14 Calling External Procedures

In situations where a particular language does not provide the features you need, or when you want to reuse existing code written in another language, you can use code written in some other language by calling external procedures.

This chapter discusses the following topics:

Overview of Multi-Language Programs

Oracle Database lets you work in different languages:

How should you choose between these different implementation possibilities? Each of these languages offers different advantages: ease of use, the availability of programmers with specific expertise, the need for portability, and the existence of legacy code are powerful determinants.

The choice may narrow depending on how your application needs to work with Oracle Database:

  • PL/SQL is a powerful development tool, specialized for SQL transaction processing.

  • Some computation-intensive tasks are executed most efficiently in a lower level language, such as C.

  • The need for portability, together with the need for security, may influence you to select Java.

Most significantly, from the point of view of performance, you should note that only PL/SQL and Java methods run within the address space of the server. C/C++ methods are dispatched as external procedures, and run on the server machine but outside the address space of the database server. Pro*COBOL and Pro*C/C++ are precompilers, and Visual Basic accesses Oracle Database through the OCI, which is implemented in C.

Taking all these factors into account suggests that there may be a number of situations in which you may need to implement your application in more than one language. For instance, the introduction of Java running within the address space of the server suggest that you may want to import existing Java applications into the database, and then leverage this technology by calling Java functions from PL/SQL and SQL.

PL/SQL external procedures allow you to write C function calls as PL/SQL bodies. These C functions are callable directly from PL/SQL, and from SQL through PL/SQL procedure calls. The database provides a special-purpose interface, the call specification, that lets you call external procedures from other languages. While this service is designed for intercommunication between SQL, PL/SQL, C, and Java, it is accessible from any base language that can call these languages. For example, your procedure can be written in a language other than Java or C and still be usable by SQL or PL/SQL, as long as your procedure is callable by C. Therefore, if you have a candidate C++ procedure, you would use a C++ extern "C" statement in that procedure to make it callable by C.

This means that the strengths and capabilities of different languages are available to you, regardless of your programmatic environment. You are not restricted to one language with its inherent limitations. External procedures promote reusability and modularity because you can deploy specific languages for specific purposes.

What Is an External Procedure?

An external procedure, also sometimes referred to as an external routine, is a procedure stored in a dynamic link library (DLL), or libunit in the case of a Java class method. You register the procedure with the base language, and then call it to perform special-purpose processing.

For instance, when you work in PL/SQL, the language loads the library dynamically at runtime, and then calls the procedure as if it were a PL/SQL subprogram. These procedures participate fully in the current transaction and can call back to the database to perform SQL operations.

The procedures are loaded only when necessary, so memory is conserved. Because the decoupling of the call specification from its implementation body means that the procedures can be enhanced without affecting the calling programs.

External procedures let you:

  • Isolate execution of client applications and processes from the database instance to ensure that any problems on the client side do not adversely impact the database.

  • Move computation-bound programs from client to server where they execute faster (because they avoid the round-trips of network communication)

  • Interface the database server with external systems and data sources

  • Extend the functionality of the database server itself

Overview of The Call Specification for External Procedures

You publish external procedures through call specifications, which provide a superset of the AS EXTERNAL function through the AS LANGUAGE clause. AS LANGUAGE call specifications allow the publishing of external C procedures, but also Java class methods.

Note:

To support legacy applications, call specifications also allow you to publish with the AS EXTERNAL clause. For new application development, however, using the AS LANGUAGE clause is recommended.

In general, call specifications enable:

  • Dispatching the appropriate C or Java target procedure

  • Datatype conversions

  • Parameter mode mappings

  • Automatic memory allocation and cleanup

  • Purity constraints to be specified, where necessary, for packaged functions called from SQL.

  • Calling Java methods or C procedures from database triggers

  • Location flexibility: you can put AS LANGUAGE call specifications in package or type specifications, or package (or type) bodies to optimize performance and hide implementation details

To use an already-existing program as an external procedure, load, publish, and then call it.

Loading External Procedures

To make your external C procedures or Java methods available to PL/SQL, you must first load them. The manner of doing this depends upon whether the procedure is written in C or Java.

Loading Java Class Methods

One way to load Java programs is to use the CREATE JAVA statement, which you can execute interactively from SQL*Plus. When implicitly invoked by the CREATE JAVA statement, the Java Virtual Machine (JVM)] library manager loads Java binaries (.class files) and resources from local BFILEs or LOB columns into RDBMS libunits.

Suppose a compiled Java class is stored in the following operating system file:

/home/java/bin/Agent.class

Creating a class libunit in schema scott from file Agent.class requires two steps: First, create a directory object on the server's file system. The name of the directory object is an alias for the directory path leading to Agent.class.

To create the directory object, you must grant user scott the CREATE ANY DIRECTORY privilege, then execute the CREATE DIRECTORY statement, as follows:

CONNECT System/Manager
GRANT CREATE ANY DIRECTORY TO Scott IDENTIFIED BY Tiger;
CONNECT Scott/Tiger
CREATE DIRECTORY Bfile_dir AS '/home/java/bin';

You are ready to create the class libunit, as follows:

CREATE JAVA CLASS USING BFILE (Bfile_dir, 'Agent.class');

The name of the libunit is derived from the name of the class.

Alternatively, you can use the command-line utility LoadJava. This uploads Java binaries and resources into a system-generated database table, then uses the CREATE JAVA statement to load the Java files into RDBMS libunits. You can upload Java files from file systems, Java IDEs, intranets, or the Internet.

Loading External C Procedures

In order to set up your database configuration to use external procedures that are written in C, or can be called from C applications, you and your database administrator must take the following steps:

Note:

  • This feature is available only on platforms that support dynamically linked libraries (DLLs) or dynamically loadable shared libraries such as Solaris .so libraries.

  • The external procedure agent can call procedures in any library that complies with the calling standard used. The supported calling standard is C. Refer to "CALLING STANDARD" for more information on the calling standard sub clause used with external procedures in PL/SQL.

  • Define the C procedures using either the 1) K & R-style prototypes or 2) the ISO/ANSI prototypes without numeric datatypes that are less than full width (such as float, short, char). Other datatypes work in ISO/ANSI prototypes if they do not change size under "default argument promotions."

    /* Supported K & R */
    void C_findRoot(x)
     float x;
    ...
    
    /* Supported ISO/ANSI */
    void C_findRoot(double x)
    ...
    
    /* Not supported ISO/ANSI */
    void C_findRoot(float x)
    ...
    

Step 1 Set Up the Environment

Your database administrator must perform the following tasks to configure your database to use external procedures that are written in C, or can be called from C applications:

  • Set configuration parameters for the agent, named extproc by default, in the configuration files tnsnames.ora and listener.ora. This establishes the connection for the external procedure agent when the database is started.

  • Start a listener process exclusively for external procedures.

    The Listener sets a few required environment variables (such as ORACLE_HOME, ORACLE_SID, and LD_LIBRARY_PATH) for the external procedure agent. It can also define specific environment variables in the ENVS section of its listener.ora entry, and these variables are passed to the agent process. Otherwise, it provides the agent with a "clean" environment. The environment variables set for the agent are independent of those set for the client and server. Therefore, external procedures, which run in the agent process, cannot read environment variables set for the client or server processes.

    Note:

    It is possible for you to set and read environment variables themselves by using the standard C procedures setenv() and getenv(), respectively. Environment variables, set this way, are specific to the agent process, which means that they can be read by all functions executed in that process, but not by any other process running on the same host.
  • Determine whether the agent for your external procedure will run in dedicated mode (the default mode) or multithreaded mode. In dedicated mode, one "dedicated" agent is launched for each user. In multithreaded mode, a single multithreaded agent is launched. The multithreaded agent handles calls using different threads for different users. In a configuration where many users will call the external procedures, using a multithreaded agent is recommended to conserve system resources.

    If the agent will run in dedicated mode, additional configuration of the agent process is not necessary.

    If the agent will run in multithreaded mode, your database administrator must configure the database system to start the agent in multithreaded mode (as a multithreaded agent). This is done using the agent control utility agtctl. For example, start the agent using the agent control utility startup command:

    agtctl startup extproc agent_sid 
    
    

    where agent_sid is the system identifier which this agent will service. An entry for this system identifier is typically added as an entry in the file tnsnames.ora. Details on the agent control utility are in the Oracle Database Heterogeneous Connectivity Administrator's Guide.

Note:

  • If you use a multithreaded agent, the library you call must be thread safe—to avoid errors such as a corrupt call stack.

  • The database server, the agent process, and the listener process that spawns the agent process must all reside on the same host.

  • By default, the agent process runs on the same database instance as your main application. In situations where reliability is critical, you might want to run the agent process for the external procedure on a separate database instance (still on the same host), so that any problems in the agent do not affect the primary database server. To do so, specify the separate database instance using a database link.

Figure 14-1 illustrates the architecture of the multithreaded external procedure agent. User sessions 1 and 2 issue requests for callouts to functions in some DLLs. These requests get serviced through heterogeneous services to the multithreaded extproc agent. These requests get handled by the agent's dispatcher threads, which then pass them on to the task threads. The task thread that is actually handling a request is responsible for loading the respective DLL and calling the function therein.

  • All requests from a user session get handled by the same dispatcher thread. For example, dispatcher 1 handles communication with user session 1, and dispatcher 2 handles communication with user session 2. This is the case for the lifetime of the session.

  • The individual requests can, however, be serviced by different task threads. For example, task thread 1 could handle the request from user session 1 at one time, and handle the request from user session 2 at another time.

Figure 14-1 Multithreaded External Procedure Agent Architecture

Description of Figure 14-1 follows
Description of "Figure 14-1 Multithreaded External Procedure Agent Architecture "

See Also:

Step 2 Identify the DLL

In this context, a DLL is any dynamically loadable operating-system file that stores external procedures.

For security reasons, your DBA controls access to the DLL. Using the CREATE LIBRARY statement, the DBA creates a schema object called an alias library, which represents the DLL. Then, if you are an authorized user, the DBA grants you EXECUTE privileges on the alias library. Alternatively, the DBA may grant you CREATE ANY LIBRARY privileges, in which case you can create your own alias libraries using the following syntax:

CREATE LIBRARY [schema_name.]library_name
  {IS | AS} 'file_path'
  [AGENT 'agent_link'];

It is recommended that you specify the full path to the DLL, rather than just the DLL name. In the following example, you create alias library c_utils, which represents DLL utils.so:

CREATE LIBRARY C_utils AS '/DLLs/utils.so';

To allow flexibility in specifying the DLLs, you can specify the root part of the path as an environment variable using the notation ${VAR_NAME}, and set up that variable in the ENVS section of the listener.ora entry.

In the following example, the agent specified by the name agent_link is used to run any external procedure in the library C_Utils. The environment variable EP_LIB_HOME is expanded by the agent to the appropriate path for that instance, such as /usr/bin/dll. Variable EP_LIB_HOME must be set in the file listener.ora, for the agent to be able to access it.

create or replace database link agent_link using 'agent_tns_alias';
create or replace library C_utils is
  '${EP_LIB_HOME}/utils.so' agent 'agent_link';

For security reasons, EXTPROC, by default, will only load DLLs that are in directory $ORACLE_HOME/bin or $ORACLE_HOME/lib. Also, only local sessions—that is, Oracle Database client processes that are running on the same machine—are allowed to connect to EXTPROC.

To load DLLs from other directories, the environment variable EXTPROC_DLLS should be set. The value for this environment variable is a colon- separated (:) list of DLL names qualified with the complete path. For example:

EXTPROC_DLLS=/private1/home/scott/dll/myDll.so:/private1/home/scott/dll/newDll.so

The preferred method to set this environment variable is through the ENVS parameter in the file listener.ora. Refer to the Oracle Net manual for more information on the EXTPROC feature.

Note the following:

  • On a Windows system, you would specify the path using a drive letter and backslash characters (\) in the path.

  • This technique is not applicable for VMS systems, where the ENVS section of listener.ora is not supported.

Step 3 Publish the External Procedure

You find or write a new external C procedure, then add it to the DLL. When the procedure is in the DLL, you publish it using the call specification mechanism described in the following section.

Publishing External Procedures

Oracle Database can only use external procedures that are published through a call specification, which maps names, parameter types, and return types for your Java class method or C external procedure to their SQL counterparts. It is written like any other PL/SQL stored subprogram except that, in its body, instead of declarations and a BEGIN ... END block, you code the AS LANGUAGE clause.

The AS LANGUAGE clause specifies:

  • Which language the procedure is written in.

  • For a Java method:

    • The signature of the Java method.

  • For a C procedure:

    • The alias library corresponding to the DLL for a C procedure.

    • The name of the C procedure in a DLL.

    • Various options for specifying how parameters are passed.

    • Which parameter (if any) holds the name of the external procedure agent, for running the procedure on a different machine.

You begin the declaration using the normal CREATE OR REPLACE syntax for a procedure, function, package specification, package body, type specification, or type body.

The call specification follows the name and parameter declarations. Its syntax is:

{IS | AS} LANGUAGE {C | JAVA}

Note:

Oracle Database uses a PL/SQL variant of the ANSI SQL92 External Procedure, but replaces the ANSI keyword AS EXTERNAL with this call specification syntax. This new syntax, first introduced for Java class methods, has been extended to C procedures.

This is then followed by either:

NAME  java_string_literal_name

Where java_string_literal_name is the signature of your Java method, or by:

LIBRARY library_name
[NAME c_string_literal_name]
[WITH CONTEXT]
[PARAMETERS (external_parameter[, external_parameter]...)];

Where library_name is the name of your alias library, c_string_literal_name is the name of your external C procedure, and external_parameter stands for:

{  CONTEXT 
 | SELF [{TDO | property}]
 | {parameter_name | RETURN} [property] [BY REFERENCE] [external_datatype]}

property stands for:

{INDICATOR [{STRUCT | TDO}] | LENGTH | DURATION | MAXLEN | CHARSETID | CHARSETFORM}

Note:

Unlike Java, C does not understand SQL types; therefore, the syntax is more intricate

The AS LANGUAGE Clause for Java Class Methods

The AS LANGUAGE clause is the interface between PL/SQL and a Java class method.

The AS LANGUAGE Clause for External C Procedures

The following subclauses tell PL/SQL where to locate the external C procedure, how to call it, and what to pass to it. Only the LIBRARY subclause is required.

LIBRARY

Specifies a local alias library. (You cannot use a database link to specify a remote library.) The library name is a PL/SQL identifier. Therefore, if you enclose the name in double quotes, then it becomes case sensitive. (By default, the name is stored in upper case.) You must have EXECUTE privileges on the alias library.

NAME

Specifies the external C procedure to be called. If you enclose the procedure name in double quotes, then it becomes case sensitive. (By default, the name is stored in upper case.) If you omit this subclause, then the procedure name defaults to the upper-case name of the PL/SQL subprogram.

Note:

The terms LANGUAGE and CALLING STANDARD apply only to the superseded AS EXTERNAL clause.

LANGUAGE

Specifies the third-generation language in which the external procedure was written. If you omit this subclause, then the language name defaults to C.

CALLING STANDARD

Specifies the calling standard under which the external procedure was compiled. The supported calling standard is C. If you omit this subclause, then the calling standard defaults to C.

WITH CONTEXT

Specifies that a context pointer will be passed to the external procedure. The context data structure is opaque to the external procedure but is available to service procedures called by the external procedure.

PARAMETERS

Specifies the positions and datatypes of parameters passed to the external procedure. It can also specify parameter properties, such as current length and maximum length, and the preferred parameter passing method (by value or by reference).

AGENT IN

Specifies which parameter holds the name of the agent process that should run this procedure. This is intended for situations where external procedure agents are run using multiple agent processes, to ensure robustness if the agent process of one external procedure fails. You can pass the name of the agent process (corresponding to the name of a database link), and if tnsnames.ora and listener.ora are set up properly across both instances, the external procedure is invoked on the other instance. Both instances must be on the same host.

This is similar to the AGENT clause of the CREATE LIBRARY statement; specifying the value at runtime through AGENT IN allows greater flexibility.

When the agent name is specified this way, it overrides any agent name declared in the alias library. If no agent name is specified, the default is the extproc agent on the same instance as the calling program.

Publishing Java Class Methods

Java classes and their methods are stored in RDBMS libunits in which you can load Java sources, binaries and resources using the LOADJAVA utility or the CREATEJAVA SQL statements. Libunits can be considered analogous to DLLs written, for example, in C—although they map one-to-one with Java classes, whereas DLLs can contain more than one procedure.

The NAME-clause string uniquely identifies the Java method. The PL/SQL function or procedure and Java must correspond with regard to parameters. If the Java method takes no parameters, then you must code an empty parameter list for it.

When you load Java classes into the RDBMS, they are not published to SQL automatically. This is because the methods of many Java classes are called only from other Java classes, or take parameters for which there is no appropriate SQL type.

Suppose you want to publish the following Java method named J_calcFactorial, which returns the factorial of its argument:

package myRoutines.math;
public class Factorial {
   public static int J_calcFactorial (int n) {
      if (n == 1) return 1;
      else return n * J_calcFactorial(n - 1);
   }
}

The following call specification publishes Java method J_calcFactorial as PL/SQL stored function plsToJavaFac_func, using SQL*Plus:

CREATE OR REPLACE FUNCTION Plstojavafac_func (N NUMBER) RETURN NUMBER AS
   LANGUAGE JAVA
   NAME 'myRoutines.math.Factorial.J_calcFactorial(int) return int';

Publishing External C Procedures

In the following example, you write a PL/SQL standalone function named plsCallsCdivisor_func that publishes C function Cdivisor_func as an external function:

CREATE OR REPLACE FUNCTION Plscallscdivisor_func (
/* Find greatest common divisor of x and y: */
   x     BINARY_INTEGER, 
   y     BINARY_INTEGER) 
RETURN BINARY_INTEGER 
AS LANGUAGE C
   LIBRARY C_utils
   NAME "Cdivisor_func"; /* Quotation marks preserve case. */

Locations of Call Specifications

For both Java class methods and external C procedures, call specifications can be specified in any of the following locations:

  • Standalone PL/SQL Procedures and Functions

  • PL/SQL Package Specifications

  • PL/SQL Package Bodies

  • Object Type Specifications

  • Object Type Bodies

    Note:

    Under Oracle Database version 8.0, AS EXTERNAL call specifications could not be placed in package or type bodies.

We have already shown an example of call specification located in a standalone PL/SQL function. Here are some examples showing some of the other locations.

Note:

In the following examples, the AUTHID and SQL_NAME_RESOLVE clauses may or may not be required to fully stipulate a call specification.

See the Oracle Database PL/SQL User's Guide and Reference and the Oracle Database SQL Reference for more information.

Example: Locating a Call Specification in a PL/SQL Package

CREATE OR REPLACE PACKAGE Demo_pack 
AUTHID DEFINER 
AS
   PROCEDURE plsToC_demoExternal_proc (x BINARY_INTEGER, y VARCHAR2, z DATE) 
   AS LANGUAGE C
      NAME "C_demoExternal"
      LIBRARY SomeLib
      WITH CONTEXT
      PARAMETERS(CONTEXT, x INT, y STRING, z OCIDATE);
END;

Example: Locating a Call Specification in a PL/SQL Package Body

CREATE OR REPLACE PACKAGE Demo_pack 
   AUTHID CURRENT_USER
AS 
   PROCEDURE plsToC_demoExternal_proc(x BINARY_INTEGER, y VARCHAR2, z DATE);
END;
 
CREATE OR REPLACE PACKAGE BODY Demo_pack 
   SQL_NAME_RESOLVE CURRENT_USER
AS
   PROCEDURE plsToC_demoExternal_proc (x BINARY_INTEGER, y VARCHAR2, z DATE)
   AS LANGUAGE JAVA
      NAME 'pkg1.class4.methodProc1(int,java.lang.String,java.sql.Date)';
END;

Example: Locating a Call Specification in an Object Type Specification

Note:

You may need to set up the following data structures for certain examples to work:
CONN SYS/CHANGE_ON_INSTALL AS SYSDBA;
GRANT CREATE ANY LIBRARY TO scott;
CONNECT scott/tiger
CREATE OR REPLACE LIBRARY SOMELIB AS '/tmp/lib.so'; 
CREATE OR REPLACE TYPE Demo_typ 
AUTHID DEFINER 
AS OBJECT
   (Attribute1   VARCHAR2(2000), SomeLib varchar2(20),
   MEMBER PROCEDURE plsToC_demoExternal_proc (x BINARY_INTEGER, y VARCHAR2, z DATE) 
   AS LANGUAGE C
      NAME "C_demoExternal"
      LIBRARY SomeLib
      WITH CONTEXT
    --  PARAMETERS(CONTEXT, x INT, y STRING, z OCIDATE)
      PARAMETERS(CONTEXT, x INT, y STRING, z OCIDATE, SELF)
);

Example: Locating a Call Specification in an Object Type Body

CREATE OR REPLACE TYPE Demo_typ 
AUTHID CURRENT_USER 
AS OBJECT
   (attribute1 NUMBER,
   MEMBER PROCEDURE plsToJ_demoExternal_proc (x BINARY_INTEGER, y VARCHAR2, z DATE)
);

CREATE OR REPLACE TYPE BODY Demo_typ 
AS
   MEMBER PROCEDURE plsToJ_demoExternal_proc (x BINARY_INTEGER, y VARCHAR2, z DATE)
   AS LANGUAGE JAVA
      NAME 'pkg1.class4.J_demoExternal(int,java.lang.String,java.sql.Date)';
END;

Example: Java with AUTHID

Here is an example of a publishing a Java class method in a standalonePL/SQL subprogram.

CREATE OR REPLACE PROCEDURE plsToJ_demoExternal_proc (x BINARY_INTEGER, y VARCHAR2, z DATE)
   AUTHID CURRENT_USER 
AS LANGUAGE JAVA
   NAME 'pkg1.class4.methodProc1(int,java.lang.String,java.sql.Date)';

Example: C with Optional AUTHID

Here is an example of AS EXTERNAL publishing a C procedure in a standalone PL/SQL program, in which the AUTHID clause is optional. This maintains compatibility with the external procedures of Oracle Database version 8.0.

CREATE OR REPLACE PROCEDURE plsToC_demoExternal_proc (x BINARY_INTEGER, y VARCHAR2, z DATE) 
AS 
   EXTERNAL
   LANGUAGE C
   NAME "C_demoExternal"
   LIBRARY SomeLib
   WITH CONTEXT
   PARAMETERS(CONTEXT, x INT, y STRING, z OCIDATE);

Example: Mixing Call Specifications in a Package

CREATE OR REPLACE PACKAGE Demo_pack 
AUTHID DEFINER 
AS 
   PROCEDURE plsToC_InBodyOld_proc (x BINARY_INTEGER, y VARCHAR2, z DATE);
   PROCEDURE plsToC_demoExternal_proc (x BINARY_INTEGER, y VARCHAR2, z DATE); 
   PROCEDURE plsToC_InBody_proc (x BINARY_INTEGER, y VARCHAR2, z DATE);
   PROCEDURE plsToJ_InBody_proc (x BINARY_INTEGER, y VARCHAR2, z DATE);

   PROCEDURE plsToJ_InSpec_proc (x BINARY_INTEGER, y VARCHAR2, z DATE)
   IS LANGUAGE JAVA
      NAME 'pkg1.class4.J_InSpec_meth(int,java.lang.String,java.sql.Date)';

PROCEDURE C_InSpec_proc (x BINARY_INTEGER, y VARCHAR2, z DATE) 
   AS LANGUAGE C
      NAME "C_demoExternal"
      LIBRARY SomeLib
      WITH CONTEXT
      PARAMETERS(CONTEXT, x INT, y STRING, z OCIDATE);
END;

CREATE OR REPLACE PACKAGE BODY Demo_pack 
AS 
PROCEDURE plsToC_InBodyOld_proc (x BINARY_INTEGER, y VARCHAR2, z DATE) 
   AS EXTERNAL
      LANGUAGE C
      NAME "C_InBodyOld"
      LIBRARY SomeLib
      WITH CONTEXT
      PARAMETERS(CONTEXT, x INT, y STRING, z OCIDATE); 
PROCEDURE plsToC_demoExternal_proc (x BINARY_INTEGER, y VARCHAR2, z DATE) 
   AS LANGUAGE C
      NAME "C_demoExternal"
      LIBRARY SomeLib
      WITH CONTEXT
      PARAMETERS(CONTEXT, x INT, y STRING, z OCIDATE);
   
PROCEDURE plsToC_InBody_proc (x BINARY_INTEGER, y VARCHAR2, z DATE) 
   AS LANGUAGE C
      NAME "C_InBody"
      LIBRARY SomeLib
      WITH CONTEXT
      PARAMETERS(CONTEXT, x INT, y STRING, z OCIDATE);
PROCEDURE plsToJ_InBody_proc (x BINARY_INTEGER, y VARCHAR2, z DATE)
   IS LANGUAGE JAVA
      NAME 'pkg1.class4.J_InBody_meth(int,java.lang.String,java.sql.Date)';
END;

Passing Parameters to External C Procedures with Call Specifications

Call specifications allows a mapping between PL/SQL and C datatypes. Refer to "Specifying Datatypes" for datatype mappings.

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

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

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

  • 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.

Note:

The maximum number of parameters that you can pass to a C external procedure is 128. However, if you pass float or double parameters by value, then the maximum is less than 128. How much less depends on the number of such parameters and your operating system. To get a rough estimate, count each float or double passed by value as two parameters.

Specifying Datatypes

Do not pass parameters to an external procedure directly. Instead, pass them to the PL/SQL subprogram that published the external procedure. Therefore, you must specify PL/SQL datatypes for the parameters. PL/SQL datatypes map to default external datatypes, as shown in Table 14-1.

Table 14-1 Parameter Datatype Mappings

PL/SQL Datatype Supported External Types Default External Type
BINARY_INTEGER
BOOLEAN
PLS_INTEGER 
[UNSIGNED] CHAR
[UNSIGNED] SHORT
[UNSIGNED] INT
[UNSIGNED] LONG
SB1, SB2, SB4
UB1, UB2, UB4
SIZE_T
INT
NATURALFoot 1 
NATURALNFootref 1
POSITIVEFootref 1
POSITIVENFootref 1
SIGNTYPEFootref 1
[UNSIGNED] CHAR
[UNSIGNED] SHORT
[UNSIGNED] INT
[UNSIGNED] LONG
SB1, SB2, SB4
UB1, UB2, UB4
SIZE_T
UNSIGNED INT
FLOAT
REAL
FLOAT
FLOAT
DOUBLE PRECISION
DOUBLE
DOUBLE
CHAR 
CHARACTER
LONG
NCHAR
NVARCHAR2
ROWID
VARCHAR 
VARCHAR2 
STRING
OCISTRING
STRING
LONG RAW 
RAW
RAW
OCIRAW
RAW
BFILE 
BLOB 
CLOB
NCLOB
OCILOBLOCATOR
OCILOBLOCATOR
NUMBER
DECFootref 1
DECIMALFootref 1
INTFootref 1
INTEGERFootref 1
NUMERICFootref 1
SMALLINTFootref 1
OCINUMBER
OCINUMBER
DATE
OCIDATE
OCIDATE
TIMESTAMP
TIMESTAMP WITH TIME ZONE
TIMESTAMP WITH LOCAL TIME ZONE
OCIDateTime
OCIDateTime
INTERVAL DAY TO SECOND
INTERVAL YEAR TO MONTH
OCIInterval
OCIInterval
composite object types: ADTs
dvoid
dvoid
composite object types: collections (varrays, nested tables)
OCICOLL
OCICOLL

Footnote 1 This PL/SQL type will only compile if you use AS EXTERNAL in your callspec.

External Datatype Mappings

Each external datatype maps to a C datatype, and the datatype conversions are performed implicitly. To avoid errors when declaring C prototype parameters, refer to Table 14-2, 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 STRING, then specify the datatype char * in your C prototype.

Table 14-2 External Datatype Mappings

External Datatype Corresponding to PL/SL Type If Mode is IN or RETURN, Specify in C Prototype... If Mode is IN by Reference or RETURN by Reference, Specify in C Prototype... If Mode is IN OUT or OUT, Specify in C Prototype...
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 *
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 **
OCINUMBER
OCINumber *
OCINumber *
OCINumber *
OCISTRING
OCIString *
OCIString *
OCIString *
OCIRAW
OCIRaw *
OCIRaw *
OCIRaw *
OCIDATE
OCIDate *
OCIDate *
OCIDate *
OCICOLL
OCIColl * or OCIArray * or OCITable *
OCIColl **
or OCIArray **
or OCITable **
OCIColl ** or OCIArray ** or OCITable **
OCITYPE
OCIType *
OCIType *
OCIType *
TDO
OCIType *
OCIType *
OCIType *
ADT
(final types)
dvoid*
dvoid*
dvoid*
ADT (non-final types)
dvoid*
dvoid*
dvoid**

Composite object types are not self describing. Their description is stored in a Type Descriptor Object (TDO). Objects and indicator structs for objects have no predefined OCI datatype, but must use the datatypes generated by Oracle Database's Object Type Translator (OTT). The optional TDO argument for INDICATOR, and for composite objects, in general, has the C datatype, OCIType *.

OCICOLL for REF and collection arguments is optional and only exists for the sake of completeness. You cannot map REFs or collections onto any other datatype and vice versa.

BY VALUE/REFERENCE for IN and IN OUT Parameter Modes

If you specify BY VALUE, then scalar IN and RETURN arguments are passed by value (which is also the default). Alternatively, you may have them passed by reference by specifying BY REFERENCE.

By default, or if you specify BY REFERENCE, then scalar IN OUT, and OUT arguments are passed by reference. Specifying BY VALUE for IN OUT, and OUT arguments is not supported for C. The usefulness of the BY REFERENCE/VALUE clause is restricted to external datatypes that are, by default, passed by value. This is true for IN, and RETURN arguments of the following external types:

[UNSIGNED] CHAR
[UNSIGNED] SHORT
[UNSIGNED] INT
[UNSIGNED] LONG
SIZE_T
SB1
SB2
SB4
UB1
UB2
UB4
FLOAT
DOUBLE

All IN and RETURN arguments of external types not on this list, all IN OUT arguments, and all OUT arguments are passed by reference.

The PARAMETERS Clause

Generally, the PL/SQL subprogram that publishes an external procedure declares a list of formal parameters, as the following example shows:

Note:

You may need to set up the following data structures for certain examples to work:
CREATE LIBRARY MathLib AS '/tmp/math.so';
CREATE OR REPLACE FUNCTION Interp_func (
/* Find the value of y at x degrees using Lagrange interpolation: */ 
   x    IN FLOAT, 
   y    IN FLOAT) 
RETURN FLOAT AS 
   LANGUAGE C
   NAME "Interp_func"
   LIBRARY MathLib;

Each formal parameter declaration specifies a name, parameter mode, and PL/SQL datatype (which maps to the default external datatype). That might be all the information the external procedure needs. If not, then you can provide more information using the PARAMETERS clause, which lets you specify the following:

  • Nondefault external datatypes

  • The current or maximum length of a parameter

  • NULL/NOT NULL indicators for parameters

  • Character set IDs and forms

  • The position of parameters in the list

  • How IN parameters are passed (by value or by reference)

If you decide to use the PARAMETERS clause, keep in mind:

  • For every formal parameter, there must be a corresponding parameter in the PARAMETERS clause.

  • If you include the WITH CONTEXT clause, then you must specify the parameter CONTEXT, which shows the position of the context pointer in the parameter list.

  • If the external procedure is a function, then you may specify the RETURN parameter, but it must be in the last position. If RETURN is not specified, the default external type is used.

Overriding Default Datatype Mapping

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.

Specifying Properties

You can also use the PARAMETERS clause to pass additional information about PL/SQL formal parameters and function results to an external procedure. Do this by specifying one or more of the following properties:

INDICATOR [{STRUCT | TDO}]
LENGTH
DURATION
MAXLEN
CHARSETID
CHARSETFORM
SELF

Table 14-3 shows the allowed and the default external datatypes, PL/SQL datatypes, and PL/SQL parameter modes allowed for a given property. Notice that MAXLEN (used to specify data returned from C back to PL/SQL) cannot be applied to an IN parameter.

Table 14-3 Properties and Datatypes

Property Allowed External Types (C) Default External Type (C) Allowed PL/SQL Types Allowed PL/SQL Modes Default PL/SQL Passing Method
INDICATOR
SHORT
SHORT
all scalars
IN
IN OUT
OUT
RETURN
BY VALUE
BY REFERENCE
BY REFERENCE
BY REFERENCE
LENGTH
[UNSIGNED] SHORT
[UNSIGNED] INT
[UNSIGNED] LONG
INT
CHAR
LONG RAW
RAW
VARCHAR2
IN
IN OUT
OUT
RETURN
BY VALUE
BY REFERENCE
BY REFERENCE
BY REFERENCE
MAXLEN
[UNSIGNED] SHORT
[UNSIGNED] INT
[UNSIGNED] LONG
INT
CHAR
LONG RAW
RAW
VARCHAR2
IN OUT
OUT
RETURN
BY REFERENCE
BY REFERENCE
BY REFERENCE
CHARSETID
CHARSETFORM
UNSIGNED SHORT
UNSIGNED INT
UNSIGNED LONG
UNSIGNED INT
CHAR
CLOB
VARCHAR2
IN
IN OUT
OUT
RETURN
BY VALUE
BY REFERENCE
BY REFERENCE
BY REFERENCE

In the following example, the PARAMETERS clause specifies properties for the PL/SQL formal parameters and function result:

CREATE OR REPLACE FUNCTION plsToCparse_func  (
   x   IN BINARY_INTEGER,
   Y   IN OUT CHAR) 
RETURN CHAR AS LANGUAGE C
   LIBRARY c_utils 
   NAME "C_parse" 
   PARAMETERS (
      x,            -- stores value of x
      x INDICATOR,  -- stores null status of x 
      y,            -- stores value of y
      y LENGTH,     -- stores current length of y
      y MAXLEN,     -- stores maximum length of y
      RETURN INDICATOR,
      RETURN);

With this PARAMETERS clause, the C prototype becomes:

char  *C_parse(x, x_ind, y, y_len, y_maxlen, retind)
int    x;
short  x_ind;
char  *y;
int   *y_len;
int   *y_maxlen;
short *retind;

A K&R prototype is needed because the indicator variable x_ind must be of datatype short and short should not be used in ISO/ANSI prototypes.

The additional parameters in the C prototype correspond to the INDICATOR (for x), LENGTH (of y), and MAXLEN (of y), as well as the INDICATOR for the function result in the PARAMETERS clause. The parameter RETURN corresponds to the C function identifier, which stores the result value.

INDICATOR

An INDICATOR is a parameter whose value indicates whether or not another parameter is NULL. PL/SQL does not need indicators, because the RDBMS concept of nullity is built into the language. However, an external procedure might need to know if a parameter or function result is NULL. Also, an external procedure might need to signal the server that a returned value is actually a NULL, and should be treated accordingly.

In such cases, you can use the property INDICATOR to associate an indicator with a formal parameter. If the PL/SQL subprogram is a function, then you can also associate an indicator with the function result, as shown earlier.

To check the value of an indicator, you can use the constants OCI_IND_NULL and OCI_IND_NOTNULL. If the indicator equals OCI_IND_NULL, then the associated parameter or function result is NULL. If the indicator equals OCI_IND_NOTNULL, then the parameter or function result is not NULL.

For IN parameters, which are inherently read-only, INDICATOR is passed by value (unless you specify BY REFERENCE) and is read-only (even if you specify BY REFERENCE). For OUT, IN OUT, and RETURN parameters, INDICATOR is passed by reference by default.

The INDICATOR can also have a STRUCT or TDO option. Because specifying INDICATOR as a property of an object is not supported, and because arguments of objects have complete indicator structs instead of INDICATOR scalars, you must specify this by using the STRUCT option. You must use the type descriptor object (TDO) option for composite objects and collections,

LENGTH and MAXLEN

In PL/SQL, there is no standard way to indicate the length of a RAW or string parameter. However, in many cases, you want to pass the length of such a parameter to and from an external procedure. Using the properties LENGTH and MAXLEN, you can specify parameters that store the current length and maximum length of a formal parameter.

Note:

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

For IN parameters, LENGTH is passed by value (unless you specify BY REFERENCE) and is read-only. For OUT, IN OUT, and RETURN parameters, LENGTH is passed by reference.

As mentioned earlier, MAXLEN does not apply to IN parameters. For OUT, IN OUT, and RETURN parameters, MAXLEN is passed by reference and is read-only.

CHARSETID and CHARSETFORM

Oracle Database provides globalization support, which lets you process single-byte and multibyte character data and convert between character sets. It also lets your applications run in different language environments.

By default, if the server and agent use the exact same $ORACLE_HOME value, the agent uses the same globalization support settings as the server (including any settings that have been specified with ALTER SESSION commands).

If the agent is running in a separate $ORACLE_HOME (even if the same location is specified by two different aliases or symbolic links), the agent uses the same globalization support settings as the server except for the character set; the default character set for the agent is defined by the NLS_LANG and NLS_NCHAR environment settings for the agent.

The properties CHARSETID and CHARSETFORM identify the nondefault character set from which the character data being passed was formed. With CHAR, CLOB, and VARCHAR2 parameters, you can use CHARSETID and CHARSETFORM to pass the character set ID and form to the external procedure.

For IN parameters, CHARSETID and CHARSETFORM are passed by value (unless you specify BY REFERENCE) and are read-only (even if you specify BY REFERENCE). For OUT, IN OUT, and RETURN parameters, CHARSETID and CHARSETFORM are passed by reference and are read-only.

The OCI attribute names for these properties are OCI_ATTR_CHARSET_ID and OCI_ATTR_CHARSET_FORM.

See Also:

Oracle Call Interface Programmer's Guide and the Oracle Database Globalization Support Guide for more information about using national language data with the OCI

Repositioning Parameters

Remember, each formal parameter of the external procedure must have a corresponding parameter in the PARAMETERS clause. Their positions can differ, because PL/SQL associates them by name, not by position. However, the PARAMETERS clause and the C prototype for the external procedure must have the same number of parameters, and they must be in the same order.

Using SELF

SELF is the always-present argument of an object type's member function or procedure, namely the object instance itself. In most cases, this argument is implicit and is not listed in the argument list of the PL/SQL procedure. However, SELF must be explicitly specified as an argument of the PARAMETERS clause.

For example, assume that a user wants to create a Person object, consisting of a person's name and date of birth, and then further a table of this object type. The user would eventually like to determine the age of each Person object in this table.

Note:

You may need to set up data structures similar to the following for certain examples to work:
CONNECT system/manager
GRANT CONNECT,RESOURCE,CREATE LIBRARY TO scott IDENTIFIED BY tiger;
CONNECT scott/tiger
CREATE OR REPLACE LIBRARY agelib UNTRUSTED IS 
  '/tmp/scott1.so';.

This example is only for Solaris; other libraries and include paths might be needed for other platforms.

In SQL*Plus, the Person object type can be created by:

CREATE OR REPLACE TYPE Person1_typ AS OBJECT
( Name      VARCHAR2(30),
  B_date    DATE,
  MEMBER FUNCTION calcAge_func RETURN NUMBER)
);

Normally, the member function would be implemented in PL/SQL, but for this example, we make it an external procedure. To realize this, the body of the member function is declared as follows:

CREATE OR REPLACE TYPE BODY Person1_typ AS 
  MEMBER FUNCTION calcAge_func RETURN NUMBER 
  AS LANGUAGE C
  NAME "age"
  LIBRARY agelib
  WITH CONTEXT
  PARAMETERS
  ( CONTEXT,
    SELF,
    SELF INDICATOR STRUCT,
    SELF TDO,
    RETURN INDICATOR
  );
END;

Notice that the calcAge_func member function does not take any arguments, but only returns a number. A member function is always invoked on an instance of the associated object type. The object instance itself always is an implicit argument of the member function. To refer to the implicit argument, the SELF keyword is used. This is incorporated into the external procedure syntax by supporting references to SELF in the parameters clause.

The matching table is created and populated.

CREATE TABLE Person_tab OF Person1_typ;

INSERT INTO Person_tab VALUES
   ('SCOTT', TO_DATE('14-MAY-85'));

INSERT INTO Person_tab VALUES
   ('TIGER', TO_DATE('22-DEC-71'));

Finally, we retrieve the information of interest from the table.

SELECT p.name, p.b_date, p.calcAge_func() FROM Person_tab p; 

NAME                           B_DATE    P.CALCAGE_ 
------------------------------ --------- ---------- 
SCOTT                          14-MAY-85          0 
TIGER                          22-DEC-71          0
 

The following is sample C code that implements the external member function and the Object-Type-Translator (OTT)-generated struct definitions:

#include <oci.h>

struct PERSON 
{ 
    OCIString   *NAME; 
    OCIDate      B_DATE; 
}; 
typedef struct PERSON PERSON; 
 
struct PERSON_ind 
{ 
    OCIInd    _atomic; 
    OCIInd    NAME; 
    OCIInd    B_DATE; 
}; 
typedef struct PERSON_ind PERSON_ind; 
 
OCINumber *age (ctx, person_obj, person_obj_ind, tdo, ret_ind) 
OCIExtProcContext *ctx; 
PERSON         *person_obj; 
PERSON_ind     *person_obj_ind; 
OCIType        *tdo; 
OCIInd         *ret_ind; 
{ 
    sword      err; 
    text       errbuf[512]; 
    OCIEnv    *envh; 
    OCISvcCtx *svch; 
    OCIError  *errh; 
    OCINumber *age; 
    int        inum = 0;
    sword      status;
  
    /* get OCI Environment */
    err = OCIExtProcGetEnv( ctx, &envh, &svch, &errh ); 

    /* initialize return age to 0 */
    age = (OCINumber *)OCIExtProcAllocCallMemory(ctx, sizeof(OCINumber));
    status = OCINumberFromInt(errh, &inum, sizeof(inum), OCI_NUMBER_SIGNED,
                              age);
    if (status != OCI_SUCCESS)
    {
      OCIExtProcRaiseExcp(ctx, (int)1476);
      return (age);
    }

    /* return NULL if the person object is null or the birthdate is null */
    if ( person_obj_ind->_atomic == OCI_IND_NULL || 
         person_obj_ind->B_DATE  == OCI_IND_NULL ) 
    { 
        *ret_ind = OCI_IND_NULL;
        return (age); 
    } 

    /* The actual implementation to calculate the age is left to the reader,
       but an easy way of doing this is a callback of the form: 
            select trunc(months_between(sysdate, person_obj->b_date) / 12) 
            from dual;   
    */ 
    *ret_ind = OCI_IND_NOTNULL;
    return (age);
} 

Passing Parameters by Reference

In C, you can pass IN scalar parameters by value (the value of the parameter is passed) or by reference (a pointer to the value is passed). When an external procedure expects a pointer to a scalar, specify BY REFERENCE phrase to pass the parameter by reference:

CREATE OR REPLACE PROCEDURE findRoot_proc (
   x IN DOUBLE PRECISION)
AS LANGUAGE C
   LIBRARY c_utils
   NAME "C_findRoot"
   PARAMETERS (
      x BY REFERENCE);

In this case, the C prototype would be:

void C_findRoot(double *x);

This is rather than the default, which would be used when there is no PARAMETERS clause:

void C_findRoot(double x);

WITH CONTEXT

By including the WITH CONTEXT clause, you can give an external procedure access to information about parameters, exceptions, memory allocation, and the user environment. The WITH CONTEXT clause specifies that a context pointer will be passed to the external procedure. For example, if you write the following PL/SQL function:

CREATE OR REPLACE FUNCTION getNum_func (
   x IN REAL) 
RETURN BINARY_INTEGER AS LANGUAGE C
   LIBRARY c_utils
   NAME "C_getNum"
   WITH CONTEXT
   PARAMETERS (
      CONTEXT,
      x BY REFERENCE,
      RETURN INDICATOR);

Then, the C prototype would be:

int C_getNum(
   OCIExtProcContext *with_context, 
   float *x, 
   short *retind);

The context data structure is opaque to the external procedure; but, is available to service procedures called by the external procedure.

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

Inter-Language Parameter Mode Mappings

PL/SQL supports the IN, IN OUT, and OUT parameter modes, as well as the RETURN clause for procedures returning values.

Executing External Procedures with the CALL Statement

Now that you have published your Java class method or external C procedure, you are ready to invoke it.

Do not call an external procedure directly. Instead, use the CALL statement to call the PL/SQL subprogram that published the external procedure. Refer to "CALL Statement Syntax".

Such calls, which you code in the same manner as a call to a regular PL/SQL procedure or function, can appear in the following:

  • Anonymous blocks

  • Standalone and packaged subprograms

  • Methods of an object type

  • Database triggers

  • SQL statements (calls to packaged functions only).

Any PL/SQL block or subprogram executing on the server side, or on the client side, (for example, in a tool such as Oracle Forms) can call an external procedure. On the server side, the external procedure runs in a separate process address space, which safeguards your database. Figure 14-2 shows how Oracle Database and external procedures interact.

Figure 14-2 Oracle Database and External Procedures

Description of Figure 14-2 follows
Description of "Figure 14-2 Oracle Database and External Procedures"

Preconditions for External Procedures

Before calling external procedures, you should consider the privileges, permissions, and synonyms that exist in the execution environment.

Privileges of External Procedures

When external procedures are called through CALL specifications, they execute with definer's privileges, rather than invoker's privileges.

A program executing with invoker's privileges is not bound to a particular schema. It executes at the calling site and accesses database items (such as tables and views) with the caller's visibility and permissions. However, a program executing with definer's privileges is bound to the schema in which it is defined. It executes at the defining site, in the definer's schema, and accesses database items with the definer's visibility and permissions.

Managing Permissions

Note:

You may need to set up the following data structures for certain examples to work:
CONNECT system/manager
GRANT CREATE ANY DIRECTORY to scott; 
CONNECT scott/tiger
CREATE OR REPLACE DIRECTORY bfile_dir AS '/tmp';
CREATE OR REPLACE JAVA RESOURCE NAMED "appImages" USING BFILE (bfile_dir,'bfile_audio');

To call external procedures, a user must have the EXECUTE privilege on the call specification and on any resources used by the procedure.

In SQL*Plus, you can use the GRANT and REVOKE data control statements to manage permissions. For example:

GRANT EXECUTE ON plsToJ_demoExternal_proc TO Public;
REVOKE EXECUTE ON plsToJ_demoExternal_proc FROM Public;
GRANT EXECUTE ON JAVA RESOURCE "appImages" TO Public;
GRANT EXECUTE ON plsToJ_demoExternal_proc TO Scott;
REVOKE EXECUTE ON plsToJ_demoExternal_proc FROM Scott;

Creating Synonyms for External Procedures

For convenience, you or your DBA can create synonyms for external procedures using the CREATE [PUBLIC] SYNONYM statement. In the following example, your DBA creates a public synonym, which is accessible to all users. If PUBLIC is not specified, then the synonym is private and accessible only within its schema.

CREATE PUBLIC SYNONYM Rfac FOR Scott.RecursiveFactorial;

CALL Statement Syntax

Invoke the external procedure by means of the SQL CALL statement. You can execute the CALL statement interactively from SQL*Plus. The syntax is:

CALL [schema.][{object_type_name | package_name}]procedure_name[@dblink_name]
   [(parameter_list)] [INTO :host_variable][INDICATOR][:indicator_variable];

This is essentially the same as executing a procedure myproc() using a SQL statement of the form "SELECT myproc(...) FROM dual," except that the overhead associated with performing the SELECT is not incurred.

For example, here is an anonymous PL/SQL block that uses dynamic SQL to call plsToC_demoExternal_proc, which we published. PL/SQL passes three parameters to the external C procedure C_demoExternal_proc.

DECLARE 
   xx NUMBER(4); 
   yy VARCHAR2(10); 
   zz DATE; 
 BEGIN 
    EXECUTE IMMEDIATE 'CALL plsToC_demoExternal_proc(:xxx, :yyy, :zzz)' USING xx,yy,zz; 
 END; 

The semantics of the CALL statement is identical to the that of an equivalent BEGIN..END block.

Note:

CALL is the only SQL statement that cannot be put, by itself, in a PL/SQL BEGIN...END block. It can be part of an EXECUTE IMMEDIATE statement within a BEGIN...END block.

Calling Java Class Methods

Here is how you would call the J_calcFactorial class method published earlier. First, declare and initialize two SQL*Plus host variables, as follows:

VARIABLE x NUMBER
VARIABLE y NUMBER
EXECUTE :x := 5;

Call J_calcFactorial:

CALL J_calcFactorial(:x) INTO :y;
PRINT y

The result:

Y
------
   120

How the Database Server Calls External C Procedures

To call an external C procedure, PL/SQL must find the path of the appropriate DLL. The PL/SQL engine retrieves the path from the data dictionary, based on the library alias from the AS LANGUAGE clause of the procedure declaration.

Next, PL/SQL alerts a Listener process which, in turn, spawns a session-specific agent. By default, this agent is named extproc, although you can specify other names in the listener.ora file. The Listener hands over the connection to the agent, and PL/SQL passes to the agent the name of the DLL, the name of the external procedure, and any parameters.

Then, the agent loads the DLL and runs the external procedure. Also, the agent handles service calls (such as raising an exception) and callbacks to Oracle Database. Finally, the agent passes to PL/SQL any values returned by the external procedure.

Note:

Although some DLL caching takes place, there is no guarantee that your DLL will remain in the cache; therefore, do not store global variables in your DLL.

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

Here, we call PL/SQL function plsCallsCdivisor_func, which we published previously, from an anonymous block. PL/SQL passes the two integer parameters to external function Cdivisor_func, which returns their greatest common divisor.

DECLARE
   g    BINARY_INTEGER;
   a    BINARY_INTEGER;
   b    BINARY_INTEGER;
CALL plsCallsCdivisor_func(a, b); 
IF g IN (2,4,8) THEN ... 

Handling Errors and Exceptions in Multi-Language Programs

The PL/SQL compiler raises compile time errors if an AS EXTERNAL call specification is found in a TYPE or PACKAGE specification.

C programs can raise exceptions through the OCIExtproc... functions.

Using Service Procedures with External C Procedures

When called from an external procedure, a service routine can raise exceptions, allocate memory, and invoke OCI handles for callbacks to the server. To use a service routine, you must specify the WITH CONTEXT clause, which lets you pass a context structure to the external procedure. The context structure is declared in header file ociextp.h as follows:

typedef struct OCIExtProcContext OCIExtProcContext;

Note:

ociextp.h is located in $ORACLE_HOME/plsql/public on UNIX.

OCIExtProcAllocCallMemory()

This service routine allocates n bytes of memory for the duration of the external procedure call. Any memory allocated by the function is freed automatically as soon as control returns to PL/SQL.

Note:

The external procedure does not need to (and should not) call the C function free() to free memory allocated by this service routine as this is handled automatically.

The C prototype for this function is as follows:

dvoid *OCIExtProcAllocCallMemory(
   OCIExtProcContext *with_context, 
   size_t amount);

The parameters with_context and amount are the context pointer and number of bytes to allocate, respectively. The function returns an untyped pointer to the allocated memory. A return value of zero indicates failure.

In SQL*Plus, suppose you publish external function plsToC_concat_func, as follows:

Note:

You may need to set up data structures similar to the following for certain examples to work:
CONNECT system/manager
DROP USER y CASCADE;
GRANT CONNECT,RESOURCE,CREATE LIBRARY TO y IDENTIFIED BY y;
CONNECT y/y
CREATE LIBRARY stringlib AS  '/private/varora/ilmswork/Cexamples/john2.so';
CREATE OR REPLACE FUNCTION plsToC_concat_func ( 
   str1 IN VARCHAR2,  
   str2 IN VARCHAR2)  
RETURN VARCHAR2 AS LANGUAGE C 
NAME "concat" 
LIBRARY stringlib 
WITH CONTEXT 
PARAMETERS ( 
CONTEXT,  
str1   STRING,  
str1   INDICATOR short,  
str2   STRING,  
str2   INDICATOR short,  
RETURN INDICATOR short,  
RETURN LENGTH short,  
RETURN STRING); 

When called, C_concat concatenates two strings, then returns the result:

select plsToC_concat_func('hello ', 'world') from dual; 
PLSTOC_CONCAT_FUNC('HELLO','WORLD') 
-----------------------------------------------------------------------------
hello world

If either string is NULL, the result is also NULL. As the following example shows, C_concat uses OCIExtProcAllocCallMemory() to allocate memory for the result string:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <oci.h>
#include <ociextp.h>

char *concat(ctx, str1, str1_i, str2, str2_i, ret_i, ret_l) 
OCIExtProcContext *ctx; 
char   *str1; 
short  str1_i; 
char   *str2; 
short  str2_i; 
short  *ret_i; 
short  *ret_l; 
{ 
  char *tmp; 
  short len; 
  /* Check for null inputs. */ 
  if ((str1_i == OCI_IND_NULL) || (str2_i == OCI_IND_NULL)) 
  { 
      *ret_i = (short)OCI_IND_NULL; 
      /* PL/SQL has no notion of a NULL ptr, so return a zero-byte string. */  
      tmp = OCIExtProcAllocCallMemory(ctx, 1);  
      tmp[0] = '\0';  
      return(tmp);  
  } 
  /* Allocate memory for result string, including NULL terminator. */ 
  len = strlen(str1) + strlen(str2); 
  tmp = OCIExtProcAllocCallMemory(ctx, len + 1); 
 
  strcpy(tmp, str1); 
  strcat(tmp, str2); 
 
  /* Set NULL indicator and length. */ 
  *ret_i = (short)OCI_IND_NOTNULL; 
  *ret_l = len; 
  /* Return pointer, which PL/SQL frees later. */ 
  return(tmp); 
} 

#ifdef LATER
static void checkerr (/*_ OCIError *errhp, sword status _*/);

void checkerr(errhp, status)
OCIError *errhp;
sword status;
{
  text errbuf[512];
  sb4 errcode = 0;

  switch (status)
  {
  case OCI_SUCCESS:
    break;
  case OCI_SUCCESS_WITH_INFO:
    (void) printf("Error - OCI_SUCCESS_WITH_INFO\n");
    break;
  case OCI_NEED_DATA:
    (void) printf("Error - OCI_NEED_DATA\n");
    break;
  case OCI_NO_DATA:
    (void) printf("Error - OCI_NODATA\n");
    break;
  case OCI_ERROR:
    (void) OCIErrorGet((dvoid *)errhp, (ub4) 1, (text *) NULL, &errcode,
                        errbuf, (ub4) sizeof(errbuf), OCI_HTYPE_ERROR);
    (void) printf("Error - %.*s\n", 512, errbuf);
    break;
  case OCI_INVALID_HANDLE:
    (void) printf("Error - OCI_INVALID_HANDLE\n");
    break;
  case OCI_STILL_EXECUTING:
    (void) printf("Error - OCI_STILL_EXECUTE\n");
    break;
  case OCI_CONTINUE:
    (void) printf("Error - OCI_CONTINUE\n");
    break;
  default:
    break;
  }
}

char *concat(ctx, str1, str1_i, str2, str2_i, ret_i, ret_l)
OCIExtProcContext *ctx;
char   *str1;
short  str1_i;
char   *str2;
short  str2_i;
short  *ret_i;
short  *ret_l;
{
  char *tmp;
  short len;
  /* Check for null inputs. */
  if ((str1_i == OCI_IND_NULL) || (str2_i == OCI_IND_NULL))
  {
      *ret_i = (short)OCI_IND_NULL;
      /* PL/SQL has no notion of a NULL ptr, so return a zero-byte string. */ 
      tmp = OCIExtProcAllocCallMemory(ctx, 1); 
      tmp[0] = '\0'; 
      return(tmp); 
  }
  /* Allocate memory for result string, including NULL terminator. */
  len = strlen(str1) + strlen(str2);
  tmp = OCIExtProcAllocCallMemory(ctx, len + 1);

  strcpy(tmp, str1);
  strcat(tmp, str2);

  /* Set NULL indicator and length. */
  *ret_i = (short)OCI_IND_NOTNULL;
  *ret_l = len;
  /* Return pointer, which PL/SQL frees later. */
  return(tmp);
}

/*======================================================================*/
int main(char *argv, int argc)
{
  OCIExtProcContext *ctx;
  char           *str1;
  short          str1_i;
  char           *str2;
  short          str2_i;
  short          *ret_i;
  short          *ret_l;
  /* OCI Handles */
  OCIEnv        *envhp;
  OCIServer     *srvhp;
  OCISvcCtx     *svchp;
  OCIError      *errhp;
  OCISession    *authp;
  OCIStmt       *stmthp;
  OCILobLocator *clob, *blob;
  OCILobLocator *Lob_loc;

  /* Initialize and Logon */
  (void) OCIInitialize((ub4) OCI_DEFAULT, (dvoid *)0,
                       (dvoid * (*)(dvoid *, size_t)) 0,
                       (dvoid * (*)(dvoid *, dvoid *, size_t))0,
                       (void (*)(dvoid *, dvoid *)) 0 );

  (void) OCIEnvInit( (OCIEnv **) &envhp, 
                    OCI_DEFAULT, (size_t) 0, 
                    (dvoid **) 0 );

  (void) OCIHandleAlloc( (dvoid *) envhp, (dvoid **) &errhp, OCI_HTYPE_ERROR, 
                   (size_t) 0, (dvoid **) 0);

  /* Server contexts */
  (void) OCIHandleAlloc( (dvoid *) envhp, (dvoid **) &srvhp, OCI_HTYPE_SERVER,
                   (size_t) 0, (dvoid **) 0);

  /* Service context */
  (void) OCIHandleAlloc( (dvoid *) envhp, (dvoid **) &svchp, OCI_HTYPE_SVCCTX,
                   (size_t) 0, (dvoid **) 0);

  /* Attach to Oracle Database */
  (void) OCIServerAttach( srvhp, errhp, (text *)"", strlen(""), 0);

  /* Set attribute server context in the service context */
  (void) OCIAttrSet ((dvoid *) svchp, OCI_HTYPE_SVCCTX, 
                     (dvoid *)srvhp, (ub4) 0,
                    OCI_ATTR_SERVER, (OCIError *) errhp);

  (void) OCIHandleAlloc((dvoid *) envhp, 
                        (dvoid **)&authp, (ub4) OCI_HTYPE_SESSION,
                        (size_t) 0, (dvoid **) 0);
 
  (void) OCIAttrSet((dvoid *) authp, (ub4) OCI_HTYPE_SESSION,
                 (dvoid *) "samp", (ub4)4,
                 (ub4) OCI_ATTR_USERNAME, errhp);
 
  (void) OCIAttrSet((dvoid *) authp, (ub4) OCI_HTYPE_SESSION,
                 (dvoid *) "samp", (ub4) 4,
                 (ub4) OCI_ATTR_PASSWORD, errhp);

  /* Begin a User Session */
  checkerr(errhp, OCISessionBegin ( svchp,  errhp, authp, OCI_CRED_RDBMS, 
                          (ub4) OCI_DEFAULT));

  (void) OCIAttrSet((dvoid *) svchp, (ub4) OCI_HTYPE_SVCCTX,
                   (dvoid *) authp, (ub4) 0,
                   (ub4) OCI_ATTR_SESSION, errhp);

  /* -----------------------User Logged In------------------------------*/
  printf ("user logged in \n");

  /* allocate a statement handle */
  checkerr(errhp, OCIHandleAlloc( (dvoid *) envhp, (dvoid **) &stmthp,
           OCI_HTYPE_STMT, (size_t) 0, (dvoid **) 0));

  checkerr(errhp, OCIDescriptorAlloc((dvoid *)envhp, (dvoid **) &Lob_loc, 
                                     (ub4) OCI_DTYPE_LOB, 
                                     (size_t) 0, (dvoid **) 0)); 

  /* ------- subroutine called  here-----------------------*/ 
  printf ("calling concat...\n");
  concat(ctx, str1, str1_i, str2, str2_i, ret_i, ret_l);

  return 0;
}

#endif

OCIExtProcRaiseExcp

This service routine raises a predefined exception, which must have a valid Oracle Database error number in the range 1..32767. After doing any necessary cleanup, your external procedure must return immediately. (No values are assigned to OUT or IN OUT parameters.) The C prototype for this function follows:

int OCIExtProcRaiseExcp(
   OCIExtProcContext *with_context, 
   size_t errnum);

The parameters with_context and error_number are the context pointer and Oracle Database error number. The return values OCIEXTPROC_SUCCESS and OCIEXTPROC_ERROR indicate success or failure.

In SQL*Plus, suppose you publish external procedure plsTo_divide_proc, as follows:

CREATE OR REPLACE PROCEDURE plsTo_divide_proc (
   dividend IN BINARY_INTEGER, 
   divisor  IN BINARY_INTEGER, 
   result   OUT FLOAT) 
AS LANGUAGE C
   NAME "C_divide"
   LIBRARY MathLib
   WITH CONTEXT
   PARAMETERS (
      CONTEXT, 
      dividend INT, 
      divisor  INT, 
      result   FLOAT);

When called, C_divide finds the quotient of two numbers. As the following example shows, if the divisor is zero, C_divide uses OCIExtProcRaiseExcp() to raise the predefined exception ZERO_DIVIDE:

void C_divide (ctx, dividend, divisor, result)
OCIExtProcContext *ctx;
int    dividend;
int    divisor;
float  *result;
{
  /* Check for zero divisor. */
  if (divisor == (int)0) 
  {
    /* Raise exception ZERO_DIVIDE, which is Oracle error 1476. */
    if (OCIExtProcRaiseExcp(ctx, (int)1476) == OCIEXTPROC_SUCCESS)
    {
      return;
    }
    else
    {
      /* Incorrect parameters were passed. */
      assert(0);
    }
  }
  *result = (float)dividend / (float)divisor;
}

OCIExtProcRaiseExcpWithMsg

This service routine raises a user-defined exception and returns a user-defined error message. The C prototype for this function follows:

int OCIExtProcRaiseExcpWithMsg(
   OCIExtProcContext *with_context, 
   size_t  error_number,
   text   *error_message, 
   size_t  len);

The parameters with_context, error_number, and error_message are the context pointer, Oracle Database error number, and error message text. The parameter len stores the length of the error message. If the message is a null-terminated string, then len is zero. The return values OCIEXTPROC_SUCCESS and OCIEXTPROC_ERROR indicate success or failure.

In the previous example, we published external procedure plsTo_divide_proc. In the following example, you use a different implementation. With this version, if the divisor is zero, then C_divide uses OCIExtProcRaiseExcpWithMsg() to raise a user-defined exception:

void C_divide (ctx, dividend, divisor, result)
OCIExtProcContext *ctx;
int    dividend;
int    divisor;
float  *result;
  /* Check for zero divisor. */
  if (divisor == (int)0) 
  {
    /* Raise a user-defined exception, which is Oracle error 20100,
       and return a null-terminated error message. */
    if (OCIExtProcRaiseExcpWithMsg(ctx, (int)20100, 
          "divisor is zero", 0) == OCIEXTPROC_SUCCESS)
    {
      return;
    }
    else
    {
      /*  Incorrect parameters were passed. */
      assert(0);
    }
  }
  *result = dividend / divisor;

}

Doing Callbacks with External C Procedures

Here is a function that is used to enable callbacks.

OCIExtProcGetEnv()

This service routine enables OCI callbacks to the database during an external procedure call. The environment handles obtained by using this function reuse the existing connection to go back to the database. If you need to establish a new connection to the database, you cannot use these handles; instead, you must create your own.

The C prototype for this function follows:

sword OCIExtProcGetEnv ( OCIExtProcContext *with_context,
   OCIEnv envh,
   OCISvcCtx svch,
   OCIError errh )

The parameter with_context is the context pointer, and the parameters envh, svch, and errh are the OCI environment, service, and error handles, respectively. The return values OCIEXTPROC_SUCCESS and OCIEXTPROC_ERROR indicate success or failure.

Both external C procedures and Java class methods can call-back to the database to do SQL operations. For a working example, refer to "Demo Program" .

Note:

Callbacks are not necessarily a same-session phenomenon; you may execute an SQL statement in a different session through OCIlogon.

An external C procedure executing on Oracle Database can call a service routine to obtain 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. Callbacks and external procedures operate in the same user session and transaction context, and so have the same user privileges.

In SQL*Plus, suppose you run the following script:

CREATE TABLE Emp_tab (empno NUMBER(10))

CREATE PROCEDURE plsToC_insertIntoEmpTab_proc (
   empno BINARY_INTEGER)
AS LANGUAGE C
   NAME "C_insertEmpTab"
   LIBRARY insert_lib
   WITH CONTEXT
   PARAMETERS (
      CONTEXT, 
      empno LONG);

Later, you might call service routine OCIExtProcGetEnv() from external procedure plsToC_insertIntoEmpTab_proc(), as follows:

#include <stdio.h>
#include <stdlib.h>
#include <oratypes.h>
#include <oci.h>   /* includes ociextp.h */
...
void C_insertIntoEmpTab (ctx, empno) 
OCIExtProcContext *ctx; 
long empno; 
{ 
  OCIEnv    *envhp; 
  OCISvcCtx *svchp; 
  OCIError  *errhp; 
  int        err; 
  ... 
  err = OCIExtProcGetEnv(ctx, &envhp, &svchp, &errhp); 
  ... 
}

If you do not use callbacks, you do not need to include oci.h; instead, just include ociextp.h.

Object Support for OCI Callbacks

To execute object-related callbacks from your external procedures, the OCI environment in the extproc agent is fully initialized in object mode. You retrieve handles to this environment with the OCIExtProcGetEnv() procedure.

The object runtime environment lets you use static, as well as dynamic, object support provided by OCI. To utilize static support, use the OTT to generate C structs for the appropriate object types, and then use conventional C code to access the object attributes.

For those objects whose types are unknown at external procedure creation time, an alternative, dynamic, way of accessing objects is first to invoke OCIDescribeAny() to obtain attribute and method information about the type. Then, OCIObjectGetAttr() and OCIObjectSetAttr() can be called to retrieve and set attribute values.

Because the current external procedure model is stateless, OCIExtProcGetEnv() must be called in every external procedure that wants to execute callbacks, or invoke OCIExtProc...() service routines. After every external procedure invocation, the callback mechanism is cleaned up and all OCI handles are freed.

Restrictions on Callbacks

With callbacks, the following SQL commands and OCI procedures are not supported:

  • Transaction control commands such as COMMIT

  • Data definition commands such as CREATE

  • The following object-oriented OCI procedures:

    OCIObjectNew 
    OCIObjectPin 
    OCIObjectUnpin 
    OCIObjectPinCountReset 
    OCIObjectLock 
    OCIObjectMarkUpdate 
    OCIObjectUnmark 
    OCIObjectUnmarkByRef 
    OCIObjectAlwaysLatest 
    OCIObjectNotAlwaysLatest 
    OCIObjectMarkDeleteByRef 
    OCIObjectMarkDelete 
    OCIObjectFlush 
    OCIObjectFlushRefresh 
    OCIObjectGetTypeRef 
    OCIObjectGetObjectRef 
    OCIObjectExists 
    OCIObjectIsLocked 
    OCIObjectIsDirtied 
    OCIObjectIsLoaded 
    OCIObjectRefresh 
    OCIObjectPinTable 
    OCIObjectArrayPin 
    OCICacheFlush, 
    OCICacheFlushRefresh, 
    OCICacheRefresh 
    OCICacheUnpin 
    OCICacheFree 
    OCICacheUnmark 
    OCICacheGetObjects 
    OCICacheRegister 
    
  • Polling-mode OCI procedures such as OCIGetPieceInfo

  • The following OCI procedures:

    OCIEnvInit
    OCIInitialize
    OCIPasswordChange
    OCIServerAttach
    OCIServerDetach
    OCISessionBegin
    OCISessionEnd
    OCISvcCtxToLda
    OCITransCommit
    OCITransDetach
    OCITransRollback
    OCITransStart
    

Also, with OCI procedure OCIHandleAlloc, the following handle types are not supported:

OCI_HTYPE_SERVER 
OCI_HTYPE_SESSION 
OCI_HTYPE_SVCCTX 
OCI_HTYPE_TRANS

Debugging External Procedures

Usually, when an external procedure fails, its prototype is faulty. In other words, 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:

lost RPC connection to external routine agent 

This 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 the preceding tables.

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. (For the location of the directory, refer to your Oracle Database Installation or User's Guide.)

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

Note:

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

Demo Program

Also in the PL/SQL demo directory is the script extproc.sql, which demonstrates the calling of an external procedure. The companion file extproc.c contains the C source code for the external procedure.

To run the demo, follow the instructions in extproc.sql. You must use the SCOTT/TIGER account, which must have CREATE LIBRARY privileges.

Guidelines for External C Procedures

Handling Global Variables

A global variable is declared outside of a function, and its value is shared by all functions of a program. In case of external procedures, this means that all functions in a DLL share the value of the global. The usage of global variables is discouraged for two reasons:

  • Threading: In the non-threaded configuration of the agent process, there is only one function active at a time. However, in the case of multithreaded agents, multiple functions can be active at the same time. In that case, it is possible that two or more functions would try to access the global variable concurrently, with unsuccessful results.

  • DLL caching: Global variables are also used to store data that is intended to persist beyond the lifetime of a function. For example, consider two functions func1() and func2() trying to pass data to each other. Because of the DLL caching feature, it is possible that after func1()'s completion, the DLL will be unloaded, which results in all global variables losing their values. When func2() is executed, the DLL is reloaded, and all globals are initialized to 0, which will be inconsistent with their values at the completion of func1().

Handling Static Variables

There are two types of static variables: external and internal. An external static variable is a special case of a global variable, so its usage is discouraged. Internal static variables are local to a particular function, but remain in existence rather than coming and going each time the function is activated. Therefore, they provide private, permanent storage within a single function. These variables are used to pass on data to subsequent invocations of the same function. But, because of the DLL caching feature mentioned previously, the DLL might be unloaded and reloaded between invocations, which means that the internal static variable would lose its value.

See Also:

Template makefile in the RDBMS subdirectory /public for help creating a dynamic link library

Guidelines for Call Specifications and CALL Statements

When calling external procedures:

  • Never write to IN parameters or overflow the capacity of OUT parameters. (PL/SQL does no run time checks for these error conditions.)

  • Never read an OUT parameter or a function result.

  • 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, then you must specify the parameter CONTEXT, which shows the position of the context pointer in the parameter list.

  • If you include the PARAMETERS clause, and if the external procedure is a function, then you must specify the parameter RETURN 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.

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

Restrictions on External C Procedures

The following restrictions apply to external procedures:

  • This feature is available only on platforms that support DLLs.

  • Only C procedures and procedures callable from C code are supported.

  • You cannot pass PL/SQL cursor variables or records to an external procedure. For records, use instances of object types instead.

  • In the LIBRARY subclause, you cannot use a database link to specify a remote library.

  • The maximum number of parameters that you can pass to a external procedure is 128. However, if you pass float or double parameters by value, then the maximum is less than 128. How much less depends on the number of such parameters and your operating system. To get a rough estimate, count each float or double passed by value as two parameters.