Release 8.0
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| Pro*C/C++ Precompiler Programmer's Guide Release 8.0 A58233-01 |
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This chapter introduces you to the Pro*C/C++ Precompiler. You look at its role in developing application programs that manipulate Oracle data and find out what it allows your applications to do. This chapter answers the following questions:
An Oracle Precompiler is a programming tool that allows you to embed SQL statements in a high-level source program. As Figure 1-1 shows, the precompiler accepts the source program as input, translates the embedded SQL statements into standard Oracle runtime library calls, and generates a modified source program that you can compile, link, and execute in the usual way.
The Oracle Pro*C/C++ Precompiler lets you use the power and flexibility of SQL in your application programs. A convenient, easy to use interface lets your application access Oracle directly.
Unlike many application development tools, the Pro*C/C++ Precompiler lets you create highly customized applications. For example, you can create user interfaces that incorporate the latest windowing and mouse technology. You can also create applications that run in the background without the need for user interaction.
Furthermore, Pro*C/C++ helps you fine-tune your applications. It allows close monitoring of resource use, SQL statement execution, and various runtime indicators. With this information, you can tweak program parameters for maximum performance.
Although precompiling adds a step to the application development process, it saves time because the precompiler, not you, translates each embedded SQL statement into several calls to the Oracle runtime library (SQLLIB).
If you want to access and manipulate Oracle data, you need SQL. Whether you use SQL interactively through SQL*Plus or embedded in an application program depends on the job at hand. If the job requires the procedural processing power of C or C++, or must be done on a regular basis, use embedded SQL.
SQL has become the database language of choice because it is flexible, powerful, and easy to learn. Being non-procedural, it lets you specify what you want done without specifying how to do it. A few English-like statements make it easy to manipulate Oracle data one row or many rows at a time.
You can execute any SQL (not SQL*Plus) statement from an application program. For example, you can
Before embedding SQL statements in an application program, you can test them interactively using SQL*Plus. Usually, only minor changes are required to switch from interactive to embedded SQL.
An extension to SQL, PL/SQL is a transaction processing language that supports procedural constructs, variable declarations, and robust error handling. Within the same PL/SQL block, you can use SQL and all the PL/SQL extensions.
The main advantage of embedded PL/SQL is better performance. Unlike SQL, PL/SQL allows you to group SQL statements logically and send them to Oracle in a block rather than one by one. This reduces network traffic and processing overhead.
For more information about PL/SQL, including how to embed it in an application program, see Chapter 6, "Using Embedded PL/SQL".
As Figure 1-2 shows, Pro*C/C++ offers many features and benefits, which help you to develop effective, reliable applications.
For example, Pro*C/C++ allows you to
To sum it up, the Pro*C/C++ Precompiler is a full-featured tool that supports a professional approach to embedded SQL programming.
SQL has become the standard language for relational database management systems. This section describes how the Pro*C/C++ Precompiler conforms to SQL standards established by the following organizations:
These organizations have adopted SQL as defined in the following publications:
ANSI standard X3.135-1992 (known informally as SQL92) provides three levels of compliance:
ANSI standard X3.168-1992 specifies the syntax and semantics for embedding SQL statements in application programs written in a standard programming language such as Ada, C, COBOL, FORTRAN, Pascal, or PL/I.
A conforming SQL implementation must support at least Entry SQL. The Oracle Pro*C/C++ Precompiler does conform to Entry SQL92.
NIST standard FIPS PUB 127-1, which applies to RDBMS software acquired for federal use, also adopts the ANSI standards. In addition, it specifies minimum sizing parameters for database constructs and requires a "FIPS Flagger" to identify ANSI extensions.
For copies of the ANSI standards, write to
American National Standards Institute
1430 Broadway
New York, NY 10018
USA
For a copy of the ISO standard, write to the national standards office of any ISO participant. For a copy of the NIST standard, write to
National Technical Information Service
U.S. Department of Commerce
Springfield, VA 22161
USA
Under Oracle8, the Pro*C/C++ Precompiler complies 100% with current ANSI/ISO standards.
The Pro*C/C++ Precompiler also complies 100% with the NIST standard. It provides a FIPS Flagger and an option named FIPS, which enables the FIPS Flagger. For more information, see "FIPS Flagger" on page 1-9.
NIST tested the Pro*C/C++ Precompiler for ANSI SQL92 compliance using the SQL Test Suite, which consists of nearly 300 test programs. Specifically, the programs tested for conformance to the C embedded SQL standard. The result: the Oracle Pro*C/C++ Precompiler was certified 100% ANSI-compliant for Entry SQL92.
For more information about the tests, write to
National Computer Systems Laboratory
Attn: Software Standards Testing Program
National Institute of Standards and Technology
Gaithersburg, MD 20899
USA
According to FIPS PUB 127-1, "an implementation that provides additional facilities not specified by this standard shall also provide an option to flag nonconforming SQL language or conforming SQL language that may be processed in a nonconforming manner." To meet this requirement, the Pro*C/C++ Precompiler provides the FIPS Flagger, which flags ANSI extensions. An extension is any SQL element that violates ANSI format or syntax rules, except privilege enforcement rules. For a list of Oracle extensions to standard SQL, see the Oracle8 Server SQL Reference.
You can use the FIPS Flagger to identify
Thus, the FIPS Flagger helps you develop portable applications.
The FIPS precompiler option governs the FIPS Flagger. To enable the FIPS Flagger, specify FIPS=YES inline or on the command line. For more information about the FIPS option, see the section "Using the Precompiler Options" on page 9-10.
There are several semantic changes in database operations between Oracle7 and Oracle8. For information on how this affects Pro*C/C++ applications, see the section "Migrating From Earlier Pro*C/C++ Releases" on page 3-9, and the discussion of the DBMS precompiler option on "DBMS" on page 9-14.
This section presents some questions that are frequently asked about Pro*C/C++, and about Oracle8 in relation to Pro*C/C++. The answers are more informal than the documentation in the rest of this Guide, but do provide references to places where you can find the reference material.
I'm confused by VARCHAR. What's a VARCHAR?
Here's a short description of VARCHARs:
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VARCHAR2 |
A kind of column in the database that contains variable-length character data. This is what Oracle calls an "internal datatype", because it's a possible column type. See "VARCHAR2" on page 3-20. |
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VARCHAR |
An Oracle "external datatype" (datatype code 9). You use this only if you're doing dynamic SQL Method 4, or datatype equivalencing. See "VARCHAR" on page 3-22 for datatype equivalencing, and Chapter 14, "Using Dynamic SQL: Advanced Concepts". |
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VARCHAR[n] varchar[n] |
This is a Pro*C/C++ "pseudotype" that you can declare as a host variable in your Pro*C/C++ program. It's actually generated by Pro*C/C++ as a struct, with a 2-byte length element, and a [n]-byte character array. See "Declaring VARCHAR Variables" on page 3-43. |
Does Pro*C/C++ generate calls to the Oracle Call Interface (OCI)?
No. Pro*C/C++ generates data structures, and calls to its runtime library: SQLLIB (libsql.a in UNIX).
Then why not just code using SQLLIB calls, and not use Pro*C/C++?
SQLLIB is not externally documented, is unsupported, and might change from release to release. Also, Pro*C/C++ is an ANSI/ISO compliant product, that follows the standard requirements for embedded SQL.
If you need to do low-level coding, use the OCI. It is supported, and Oracle is committed to supporting it.
You can also mix OCI and Pro*C/C++. See "SQLLIB Extensions for OCI Release 8 Interoperability" on page 4-55.
Can I call a PL/SQL stored procedure from a Pro*C/C++ program?
Certainly. See Chapter 6, "Using Embedded PL/SQL". There's a demo program starting on "Calling a Stored Subprogram" on page 6-23.
Can I write C++ code, and precompile it using Pro*C/C++?
Yes. Since Pro*C/C++ release 2.1, you can precompile C++ applications. See Chapter 7, "Using C++".
Can I use bind variables anyplace in a SQL statement? For example, I'd like to be able to input the name of a table in my SQL statements at runtime. But when I use host variables, I get precompiler errors.
In general, you can use host variables at any place in a SQL, or PL/SQL, statement where expressions are allowed. See "Referencing Host Variables" on page 3-31. The following SQL statement, where table_name is a host variable, is illegal:
EXEC SQL SELECT ename,sal INTO :name, :salary FROM :table_name;
To solve your problem, you need to use dynamic SQL. See Chapter 13, "Using Dynamic SQL". There is a demo program that you can adapt to do this starting on "Sample Program: Dynamic SQL Method 1" on page 13-9.
I am confused by character handling in Pro*C/C++. It seems that there are many options. Can you help?
There are many options, but we can simplify. First of all, if you need compatibility with previous V1.x precompilers, and with both Oracle V6 and Oracle7, the safest thing to do is use VARCHAR[n] host variables. See "Declaring VARCHAR Variables" on page 3-43.
The default datatype for all other character variables in Pro*C/C++ is CHARZ; see "CHARZ" on page 3-25. Briefly, this means that you must null-terminate the string on input, and it is both blank-padded and null-terminated on output.
In release 8.0, the CHAR_MAP precompiler option was introduced to specify the default mapping of char variables. See "Precompiler Option CHAR_MAP" on page 3-50.
If neither VARCHAR nor CHARZ works for your application, and you need total C-like behavior (null termination, absolutely no blank-padding), use the TYPE command and the C typedef statement, and use datatype equivalencing to convert your character host variables to STRING. See "User-Defined Type Equivalencing" on page 3-60. There is a sample program that shows how to use the TYPE command starting on "Sample Program: Cursor and a Host Structure" on page 3-38.
What about character pointers? Is there anything special about them?
Yes. When Pro*C/C++ binds an input or output host variable, it must know the length. When you use VARCHAR[n], or declare a host variable of type char[n], Pro*C/C++ knows the length from your declaration. But when you use a character pointer as a host variable, and use malloc() to define the buffer in your program, Pro*C/C++ has no way of knowing the length.
What you must do on output is not only allocate the buffer, but pad it out with some non-null characters, then null-terminate it. On input or output, Pro*C/C++ calls strlen() for the buffer to get the length. See "Pointer Variables" on page 3-41.
Why doesn't SPOOL work in Pro*C/C++?
SPOOL is a special command used in SQL*Plus. It is not an embedded SQL command. See "Key Concepts of Embedded SQL Programming" on page 2-2.
Where can I find the on-line versions of the sample programs?
Each Oracle installation should have a demo directory. If the directory is not there, or it does not contain the sample programs, see your system or database administrator.
How can I compile and link my application?
Compiling and linking are very platform specific. Your system-specific Oracle documentation has instructions on how to link a Pro*C/C++ application. On UNIX systems, there is a makefile called proc.mk in the demo directory. To link, say, the demo program sample1.pc, you would enter the command line
make -f proc.mk sample1
If you need to use special precompiler options, you can run Pro*C/C++ separately, then do the make. Or, you can create your own custom makefile. For example, if your program contains embedded PL/SQL code, you can enter
proc cv_demo userid=scott/tiger sqlcheck=semantics make -f proc.mk cv_demo
On VMS systems, there is a script called LNPROC that you use to link your Pro*C/C++ applications.
I have been told that Pro*C/C++ now supports using structures as host variables. How does this work with the array interface?
You can use arrays inside a single structure, or an array of structures with the array interface. See "Host Structures" on page 3-34 and "Pointer Variables" on page 3-41.
Is it possible to have recursive functions in Pro*C/C++, if I use embedded SQL in the function?
Yes. With Pro*C/C++, you can also use cursor variables in recursive functions.
Can I use any release of the Oracle Pro*C or Pro*C/C++ Precompiler with any version of the Oracle Server?
No. You can use an older version of Pro*C or Pro*C/C++ with a newer version of the server, but you cannot use a newer version of Pro*C/C++ with an older version of the server.
For example, you can use release 2.2 of Pro*C/C++ with Oracle8, but you cannot use Pro*C/C++ release8.0 with the Oracle7 server.
When my application runs under Oracle8, I keep getting an ORA-1405 error (fetched column value is null). It worked fine under Oracle V7. What is happening?
You are selecting a null into a host variable that does not have an associated indicator variable. This is not in compliance with the ANSI/ISO standards, and was changed beginning with Oracle7.
If possible, rewrite your program using indicator variables, and use indicators in future development. Indicator variables are described on "Indicator Variables" on page 3-32.
Alternatively, if precompiling with MODE=ORACLE and DBMS=V7 or V8, specify UNSAFE_NULL=YES on the command line (see "UNSAFE_NULL" on page 9-37 for more information) to disable the ORA-01405 message, or precompile with DBMS=V6.
Are all SQLLIB functions private?
No. There are some SQLLIB functions that you can call to get information about your program, or its data. The SQLLIB public functions are shown here:
|
SQLSQLDAAlloc() |
Used to allocate a SQL descriptor array (SQLDA) for dynamic SQL Method 4.See "How is the SQLDA Referenced?" on page 14-3. |
|
SQLCDAFromResultSetCursor() |
Used to convert a Pro*C/C++ cursor variable to an OCI cursor data area. See "SQLLIB Public Functions -- New Names" on page 4-36. |
|
SQLSQLDAFree() |
Used to free a SQLDA allocated using SQLSQLDAAlloc(). See "SQLLIB Public Functions -- New Names" on page 4-36 . |
|
SQLCDAToResultSetCursor() |
Used to convert an OCI cursor data area to a Pro*C/C++ cursor variable. See "SQLLIB Public Functions -- New Names" on page 4-36. |
|
SQLErrorGetText() |
Returns a long error message. See "sqlerrm" on page 11-20. |
|
SQLStmtGetText) |
Used to return the text of the most recently executed SQL statement. See "Obtaining the Text of SQL Statements" on page 11-32. |
|
SQLLDAGetNamed() |
Used to obtain a valid Logon Data Area for a named connection, when OCI calls are used in a Pro*C/C++ program. See "SQLLIB Public Functions -- New Names" on page 4-36. |
|
SQLLDAGetCurrent() |
Used to obtain a valid Logon Data Area for the most recent connection, when OCI calls are used in a Pro*C/C++ program. See "SQLLIB Public Functions -- New Names" on page 4-36. |
|
SQLColumnNullCheck() |
Returns an indication of null status for dynamic SQL Method 4. See "Handling Null/Not Null Datatypes" on page 14-17. |
|
SQLNumberPrecV6() |
Returns precision and scale of numbers. See "Extracting Precision and Scale" on page 14-16. |
|
SQLNumberPrecV7() |
A variant of SQLNumberPrecV6(). See "Extracting Precision and Scale" on page 14-16. |
|
SQLVarcharGetLength() |
Used for obtaining the padded size of a VARCHAR[n]. See "Finding the Length of the VARCHAR Array Component" on page 3-46. |
In the preceding list, the functions are thread-safe SQLLIB public functions. Use these functions in multi-threaded applications. The names of the functions have been changed for release 8.0, but the old names are still supported in Pro*C/C++. For more information about these thread-safe public functions (including their old names), see the table "SQLLIB Public Functions -- New Names" on page 4-36.
How does Oracle8 support the new Object Types?
See the chapters Chapter 8, "Object Support in Pro*C/C++" and Chapter 16, "Using the Object Type Translator" for how to use Object types in Pro*C/C++ applications.
