATETIME | SQLDATETIME or SQLDATETIMN |
Note that the SQL Server type in the preceding table is listed merely for your information. The vartype you specify does not necessarily have to correspond to a particular SQL Server data type because dbbind converts SQL Server data into the specified vartype .
The following table lists the four representations for character and text data. They differ according to whether the data is blank-padded or null-terminated.
| vartype | varaddr | Padding | Terminator |
|---|---|---|---|
| CHARBIND | DBCHAR | blanks | none |
| STRINGBIND | DBCHAR | blanks | /0 |
| NTBSTRINGBIND | DBCHAR | none | /0 |
| VARYCHARBIND | DBVARYCHAR | none | none |
Note that "/0" is the null terminator character. Similarly, binary and image data can be stored in two different ways.
| vartype | varaddr | Padding |
|---|---|---|
| BINARYBIND | DBBINARY | nulls |
| VARYBINBIND | DBVARBINARY | none |
When the source column specified by the column parameter has a type of SQLDECIMAL or SQLNUMERIC, you can keep the same precision and scale in your bound C variable by using SRCDECIMALBIND or SRCNUMERICBIND.
varlen
Is the length of the varaddr program variable in bytes. For fixed-length vartypes , such as MONEYBIND or FLT8BIND, this length is ignored. For character, text, binary, and image types, varlen must describe the total length of the available destination buffer space, including any space that can be required for special terminating bytes, such as a null terminator. If varlen is 0, the number of bytes available is copied into the program variable. (For char and binary SQL Server data, the number of bytes available is equal to the defined length of the database column, including any blank padding. For varchar , varbinary , text , and image data, the number of bytes available is equal to the actual data contained in the column.) Therefore, if you are sure that your program variable is large enough to handle the results, you can set varlen to 0.
In some cases, DB-Library issues a message indicating that data conversion resulted in an overflow. This is usually caused by a varlen specification being too small for the data being received from SQL Server. For example, if varlen is set to 5, vartype is set to VARYCHARBIND, and the SQL Server column being bound is of type VARCHAR with a length of 20. When the bind occurs (using dbnextrow ), the overflow message is issued. Note however that five bytes of data will be bound. Other types of binds also can cause the overflow message to be issued. For information about data type conversions, see dbconvert .
varaddr
Is the address of the program variable to which the data is copied. Calling dbbind with a NULL varaddr parameter breaks a previously set binding.
When binding using DECIMALBIND or NUMERICBIND, the varaddr parameter must be a pointer to a DBNUMERIC or DBDECIMAL C variable, respectively, with the precision and scale fields of the structure already set to the desired values. You can use DEFAULTPRECISION to specify a default precision and DEFAULTSCALE to specify a default scale.
Returns
SUCCEED or FAIL. The dbbind function returns FAIL if the column number given isn't valid, if the vartype isn't compatible with the SQL Server data type being returned, or if varaddr is NULL.
Remarks
Data comes back from SQL Server one row at a time. This function directs DB-Library to copy the data for a regular column (designated in a SELECT statement's select list) into a program variable. When each new row containing regular (not compute) data is read using dbnextrow or dbgetrow , the data from the designated column in that row is copied into the program variable with the address varaddr . There must be a separate dbbind call for each regular column to be copied. It is not necessary to bind every column to a program variable. A result column can be bound to only one program variable.
SQL Server can return two types of rows: regular rows and compute rows resulting from the COMPUTE clause of a SELECT statement. The dbbind function binds data from regular rows. Use dbaltbind for binding data from compute rows.
Calls to dbbind must be made after a call to dbresults and before the first call to dbnextrow .
Using dbbind causes some overhead because it copies the row data into the designated program variable. To avoid this copying, the returned data can be accessed more directly with dbdatlen and dbdata .
Because null values can be returned from SQL Server, there is a set of default values, one for each data type that will be substituted when binding null values. You can explicitly set your own values to be substituted for the default null value with the dbsetnull function. (For more information about a list of the default substitution values, see dbsetnull .)
For the Microsoft Windows® operating system, DB-Library retrieves information about date, time, numeric, and currency formatting from the Sqlcommn.loc file. The location of Sqlcommn.loc is pointed to by the SQLLocalizationFile key in the Windows initialization file (Win.ini) under the [SQLSERVER ] application heading. For example:
[SQLSERVER]
SQLLocalizationFile=C:/SQL60/BIN/SQLCOMMN.LOC
For the Microsoft Windows NT® 4.0 operating system, you set the date, time, numeric, and currency formatting using the International application in Control Panel. Use the SQL Server Client Network Utility Use International Settings option to activate this for DB-Library.
Examples
This example shows the typical sequence of calls:
DBINT xvariable;
DBCHAR yvariable[10];
// Read the query into the command buffer.
dbcmd(dbproc, "SELECT x = 100, y = 'hello'");
// Send the query to SQL Server.
dbsqlexec(dbproc);
// Get ready to process the results of the query.
dbresults(dbproc);
// Bind column data to program variables.
dbbind(dbproc, 1, INTBIND, (DBINT) 0, (BYTE *) &xvariable);
dbbind(dbproc, 2, STRINGBIND, (DBINT) 0, yvariable);
// Now process each row.
while (dbnextrow(dbproc) != NO_MORE_ROWS)
{
// C-code to print or process row data
}