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Oracle® Database Data Cartridge Developer's Guide
11g Release 1 (11.1)

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5 Implementing Data Cartridges in C, C++ and Java

This chapter describes how to use C, C++, and Java to implement the methods of a data cartridge. Methods are procedures and functions that define the operations permitted on data defined using the data cartridge. The focus is on issues related to developing and debugging external procedures.

This chapter contains these topics:

Using External Procedures

PL/SQL is a powerful language for database programming, but some methods are too complex to code optimally in PL/SQL. For example, a routine to perform numerical integration will probably run faster if it is implemented in C rather than PL/SQL.

To support such special-purpose processing, PL/SQL provides an interface for calling routines written in other languages. This makes the strengths and capabilities of 3GLs, like C, available through calls from a database server. Such a 3GL routine is called an external procedure; it is stored in a shared library, registered with PL/SQL, and called from PL/SQL at runtime.

External procedures are an important tool for data cartridge developers. They can be used not only to write fast, efficient, computation-intensive routines for cartridge types, but also to integrate existing code with the database as data cartridges. Existing shared libraries from other languages, such as a Windows NT DLL with C routines to perform format conversions for audio files, can be called directly from a method in a type implemented by an audio cartridge. Similarly, you can use external procedures to process signals, drive devices, analyze data streams, render graphics, or process numerical data.

See Also:

PL/SQL User's Guide and Reference for details on external procedures and their use

Using Shared Libraries

A shared library is an operating system file, such as a Windows DLL or a Solaris shared object, that stores the coded implementation of external procedures. You can access to the shared library from Oracle by using an alias library, which is a schema object that represents the library within PL/SQL. For security reasons, you need DBA privileges to create an alias library.

To create the alias library, you must decide on the operating system location for the library, log in as a DBA or as a user with the CREATE LIBRARY privilege, and then enter the statement in Example 5-1. This creates the alias library schema object in the database. After the alias library is created, you can refer to the shared library by the name DS_Lib from PL/SQL.

Example 5-1 How to Create an Alias Library

CREATE OR REPLACE LIBRARY DS_Lib AS  
     '/data_cartridge_dir/libdatastream.so';

Example 5-1 specifies an absolute path for the library. If you have copies of the library on multiple systems, to support distributed execution of external procedures by designated or dedicated agents, use an environment variable to specify the location of the libraries more generally, as in Example 5-2. This statement uses the environment variable ${DS_LIB_HOME} to specify a common point of reference or root directory from which the library can be found on all systems. The string following the AGENT keyword specifies the agent (actually, a database link) that will be used to run any external procedure declared to be in library DS_Lib.

Example 5-2 How to Specify the Location of the Library Using an Environment Variable

CREATE OR REPLACE LIBRARY DS_Lib AS 
  '${DS_LIB_HOME}/libdatastream.so' AGENT 'agent_link';

See Also:

Oracle Database PL/SQL Language Reference for more information on using dedicated external procedure agents

Registering an External Procedure

To call an external procedure, you must not only instruct PL/SQL regarding the alias library where the external procedure is defined, but also how to call this procedure and what arguments to pass to it.

The DataStream type was defined in Example 3-1, and Example 3-2 defined methods o f DataStream by calling functions from the DS_Package package, which is specified in Example 4-9. Example 5-3 defines the body of this package.

Example 5-3 How to Define the Body of a Package

CREATE OR REPLACE PACKAGE BODY DS_Package AS 
     FUNCTION DS_Findmin(data  CLOB) RETURN PLS_INTEGER IS EXTERNAL 
     NAME "c_findmin" LIBRARY DS_Lib LANGUAGE C WITH CONTEXT; 
     FUNCTION DS_Findmax(data CLOB) RETURN PLS_INTEGER IS EXTERNAL 
     NAME "c_findmax" LIBRARY DS_Lib LANGUAGE C WITH CONTEXT; 
   END;

Note that in the PACKAGE BODY declaration clause, the package functions are tied to external procedures in a shared library. The EXTERNAL clause in the function declaration registers information about the external procedure, such as its name (found after the NAME keyword), its location (which must be an alias library, following the LIBRARY keyword), the language in which the external procedure is written (following the LANGUAGE keyword), and so on.

The final part of the EXTERNAL clause in the example is the WITH CONTEXT specification. This means that a context pointer is passed to the external procedure. The context pointer is opaque to the external procedure, but is available so that the external procedure can call back to the Oracle server, to potentially access more data in the same transaction context.

Although the example describes external procedure calls from object type methods, a data cartridge can use external procedures from a variety of other places in PL/SQL. External procedure calls can appear in:

See Also:

How PL/SQL Calls an External Procedure

To call an external procedure, PL/SQL must know the DLL or shared library in which the procedure resides. PL/SQL looks up the alias library in the EXTERNAL clause of the subprogram that registered the external procedure. The data dictionary is used to determine the actual path to the operating system shared library or DLL.

PL/SQL alerts a Listener process, which in turn starts a session-specific agent. Unless some other particular agent has been designated either in the CREATE LIBRARY statement for the procedure's specified library or in the agent argument of the CREATE PROCEDURE statement, the default agent extproc is launched. The Listener hands over the connection to the agent. PL/SQL passes the agent the name of the DLL, the name of the external procedure, and any parameters passed in by the caller. The rest of this account assumes that the agent launched is the default agent extproc.

After receiving the name of the DLL and the external procedure, extproc loads the DLL and runs the external procedure. Also, extproc handles service calls, such as raising an exception, and callbacks to the Oracle server. Finally, extproc passes to PL/SQL any values returned by the external procedure. Figure 5-1 shows the flow of control.

Figure 5-1 How to Call an External Procedure

Description of Figure 5-1 follows
Description of "Figure 5-1 How to Call an External Procedure"

After the external procedure completes, extproc remains active throughout your Oracle session. Thus, you incur the cost of spawning extproc only once, no matter how many calls you make. Still, you should call an external procedure only when the computational benefits outweigh the cost. When you log off, extproc is killed.

Note that the Listener must start extproc on the system that runs the Oracle server. Starting extproc on a different system is not supported.

See Also:

Configuration Files for External Procedures

The configuration files listener.ora and tnsnames.ora must have appropriate entries, so that the Listener can dispatch the external procedures.

The Listener configuration file listener.ora must have a SID_DESC entry for the external procedure, as demonstrated in Example 5-4.

Example 5-4 How to Set the SID_DESC Entry in the Listener Configuration FIle

# Listener configuration file  
# This file is generated by stkconf.tsc  
 
CONNECT_TIMEOUT_LISTENER = 0  
 
LISTENER = (ADDRESS_LIST=  
  (ADDRESS=(PROTOCOL=ipc)(KEY=o8))  
  (ADDRESS=(PROTOCOL=tcp)(HOST=unix123)(PORT=1521))  
)

SID_LIST_LISTENER = (SID_LIST=   
  SID_DESC=(SID_NAME=o8)(ORACLE_HOME=/rdbms/u01/app/oracle/product/8.0.3))  
 
(SID_DESC=(SID_NAME=extproc)(ORACLE_HOME=/rdbms/u01/app/oracle/product/8.0.3)
  (PROGRAM=extproc))  
)

Example 5-4 assumes the following:

The tnsnames.ora file is the network substrate configuration file, and it must also be updated to refer to the external procedure, as demonstrated in Example 5-5:

Example 5-5 How to Update the Network Substrate Configuration File to Refer to External Procedures

o8 = (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=unix123)(PORT=1521))
  (CONNECT_DATA=(SID=o8)))
extproc_connection_data = (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=o8))
  CONNECT_DATA=(SID=extproc)))

Example 5-5 assumes that IPC mechanisms are used to communicate with the external procedure. You can also use, for example, TCP/IP for communication, in which case the PROTOCOL parameter must be set to tcp.

See Also:

Oracle Database Administrator's Guide for more information about configuring the listener.ora and tnsnames.ora files

Passing Parameters to an External Procedure

Passing parameters to an external procedure is complicated by several circumstances:

  • The set of PL/SQL datatypes does not correspond one-to-one with the set of C datatypes.

  • PL/SQL parameters can be null, whereas C parameters cannot. Unlike C, PL/SQL includes the RDBMS concept of nullity.

  • The external procedure might need the current length or maximum length of CHAR, LONG RAW, RAW, and VARCHAR2 parameters.

  • The external procedure might need character set information about CHAR, VARCHAR2, and CLOB parameters.

  • PL/SQL might need the current length, maximum length, or null status of values returned by the external procedure.

In the following sections, you learn how to specify a parameter list that deals with these circumstances. An example of parameter passing is shown in Example 5-6, where the package function DS_Findmin(data CLOB) calls the C routine c_findmin and the CLOB argument is passed to the C routine as an OCILobLocator().

Specifying Datatypes

You do not pass parameters to an external procedure directly. Instead, you pass them to the PL/SQL subprogram that registered the external procedure. So, you must specify PL/SQL datatypes for the parameters. Table 5-1 maps each PL/SQL datatype to a default external datatype. The external datatypes map to C datatype.

Table 5-1 Parameter Datatype Mappings

PL/SQL Type Supported External Types Default External Type
BINARY_INTEGER,
BOOLEAN,
PLS_INTEGER 
CHAR, UNSIGNED CHAR, SHORT, UNSIGNED
 SHORT, INT, UNSIGNED INT, LONG,
 UNSIGNED LONG, SB1, UB1, SB2, UB2, 
 SB4, UB4, SIZE_T
INT
NATURAL, NATURALN, 
POSITIVE,
POSITIVEN, 
SIGNTYPE
CHAR, UNSIGNED CHAR, SHORT, UNSIGNED
 SHORT, INT, UNSIGNED INT, LONG,
 UNSIGNED LONG, SB1, UB1, SB2 ,UB2,
 SB4, UB4, SIZE_T
UNSIGNED INT
FLOAT, REAL
FLOAT
FLOAT
DOUBLE PRECISION
DOUBLE
DOUBLE
CHAR, CHARACTER,
LONG, ROWID, VARCHAR, VARCHAR2 
STRING
STRING
LONG RAW, RAW
RAW
RAW
BFILE, BLOB, CLOB
OCILOBLOCATOR
OCILOBLOCATOR

In some cases, you can use the PARAMETERS clause to override the default datatype mappings. For example, you can re-map the PL/SQL datatype BOOLEAN from external datatype INT to external datatype CHAR.

To avoid errors when declaring C prototype parameters, refer to Table 5-2, which shows the C datatype to specify for a given external datatype and PL/SQL parameter mode. For example, if the external datatype of an OUT parameter is CHAR, specify the datatype char* in your C prototype.

Table 5-2 External Datatype Mappings

External Datatype IN, RETURN IN by Reference, RETURN by Reference IN OUT, OUT
CHAR
char
char *
char *
UNSIGNED CHAR
unsigned char
unsigned char *
unsigned char *
SHORT
short
short *
short *
UNSIGNED SHORT
unsigned short
unsigned short *
unsigned short *
INT
int
int *
int *
UNSIGNED INT
unsigned int
unsigned int *
unsigned int *
LONG
long
long *
long *
UNSIGNED LONG
unsigned long
unsigned long *
unsigned long *
SIZE_T
size_t
size_t *
size_t *
SB1
sb1
sb1 *
sb1 *
UB1
ub1
ub1 *
ub1 *
SB2
sb2
sb2 *
sb2 *
UB2
ub2
ub2 *
ub2 *
SB4
sb4
sb4 *
sb4 *
UB4
ub4
ub4 *
ub4 *
FLOAT
float
float *
float *
DOUBLE
double
double *
double *
STRING
char *
char *
char *
RAW
unsigned char *
unsigned char *
unsigned char *
OCILOBLOCATOR
OCILobLocator *
OCILobLocator *
OCILobLocator **

Using the Parameters Clause

You can optionally use the PARAMETERS clause to pass additional information about PL/SQL formal parameters and function return values to an external procedure. You can also use this clause to reposition parameters.

Using the WITH CONTEXT Clause

Once launched, an external procedure may need to access the database. For example, DS_Findmin does not copy the entire CLOB data over to c_findmin, because doing so would vastly increase the amount of stack that the C routine needs. Instead, the PL/SQL function just passes a LOB locator to the C routine, with the intent that the database will be re-accessed from C to read the actual LOB data.

When the C routine reads the data, it can use the OCI buffering and streaming interfaces associated with LOBs, so that only incremental amounts of stack are needed. Such re-access of the database from an external procedure is known as a callback.

To be able to call back to a database, you need to use the WITH CONTEXT clause to give the external procedure access to the database environment, service, and error handles. When an external procedure is called using WITH CONTEXT, the corresponding C routine automatically gets an argument of type OCIExtProcContext* as its first parameter. The order of the parameters can be changed using the PARAMETERS clause. You can use this context pointer to fetch the handles using the OCIExtProcGetEnv call, and then call back to the database. This procedure is shown in Example 5-6.

See Also:

Oracle Call Interface Programmer's Guide for details about OCI callbacks

Doing Callbacks

An external procedure that runs on the Oracle server can call the access function OCIExtProcGetEnv() to obtain the OCI environment and service handles. With the OCI, you can use callbacks to execute SQL statements and PL/SQL subprograms, fetch data, and manipulate LOBs. Moreover, callbacks and external procedures operate in the same user session and transaction context, so they have the same user privileges.

Example 5-6 is a version of c_findmin that is simplified to illustrate callbacks.

Example 5-6 How to Use Callbacks

Static  OCIEnv   *envhp;
Static  OCISvcCtx  *svchp;
Static OCIError   *errhp;
Int   c_findmin (OCIExtProcContext *ctx, OCILobLocator  *lobl) {
sword  retval;
retval = OCIExtProcGetEnv (ctx, &envhp, &svchp, &errhp);
if ((retval != OCI_SUCCESS) && (retval !=  OCI_SUCCESS_WITH_INFO))
   exit(-1);
   /* Use lobl to read the CLOB, compute the minimum, and store the value
       in retval. */
return retval;
}

Restrictions on Callbacks

With callbacks, the following SQL statements and OCI routines are not supported:

  • Transaction control statements such as COMMIT

  • Data definition statements such as CREATE

  • Object-oriented OCI routines such as OCIRefClear

  • Polling-mode OCI routines such as OCIGetPieceInfo

  • The following OCI routines:

    • OCIEnvInit()

    • OCIInitialize()

    • OCIPasswordChange()

    • OCIServerAttach()

    • OCIServerDetach()

    • OCISessionBegin ()

    • OCISessionEnd ()

    • OCISvcCtxToLda()

    • OCITransCommit()

    • OCITransDetach()

    • OCITransRollback()

    • OCITransStart()

  • Also, with OCI routine OCIHandleAlloc(), the following handle types are not supported:

    • OCI_HTYPE_SERVER

    • OCI_HTYPE_SESSION

    • OCI_HTYPE_SVCCTX

    • OCI_HTYPE_TRANS

Common Potential Errors

This section presents several kinds of errors you might encounter when running external procedures.

Calls to External Functions

Can't Find DLL
ORA-06520: PL/SQL: Error loading external library
ORA-06522: Unable to load DLL
ORA-06512: at "<name>", line <number>
ORA-06512: at "<name>", line <number>
ORA-06512: at line <number>

You may have specified the wrong path or wrong name for the DLL file, or you may have tried to use a DLL on a network mounted drive (a remote drive).

RPC Time Out

ORA-28576: lost RPC connection to external procedure agent
ORA-06512: at "<name>", line <number>
ORA-06512: at "<name>", line <number>
ORA-06512: at line <number>

This error might occur after you exit a debugger while debugging a shared library or DLL. Simply disconnect your client and reconnect to the database.

Debugging External Procedures

Usually, when an external procedure fails, its C prototype is faulty. That is, the prototype does not match the one generated internally by PL/SQL. This can happen if you specify an incompatible C datatype. For example, to pass an OUT parameter of type REAL, you must specify float *. Specifying float, double *, or any other C datatype will result in a mismatch.

In such cases, you might get a lost RPC connection to external procedure agent error, which means that agent extproc terminated abnormally because the external procedure caused a core dump. To avoid errors when declaring C prototype parameters, refer to Table 5-2.

Using Package DEBUG_EXTPROC

To help you debug external procedures, PL/SQL provides the utility package DEBUG_EXTPROC. To install the package, run the script dbgextp.sql, which you can find in the PL/SQL demo directory.

To use the package, follow the instructions in dbgextp.sql. Your Oracle account must have EXECUTE privileges on the package and CREATE LIBRARY privileges.

Note that DEBUG_EXTPROC works only on platforms with debuggers that can attach to a running process.

Debugging C Code in DLLs on Windows NT Systems

If you are developing on a Windows NT system, you may perform the following additional actions to debug external procedures:

  1. Invoke the Windows NT Task Manager; press Ctrl+Alt+Del and select Task Manager.

  2. In the Processes display, select ExtProc.exe.

  3. Right click, and select Debug.

  4. Select OK in the message box.

    At this point, if you have built your DLL in a debug fashion with Microsoft Visual C++, Visual C++ is activated.

  5. In the Visual C++ window, select Edit > Breakpoints.

  6. Use the breakpoint identified in dbgextp.sql in the PL/SQL demo directory.

Guidelines for Using External Procedures with Data Cartridges

Make sure to write thread-safe external procedures. In particular, avoid using static variables, which can be shared by routines running in separate threads.

For help in creating a dynamic link library, look in the RDBMS subdirectory /public, where a template makefile can be found.

When calling external procedures, never write to IN parameters or overflow the capacity of OUT parameters. PL/SQL does no runtime checks for these error conditions. Likewise, never read an OUT parameter or a function result. Also, always assign a value to IN OUT and OUT parameters and to function results. Otherwise, your external procedure will not return successfully.

If you include the WITH CONTEXT and PARAMETERS clauses, you must specify the parameter CONTEXT, which shows the position of the context pointer in the parameter list. If you omit the PARAMETERS clause, the context pointer is the first parameter passed to the external procedure.

If you include the PARAMETERS clause and the external procedure is a function, you must specify the parameter RETURN (not RETURN property) in the last position.

For every formal parameter, there must be a corresponding parameter in the PARAMETERS clause. Also, make sure that the datatypes of parameters in the PARAMETERS clause are compatible with those in the C prototype because no implicit conversions are done.

A parameter for which you specify INDICATOR or LENGTH has the same parameter mode as the corresponding formal parameter. However, a parameter for which you specify MAXLEN, CHARSETID, or CHARSETFORM is always treated like an IN parameter, even if you also specify BY REFERENCE.

With a parameter of type CHAR, LONG RAW, RAW, or VARCHAR2, you must use the property LENGTH. Also, if that parameter is IN OUT or OUT and null, you must set the length of the corresponding C parameter to zero.

See Also:

For more information about multithreading, see the Oracle Database Heterogeneous Connectivity Administrator's Guide.

Java Methods

In order to utilize Java Data Cartridges, it is important that you know how to load Java class definitions, about how to call stored procedures, and about context management. Information on ODCI classes can also be found in Chapter 18, "Cartridge Services Using C, C++ and Java" of this manual.