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Error handling in a dblib program
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main() { DBPROCESS *dbproc; /* data structure for connection */ /* Install our own error handling function */ dberrhandle(msg_rtn) /* Execute a DELETE statement */ dbcmd(dbproc, delete from salesreps where quota < 100000.00"); dbsqlexec(dbproc); dbresults(dbproc);
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Figure 19-6.
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Error handling in a dblib program (continued)
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dbsqlexec() or dbresults() and dblib receives the error message from SQL Server, it upcalls the msg_rtn() routine in the program, passing it five parameters: I dbproc. The connection on which the error occurred I msgno. The SQL Server error number identifying the error I msgstate. A parameter providing information about the error context I severity. A number indicating the seriousness of the error I msgtext. An error message corresponding to msgno The msg_rtn() function in this program handles the message by printing it and saving the error number in a program variable for use later in the program. When the message-handling function returns to dblib (which called it), dblib completes its own processing and then returns to the program with a FAIL status. The program can detect this return value and perform further error processing, if appropriate. The program excerpt in the figure actually presents a simplified view of SQL Server error handling. In addition to SQL statement errors detected by SQL Server, errors can also occur within the dblib API itself. For example, if the network connection to the SQL Server is lost, a dblib call may time out waiting for a response from SQL Server, resulting in an error. The API handles these errors by upcalling a separate error-handling function, which operates much like the message-handling function described here.
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A comparison of Figure 19-6 with Figures 17-10 and 17-13 illustrates the differences in error-handling techniques between dblib and embedded SQL: I In embedded SQL, the SQLCA structure is used to signal errors and warnings to the program. SQL Server communicates errors and warnings by upcalling special functions within the application program and returning a failure status for the API function that encountered the error. I In embedded SQL, error processing is synchronous. The embedded SQL statement fails, control returns to the program, and the SQLCODE or SQLSTATE value is tested. SQL Server error processing is asynchronous. When an API call fails, SQL Server calls the application program s error-handling or messagehandling function during the API call. It returns to the application program with an error status later. I Embedded SQL has only a single type of error and a single mechanism for reporting it. The SQL Server scheme has two types of errors and two parallel mechanisms. In summary, error handling in embedded SQL is simple and straightforward, but the application program can make only a limited number of responses when an error occurs. A SQL Server program has more flexibility in handling errors. However, the upcall scheme used by dblib is more sophisticated, and while it is familiar to systems programmers, it may be unfamiliar to application programmers.
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SQL Server Queries
The SQL Server technique for handling programmatic queries is very similar to its technique for handling other SQL statements. To perform a query, a program sends a SELECT statement to SQL Server and uses dblib to retrieve the query results row by row. The program in Figure 19-7 illustrates the SQL Server query-processing technique. 1. The program uses the dbcmd() and dbsqlexec() calls to pass a SELECT statement to SQL Server and request its execution. 2. When the program calls dbresults() for the SELECT statement, dblib returns the completion status for the query and also makes the query results available for processing. 3. The program calls dbbind() once for each column of query results, telling dblib where it should return the data for that particular column. The arguments to dbbind() indicate the column number, the buffer to receive its data, the size of the buffer, and the expected data type. 4. The program loops, calling dbnextrow() repeatedly to obtain the rows of query results. The API places the returned data into the data areas indicated in the previous dbbind() calls.
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