Release 8.1.5
A68022-01
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| Pro*C/C++ Precompiler Programmer's Guide Release 8.1.5 A68022-01 |
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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, Pro*C/C++ 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 7, "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, Pro*C/C++ is a full-featured tool that supports a professional approach to embedded SQL programming.
This section presents some questions that are frequently asked about Pro*C/C++, and about Oracle8i 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". |
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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" for datatype equivalencing, Chapter 14, "ANSI Dynamic SQL", and Chapter 15, "Oracle Dynamic SQL: Method 4". |
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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". |
Does Pro*C/C++ generate calls to the Oracle Call Interface?
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 Oracle Call Interface. Oracle is committed to supporting it.
You can also mix OCI and Pro*C/C++. See "SQLLIB Extensions for OCI Release 8 Interoperability".
Can I call a PL/SQL stored procedure from a Pro*C/C++ program?
Certainly. See Chapter 7, "Embedded PL/SQL". There's a demo program, "Calling a Stored PL/SQL or Java Subprogram".
Can I write C++ code, and precompile it using Pro*C/C++?
Yes. See Chapter 12, "C++ Applications".
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".
However, 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, "Oracle Dynamic SQL". There is a demo program that you can adapt to do this, "Sample Program: Dynamic SQL Method 1".
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 precompiler releases, and Oracle7, the safest thing to do is use VARCHAR[n] host variables. See "Declaring VARCHAR Variables".
The default datatype for all other character variables in Pro*C/C++ is CHARZ; see "CHARZ". 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".
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". There is a sample program that shows how to use the TYPE command starting on "Sample Program: Cursor and a Host Structure".
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".
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".
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" and "Pointer Variables".
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 Pro*C/C++ 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 Oracle8i, but you cannot use release 8 of Pro*C/C++ with the Oracle7 server.
When my application runs under Oracle8i, I keep getting an ORA-1405 error (fetched column value is NULL). 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 "Indicator Variables".
Alternatively, if precompiling with MODE=ORACLE and DBMS=V7 or V8, specify UNSAFE_NULL=YES on the command line (see "UNSAFE_NULL" for more information) to disable the ORA-01405 message.
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:
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SQLSQLDAAlloc() |
Used to allocate a SQL descriptor array (SQLDA) for dynamic SQL Method 4.See "How is the SQLDA Referenced?". |
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SQLCDAFromResultSetCursor() |
Used to convert a Pro*C/C++ cursor variable to an OCI cursor data area. See "New Names for SQLLIB Public Functions". |
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SQLSQLDAFree() |
Used to free a SQLDA allocated using SQLSQLDAAlloc(). See "New Names for SQLLIB Public Functions". |
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SQLCDAToResultSetCursor() |
Used to convert an OCI cursor data area to a Pro*C/C++ cursor variable. See "New Names for SQLLIB Public Functions". |
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SQLErrorGetText() |
Returns a long error message. See "sqlerrm". |
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SQLStmtGetText() |
Used to return the text of the most recently executed SQL statement. See "Obtaining the Text of SQL Statements". |
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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 "New Names for SQLLIB Public Functions". |
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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 "New Names for SQLLIB Public Functions". |
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SQLColumnNullCheck() |
Returns an indication of NULL status for dynamic SQL Method 4. See "Handling NULL/Not NULL Datatypes". |
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SQLNumberPrecV6() |
Returns precision and scale of numbers. See "Extracting Precision and Scale". |
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SQLNumberPrecV7() |
A variant of SQLNumberPrecV6(). See "Extracting Precision and Scale". |
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SQLVarcharGetLength() |
Used for obtaining the padded size of a VARCHAR[n]. See "Finding the Length of the VARCHAR Array Component". |
In the preceding list, the functions are thread-safe SQLLIB public functions. Use these functions in all new applications. The names of the functions were 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".
How does Oracle8i support the new Object Types?
See the chapters Chapter 17, "Objects" and Chapter 19, "The Object Type Translator" for how to use Object types in Pro*C/C++ applications.
