2
OCI Programming Basics

This chapter introduces you to the basic concepts involved in programming with the OCI. This chapter covers the following topics:

• Overview of OCI Programming
• OCI Program Structure
• OCI Data Structures
• Handles
• Descriptors
• OCI Programming Steps
• OCI Environment Initialization
• Processing SQL Statements
• Commit or Rollback
• Terminating the Application
• Error Handling
• Additional Coding Guidelines
• Using PL/SQL in an OCI Program
• OCI Globalization Support
• OCI Database Globalization Support Functions

Overview of OCI Programming

This chapter provides an introduction to the concepts and procedures involved in developing an OCI application. After reading this chapter, you should have most of the tools necessary to understand and create a basic OCI application.

This chapter is broken down into the following major sections:

• OCI Program Structure - covers the basic overall structure of an OCI application, including the major steps involved in creating one.
• OCI Data Structures - discusses handles, and descriptors.
• OCI Programming Steps - discusses in detail each of the steps involved in coding an OCI application.
• Error Handling - covers error handling in OCI applications.
• Additional Coding Guidelines - provides useful information to keep in mind when coding an OCI application.
• Nonblocking Mode - this section covers the use of nonblocking mode to connect to an Oracle database server.
• Using PL/SQL in an OCI Program - discusses some important points to keep in mind when working with PL/SQL in an OCI application.

New users should pay particular attention to the information presented in this chapter, because it forms the basis for the rest of the material presented in this guide. The information in this chapter is supplemented by information in later chapters.

OCI Program Structure

The general goal of an OCI application is to operate on behalf of multiple users. In an n-tiered configuration, multiple users are sending HTTP requests to the client application. The client application may need to perform some data operations that include exchanging data and performing data processing.

The OCI uses the following basic program structure:

1. Initialize the OCI programming environment and threads.
2. Allocate necessary handles, and establish server connections and user sessions.
3. Exchange data with the database server by executing SQL statements on the server, and perform necessary application data processing.
4. Reexecute prepared statements, or prepare a new statement for execution.
5. Terminate user sessions and server connections.
6. Free handles.

Figure 2-1, "Basic OCI Program Flow" illustrates the flow of steps in an OCI application. Each step is described in more detail in the section "OCI Programming Steps".

Figure 2-1 Basic OCI Program Flow

Keep in mind that the diagram and the list of steps present a simple generalization of OCI programming steps. Variations are possible, depending on the functionality of the program. OCI applications that include more sophisticated functionality, such as managing multiple sessions and transactions and using objects, require additional steps.

All OCI function calls are executed in the context of an environment. There can be multiple environments within an OCI process, as illustrated in Figure 2-2, "Multiple Environments Within an OCI Process". If an environment requires any process-level initialization then it is performed automatically.

Figure 2-2 Multiple Environments Within an OCI Process

OCI Data Structures

Handles and descriptors are opaque data structures which are defined in OCI applications and may be allocated directly, through specific allocate calls, or may be implicitly allocated by OCI functions.

Handles and descriptors store information pertaining to data, connections, or application behavior. Handles are defined in more detail in the following section. Descriptors are discussed in the section "Descriptors".

Handles

Almost all OCI calls include in their parameter list one or more handles. A handle is an opaque pointer to a storage area allocated by the OCI library. You use a handle to store context or connection information, (for example, an environment or service context handle), or it may store information about OCI functions or data (for example, an error or describe handle). Handles can make programming easier, because the library, rather than the application, maintains this data.

Most OCI applications need to access the information stored in handles. The get and set attribute OCI calls, OCIAttrGet() and OCIAttrSet(), access this information.

The following table lists the handles defined for the OCI. For each handle type, the C datatype and handle type constant used to identify the handle type in OCI calls are listed.

Table 2-1 OCI Handle Types  

Allocating and Freeing Handles

Your application allocates all handles (except the bind, define, and thread handles) with respect to particular environment handle. You pass the environment handle as one of the parameters to the handle allocation call. The allocated handles is then specific to that particular environment.

The bind and define handles are allocated with respect to a statement handle, and contain information about the statement represented by that handle.

Figure 2-3, "Hierarchy of Handles" illustrates the relationship between the various types of handles.

All user-allocated handles are allocated using the OCI handle allocation call, OCIHandleAlloc().

The thread handle is allocated with the OCIThreadHndInit() call.

An application must free all handles when they are no longer needed. The OCIHandleFree() function frees handles.

Figure 2-3 Hierarchy of Handles

Handles lessen the need for global variables. Handles also make error reporting easier. An error handle is used to return errors and diagnostic information.

The various handle types are described in more detail in the following sections.

Environment Handle

The environment handle defines a context in which all OCI functions are invoked. Each environment handle contains a memory cache, which allows for fast memory access. All memory allocation under the environment handle is done from this cache. Access to the cache is serialized if multiple threads try to allocate memory under the same environment handle. When multiple threads share a single environment handle, they may block on access to the cache.

The environment handle is passed as the parent parameter to the OCIHandleAlloc() call to allocate all other handle types. Bind and define handles are allocated implicitly.

Error Handle

The error handle is passed as a parameter to most OCI calls. The error handle maintains information about errors that occur during an OCI operation. If an error occurs in a call, the error handle can be passed to OCIErrorGet() to obtain additional information about the error that occurred.

Allocating the error handle is one of the first steps in an OCI application because most OCI calls require an error handle as one of its parameters.

Service Context and Associated Handles

A service context handle defines attributes that determine the operational context for OCI calls to a server. The service context contains three handles as its attributes, that represent a server connection, a user session, and a transaction. These attributes are illustrated in Figure 2-4, "Components of a Service Context":

Figure 2-4 Components of a Service Context

• A server handle identifies a connection to a database. It translates into a physical connection in a connection-oriented transport mechanism.
• A user session handle defines a user's roles and privileges (also known as the user's security domain), and the operational context on which the calls execute.
• A transaction handle defines the transaction in which the SQL operations are performed. The transaction context includes user session state information, including the fetch state and package instantiation, if any.

Breaking the service context down in this way provides scalability and enables programmers to create sophisticated three-tiered applications and transaction processing (TP) monitors to execute requests on behalf of multiple users on multiple application servers and different transaction contexts.

You must allocate and initialize the service context handle with OCIHandleAlloc() or OCILogon() before you can use it. The service context handle is allocated explicitly by OCIHandleAlloc(). It can be initialized using OCIAttrSet() with the server, session, and transaction handle. If the service context handle is allocated implicitly using OCILogon(), it is already initialized.

Applications maintaining only a single user session for each database connection at any time can call OCILogon() to get an initialized service context handle.

In applications requiring more complex session management, the service context must be explicitly allocated, and the server handle and user session handle must be explicitly set into the service context. OCIServerAttach() and OCISessionBegin(), calls initialize the server and user session handle respectively.

An application will only define a transaction explicitly if it is a global transaction or there are multiple transactions active for sessions. It will be able to work correctly with the implicit transaction created automatically by OCI when the application makes changes to the database.

Statement Handle, Bind Handle, and Define Handle

A statement handle is the context that identifies a SQL or PL/SQL statement and its associated attributes.

Figure 2-5 Statement Handles

Information about input and output bind variables is stored in bind handles. The OCI library allocates a bind handle for each placeholder bound with the OCIBindByName() or OCIBindByPos() function. The user does not need to allocate bind handles. They are implicitly allocated by the bind call.

Fetched data returned by a query (select statement) is converted and retrieved according to the specifications of the define handles. The OCI library allocates a define handle for each output variable defined with OCIDefineByPos(). The user does not need to allocate define handles. They are implicitly allocated by the define call.

Bind and define handles are freed when the statement handle is freed or when a new statement is prepared on the statement handle.

Statement context data, the data associated with a statement handle, can be shared.

Describe Handle

The describe handle is used by the OCI describe call, OCIDescribeAny(). This call obtains information about schema objects in a database (for example, functions, procedures). The call takes a describe handle as one of its parameters, along with information about the object being described. When the call completes, the describe handle is populated with information about the object. The OCI application can then obtain describe information through the attributes of parameter descriptors.

Complex Object Retrieval Handle

The complex object retrieval (COR) handle is used by some OCI applications that work with objects in an Oracle database server. This handle contains COR descriptors, which provide instructions about retrieving objects referenced by another object.

Thread Handle

For information about the thread handle, which is used in multithreaded applications, refer to "The OCIThread Package".

Subscription Handle

The subscription handle is used by an OCI client application that is interested in registering for subscriptions to receive notifications of database events or events in the AQ namespace. The subscription handle encapsulates all information related to a registration from a client.

Direct Path Handles

The direct path handles are necessary for an OCI application that utilizes the direct path load engine in the Oracle database server. The direct path load interface allows the application to access the direct block formatter of the Oracle server.

Figure 2-6 Direct Path Handles

Process Handle

The process handle is a specialized handle for OCI applications that utilize shared data structures mode to set global parameters.

Connection Pool Handle

The connection pool handle is used for applications that pool physical connections into virtual connections, by calling specific OCI functions.

Handle Attributes

All OCI handles have attributes associated with them. These attributes represent data stored in that handle. You can read handle attributes using the attribute get call, OCIAttrGet(), and you can change them with the attribute set call, OCIAttrSet().

For example, the following statements set the username in the session handle by writing to the OCI_ATTR_USERNAME attribute:

text username[] = "scott";
err = OCIAttrSet ((dvoid*) mysessp, OCI_HTYPE_SESSION, (dvoid*) username, 
      (ub4) strlen(username), OCI_ATTR_USERNAME,
      (OCIError *) myerrhp);

Some OCI functions require that particular handle attributes be set before the function is called. For example, when OCISessionBegin() is called to establish a user's login session, the username and password must be set in the user session handle before the call is made.

Other OCI functions provide useful return data in handle attributes after the function completes. For example, when OCIStmtExecute() is called to execute a SQL query, describe information relating to the select-list items is returned in the statement handle.

ub4 parmcnt; 
/* get the number of columns in the select list */ 
err = OCIAttrGet ((dvoid *)stmhp, (ub4)OCI_HTYPE_STMT, (dvoid *) 
         &parmcnt, (ub4 *) 0, (ub4)OCI_ATTR_PARAM_COUNT, errhp); 

User Memory Allocation

The OCIEnvCreate() call, which initializes the environment handle, and the generic handle allocation (OCIHandleAlloc()) and descriptor allocation (OCIDescriptorAlloc()) calls have an xtramem_sz parameter in their parameter list. This parameter is used to specify memory chunk size which is allocated along with that handle for the user. This memory is not used by OCI and is for use by the application only.

Typically, an application uses this parameter to allocate an application-defined structure, such as for an application bookkeeping or storing context information, that has the same lifetime as the handle.

Using the xtramem_sz parameter means that the application does not need to explicitly allocate and deallocate memory as each handle is allocated and deallocated. The memory is allocated along with the handle, and freeing the handle frees up the user's data structures as well.

Descriptors

OCI descriptors and locators are opaque data structures that maintain data-specific information. The following table lists them, along with their C datatype, and the OCI type constant that allocates a descriptor of that type in a call to OCIDescriptorAlloc(). The OCIDescriptorFree() function frees descriptors and locators.

Table 2-2 Descriptor Types  

The main purpose of each descriptor type is listed here, and each descriptor type is described in the following sections:

• OCISnapshot - used in statement execution
• OCILOBLocator - used for LOB (OCI_DTYPE_LOB) or FILE (OCI_DTYPE_FILE) calls
• OCIParam - used in describe calls
• OCIRowid - used for binding or defining ROWID values
• OCIDateTime and OCIInterval - used for datetime and interval datatypes
• OCIComplexObjectComp - used for complex object retrieval
• OCIAQEnqOptions, OCIAQDeqOptions, OCIAQMsgProperties, OCIAQAgent - used for Advanced Queuing
• OCIAQNotify - used for publish-subscribe notification
• OCIServerDNs - used for LDAP-based publish-subscribe notification

Snapshot Descriptor

The snapshot descriptor is an optional parameter to the execute call, OCIStmtExecute(). It indicates that a query is being executed against a particular database snapshot. A database snapshot represents the state of a database at a particular point in time.

You allocate a snapshot descriptor with a call to OCIDescriptorAlloc(), by passing OCI_DTYPE_SNAP as the type parameter.

LOB/FILE Datatype Locator

A LOB (large object) is an Oracle datatype that can hold up to 4 gigabytes of binary (BLOB) or character (CLOB) data. In the database, an opaque data structure called a LOB locator is stored in a LOB column of a database row, or in the place of a LOB attribute of an object. The locator serves as a pointer to the actual LOB value, which is stored in a separate location.

The OCI LOB locator is used to perform OCI operations against a LOB (BLOB or CLOB) or FILE (BFILE). OCILob* functions take the LOB locator as a parameter instead of the LOB value. OCI LOB functions do not take actual LOB data as parameters. These functions take the LOB locators as parameters and operate on the LOB data referenced by these locators.

Hence, the old long interface can operate on the actual LOB value. This descriptor--OCILobLocator--is also used for operations on FILEs.

The LOB locator is allocated with a call to OCIDescriptorAlloc(), by passing OCI_DTYPE_LOB as the type parameter for BLOBs or CLOBs, and OCI_DTYPE_FILE for BFILEs.

An OCI application can retrieve a LOB locator from the server by issuing a SQL statement containing a LOB column or attribute as an element in the select list. In this case, the application would first allocate the LOB locator and then use it to define an output variable. Similarly, a LOB locator can be used as part of a bind operation to create an association between a LOB and a placeholder in a SQL statement.

The LOB locator datatype (OCILobLocator) is not a valid datatype when connected to an Oracle7 Server.

Parameter Descriptor

OCI applications use parameter descriptors to obtain information about select-list columns or schema objects. This information is obtained through a describe operation.

The parameter descriptor is the one descriptor type that is not allocated using OCIDescriptorAlloc(). You can obtain it only as an attribute of a describe, statement, or complex object retrieval handle by specifying the position of the parameter using an OCIParamGet() call.

ROWID Descriptor

The ROWID descriptor, OCIRowid, is used by applications that need to retrieve and use Oracle ROWIDs. The size and structure of the ROWID has changed from Oracle release 7 to Oracle release 8, and is opaque to the user. To work with a ROWID using OCI release 8 or later, an application can define a ROWID descriptor for a rowid position in a SQL select-list, and retrieve a ROWID into the descriptor. This same descriptor can later be bound to an input variable in an INSERT statement or WHERE clause.

ROWIDs are also redirected into descriptors using OCIAttrGet() on the statement handle following an execute.

Datetime and Interval Descriptors

These descriptors are used by applications which use the datetime or interval datatypes (OCIDateTime and OCIInterval). These descriptors can be used for binding and defining, and are passed as parameters to the functions OCIDescAlloc() and OCIDescFree() to allocate and free memory.

Complex Object Descriptor

For information about the complex object descriptor and its use, refer to "Complex Object Retrieval".

Advanced Queuing Descriptors

For information about Advanced Queuing and its related descriptors, refer to "OCI and Advanced Queuing".

LDAP-based Publish-Subscribe Notification

For information about LDAP-based publish-subscribe notification, see "Publish-Subscribe Registration Functions".

User Memory Allocation

The OCIDescriptorAlloc() call has an xtramem_sz parameter in its parameter list. This parameter is used to specify an amount of user memory which should be allocated along with a descriptor or locator.

Typically, an application uses this parameter to allocate an application-defined structure that has the same lifetime as the descriptor or locator. This structure maybe used for application bookkeeping or storing context information.

Using the xtramem_sz parameter means that the application does not need to explicitly allocate and deallocate memory as each descriptor or locator is allocated and deallocated. The memory is allocated along with the descriptor or locator, and freeing the descriptor or locator (with OCIDescriptorFree()) frees up the user's data structures as well.

The OCIHandleAlloc() call has a similar parameter for allocating user memory which has the same lifetime as the handle.

The OCIEnvCreate() and OCIEnvInit() calls have a similar parameter for allocating user memory which has the same lifetime as the environment handle.

OCI Programming Steps

Each of the steps that you perform in an OCI application is described in greater detail in the following sections. Some of the steps are optional. For example, you do not need to describe or define select-list items if the statement is not a query.

The following sections describe the steps that are required of an OCI application:

• OCI Environment Initialization.
• Processing SQL Statements.
• Commit or Rollback.
• Terminating the Application.
• Error Handling.

Application-specific processing will also occur in between any and all of the OCI function steps.

OCI Environment Initialization

This section describes how to initialize the OCI environment, establish a connection to a server, and authorize a user to perform actions against a database.

First, the three main steps in initializing the OCI environment are described in the following sections:

1. Creating the OCI environment.
2. Allocating Handles and Descriptors.
3. Initializing the Application, Connection, and Session.

Creating the OCI Environment

Each OCI function call is executed in the context of an environment that is created with the OCIEnvCreate() call. This call must be invoked before any other OCI call. The only exception is when setting a process-level attribute for the OCI shared mode.

The mode parameter of OCIEnvCreate() specifies whether the application calling the OCI library functions will:

• Run in a threaded environment (mode = OCI_THREADED).
• Use objects (mode = OCI_OBJECT).
• Use shared data structures (mode = OCI_SHARED).
• Use subscriptions (mode = OCI_EVENTS).

The mode can be set independently in each environment.

Initializing in object mode is necessary if the application will be binding and defining objects, or if the application will be using the OCI's object navigation calls. The program may also choose to use none of these features (mode = OCI_DEFAULT) or some combination of them, separating the options with a vertical bar. For example if mode = (OCI_THREADED | OCI_OBJECT), then the application runs in a threaded environment and use objects.

You can also specify user-defined memory management functions for each OCI environment.

Shared Data Mode

When a SQL statement is processed, certain underlying data is associated with the statement. This data includes information about statement text and bind data, as well as define and describe information for queries. For applications where the same set of SQL statements is executed on multiple instances of the application on the same host, the data can be shared.

When an OCI application is initialized in shared mode, common statement data is shared between multiple statement handles, thus providing memory savings for the application. This savings may be particularly valuable for applications which create multiple statement handles which execute the same SQL statement on different users' sessions but in the same schema, either on the same or multiple connections.

Without the shared mode feature, each execution of the query using an OCI statement handle requires its own memory for storing the metadata. The total amount of memory required is roughly equal to the number of statements being executed in all the processes combined multiplied by the memory required for each statement handle.

A large part of the common memory in a statement handle is shared among all the processes executing the same statement with the shared mode feature. The total amount of memory in all the processes combined is much less than in the previous case for the same number of processes. The memory requirement for each statement handle is much smaller than in the case where there is no sharing, as the number of such statements increases to a large number.

Shared data structure mode can be useful in the following scenarios:

• When several instances of the same application are running on the same machine to service multiple clients. Each of these instances may be executing identical SQL statements, differentiated by different bind values.
• When an application process forks service threads to execute the same statement for different users either on the same connection or on multiple connections. The same saving as above can be realized in this scenario too.
• Where the types of applications are SQL drivers and other middle-tiered applications.

  Note: Small applications, which execute single queries non-concurrently do not benefit from this feature.

There are several ways to use the shared OCI functionality. Existing applications can quickly examine the benefits of this feature without changing any code. These applications can be initialized in OCI shared mode by setting environment variables. New applications should use OCI API calls to initialize shared mode functionality.

Using OCI Functions

To initialize OCI shared mode functionality, process handle parameters must be set and OCIEnvCreate() must be called with the mode flag set to OCI_SHARED. For example:

ub4 mode = OCI_SHARED | OCI_THREADED;
OCIEnvCreate (&envhp, mode, (CONST dvoid *)0, 0, 0, 0, (size_t)0, (dvoid **)0);

The first application that initializes OCI in shared mode starts up the shared subsystem using the parameters set by that OCI application. When subsequent applications initialize using the shared mode, they use the previously started shared subsystem.

If an OCI application has been initialized in shared mode, all statements that are prepared and executed use the shared subsystem by default. If you do not want to use the shared subsystem to execute a specific SQL statement, then you can use the OCI_NO_SHARING flag in OCIStmtPrepare(). For example:

OCIStmtPrepare(stmthp, (CONST text *)createstmt,
              (ub4)strlen((char *)updstmt), (ub4)OCI_NTV_SYNTAX,
              (ub4)OCI_NO_SHARING);

The OCI_NO_SHARING flag has no effect if the process has not been initialized in the shared mode.

To detach a process from the shared memory subsystem, use the OCITerminate() call.

Using Environment Variables

The environment variables OCI_SHARED_MODE and OCI_NUM_SHARED_PROCS can be used to set OCI shared mode functionality. However, this is not the recommended method. This procedure lets you to quickly examine the benefits of using shared mode functionality in existing applications.

OCI_SHARED_MODE

To initialize an OCI application to run in shared mode, set the environment variable OCI_SHARED_MODE before executing a OCI program. To set the variable in the C-shell under Solaris^TM Operating Environment, for example, issue the command:

setenv OCI_SHARED_MODE number

where number is the size of the shared memory address space. For example:

setenv OCI_SHARED_MODE 20000000

If the shared subsystem is not already running, setting this variable launches the subsystem by creating a shared memory address space with the size specified. The size of the shared memory required is determined by the nature of the application and depends on the size and type of the SQL statement and the underlying table(s) that it accesses.

OCI_NUM_SHARED_PROCS

To set the maximum number of processes that can connect to the shared subsystem, set the environment variable ORA_OCI_NUM_SHARED_PROCS. To set this variable, issue the command:

setenv OCI_NUM_SHARED_PROCS number

where number is the maximum number of processes. For example:

setenv OCI_NUM_SHARED_PROCS   20

ORA_OCI_NUM_SHARED_PROCS is an initialization parameter for starting the shared subsystem. It has no effect if the shared subsystem is already running.

Allocating Handles and Descriptors

Oracle provides OCI functions to allocate and deallocate handles and descriptors. You must allocate handles using OCIHandleAlloc() before passing them into an OCI call, unless the OCI call, such as OCIBindByPos(), allocates the handles for you.

You can allocate the following types of handles with OCIHandleAlloc():

• error handle.
• service context handle.
• statement handle.
• describe handle.
• server handle.
• user session handle.
• transaction handle.
• connection pool handle
• complex object retrieval handle.
• subscription handle.
• direct path context handle.
• direct path column array handle.
• direct path stream handle

Depending on the functionality of your application, it needs to allocate some or all of these handles.

Application Initialization, Connection, and Session Creation

An application must call OCIEnvCreate() to initialize the OCI environment handle.

Following this step, the application has two options for establishing a server connection and beginning a user session: Single User, Single Connection; or Multiple Sessions or Connections.

Option 1: Single User, Single Connection

This option is the simplified logon method.

If an application maintains only a single user session for each database connection at any time, the application can take advantage of the OCI's simplified logon procedure.

When an application calls OCILogon(), the OCI library initializes the service context handle that is passed to it and creates a connection to the specified server for the user whose username and password are passed to the function.

The following is an example of what a call to OCILogon() might look like:

OCILogon(envhp, errhp, &svchp, "scott", nameLen, "tiger", 
      passwdLen, "oracledb", dbnameLen);

The parameters to this call include the service context handle (which are initialized), the username, the user's password, and the name of the database that are used to establish the connection. The server and user session handles are also implicitly allocated by this function.

If an application uses this logon method, the service context, server, and user session handles will all be read-only, which means that the application cannot switch session or transaction by changing the appropriate attributes of the service context handle, using OCIAttrSet().

An application that initializes its session and authorization using OCILogon() should terminate them using OCILogoff().

Option 2: Multiple Sessions or Connections

This option uses explicit attach and begin session calls.

If an application needs to maintain multiple user sessions on a database connection, the application requires a different set of calls to set up the sessions and connections. This includes specific calls to attach to the server and begin sessions:

• OCIServerAttach() creates an access path to the data server for OCI operations.
• OCISessionBegin() establishes a session for a user against a particular server. This call is required for the user to be able to execute any operation on the server.

  Note: See "Nonblocking Mode" for information about specifying a blocking or nonblocking connection in the OCIServerAttach() call.

These calls set up an operational environment that lets you to execute SQL and PL/SQL statements against a database. The database must be up and running before the calls are made, or else they will fail.

Example of Creating and initializing an OCI Environment

The following example demonstrates the use of creating and initializing an OCI environment. In the example, a server context is created and set in the service handle. Then a user session handle is created and initialized using a database username and password. For the sake of simplicity, error checking is not included.

#include <s.h>
#include <oci.h>
...
main()
{
...
OCIEnv *myenvhp;       /* the environment handle */
OCIServer *mysrvhp;    /* the server handle */
OCIError *myerrhp;     /* the error handle */
OCISession *myusrhp;   /* user session handle */
OCISvcCtx *mysvchp;    /* the  service handle */
...
/* initialize the mode to be the threaded and object environment */
(void) OCIEnvCreate(&myenvhp, OCI_THREADED|OCI_OBJECT, (dvoid *)0, 
                               0, 0, 0, (size_t) 0, (dvoid **)0);
 
     /* allocate a server handle */
(void) OCIHandleAlloc ((dvoid *)myenvhp, (dvoid **)&mysrvhp,
      OCI_HTYPE_SERVER, 0, (dvoid **) 0);

      /* allocate an error handle */
(void) OCIHandleAlloc ((dvoid *)myenvhp, (dvoid **)&myerrhp,
      OCI_HTYPE_ERROR, 0, (dvoid **) 0);

      /* create a server context */
(void) OCIServerAttach (mysrvhp, myerrhp, (text *)"inst1_alias", 
      strlen ("inst1_alias"), OCI_DEFAULT);

     /* allocate a service handle */
(void) OCIHandleAlloc ((dvoid *)myenvhp, (dvoid **)&mysvchp,
      OCI_HTYPE_SVCCTX, 0, (dvoid **) 0);

 /* set the server attribute in the service context handle*/
(void) OCIAttrSet ((dvoid *)mysvchp, OCI_HTYPE_SVCCTX, 
       (dvoid *)mysrvhp, (ub4) 0, OCI_ATTR_SERVER, myerrhp);

      /* allocate a user session handle */
(void) OCIHandleAlloc ((dvoid *)myenvhp, (dvoid **)&myusrhp,
     OCI_HTYPE_SESSION, 0, (dvoid **) 0);

      /* set username attribute in user session handle */
 (void) OCIAttrSet ((dvoid *)myusrhp, OCI_HTYPE_SESSION,
      (dvoid *)"scott", (ub4)strlen("scott"),
      OCI_ATTR_USERNAME, myerrhp);

      /* set password attribute in user session handle */
 (void) OCIAttrSet ((dvoid *)myusrhp, OCI_HTYPE_SESSION,
      (dvoid *)"tiger", (ub4)strlen("tiger"),
      OCI_ATTR_PASSWORD, myerrhp);

 (void) OCISessionBegin ((dvoid *) mysvchp, myerrhp, myusrhp,
      OCI_CRED_RDBMS, OCI_DEFAULT);

    /* set the user session attribute in the service context handle*/
 (void) OCIAttrSet (  (dvoid *)mysvchp, OCI_HTYPE_SVCCTX, 
       (dvoid *)myusrhp, (ub4) 0, OCI_ATTR_SESSION, myerrhp);
...
}

The demonstration program cdemo81.c in the demo directory illustrates this process, with error-checking.

Processing SQL Statements

For information about processing SQL statements, refer to Chapter 4, "Using SQL Statements in OCI".

Commit or Rollback

An application commits changes to the database by calling OCITransCommit(). This call takes a service context as one of its parameters. The transaction currently associated with the service context is the one whose changes are committed. This may be a transaction explicitly created by the application or the implicit transaction created when the application modifies the database.

If you want to roll back a transaction, use the OCITransRollback() call.

If an application disconnects from Oracle in some way other than a normal logoff (for example, losing a network connection), and OCITransCommit() has not been called, all active transactions are rolled back automatically.

Terminating the Application

An OCI application should perform the following three steps before it terminates:

1. Delete the user session by calling OCISessionEnd() for each session.
2. Delete access to the data source(s) by calling OCIServerDetach() for each source.
3. Explicitly deallocate all handles by calling OCIHandleFree() for each handle.
4. Delete the environment handle, which deallocates all other handles associated with it.

   Note: When a parent OCI handle is freed, any child handles associated with it are freed automatically

The calls to OCIServerDetach() and OCISessionEnd() are not mandatory, but are recommended. If the application terminates, and OCITransCommit() (transaction commit) has not been called, any pending transactions are automatically rolled back

Error Handling

OCI function calls have a set of return codes, listed in Table 2-3, "OCI Return Codes", which indicate the success or failure of the call, such as OCI_SUCCESS or OCI_ERROR, or provide other information that may be required by the application, such as OCI_NEED_DATA or OCI_STILL_EXECUTING. Most OCI calls return one of these codes.

Table 2-3 OCI Return Codes  

If the return code indicates that an error has occurred, the application can retrieve Oracle-specific error codes and messages by calling OCIErrorGet(). One of the parameters to OCIErrorGet() is the error handle passed to the call that caused the error.

The following example code returns error information given an error handle and the return code from an OCI function call. If the return code is OCI_ERROR, the function prints out diagnostic information. OCI_SUCCESS results in no printout, and other return codes print the return code information.

STATICF void checkerr(errhp, status)
OCIError *errhp;
sword status;
{
  text errbuf[512];
  ub4 buflen;
  ub4 errcode;

  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 (errhp, (ub4) 1, (text *) NULL, &errcode,
                    errbuf, (ub4) sizeof(errbuf), OCI_HTYPE_ERROR);
    (void) printf("Error - %s\n", 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;
default:
    break;
  }
}

Return and Error Codes for Truncation and Null Data

In Table 2-4, Table 2-5, and Table 2-6, the OCI return code, Oracle error number, indicator variable, and column return code are specified when the data fetched is null or truncated.

Table 2-4 Normal Data - Not Null and Not Truncated  

OCI_SUCCESS
error = 0

OCI_SUCCESS
error = 0
indicator = 0

OCI_SUCCESS
error = 0
return code = 0

OCI_SUCCESS
error = 0
indicator = 0
return code = 0

Table 2-5 Null Data  

OCI_ERROR
error = 1405

OCI_SUCCESS
error = 0
indicator = -1

OCI_ERROR
error = 1405
return code = 1405

OCI_SUCCESS
error = 0
indicator = -1
return code = 1405

Table 2-6 Truncated Data  

OCI_ERROR
error = 1406

OCI_ERROR
error = 1406
indicator = data_len

OCI_SUCCESS_WITH_INF
O

error = 24345
return code = 1405

OCI_SUCCESS_WITH_INFO

error = 24345
indicator = data_len
return code = 1406

In Table 2-6, data_len is the actual length of the data that has been truncated if this length is less than or equal to SB2MAXVAL. Otherwise, the indicator is set to -2.

Functions Returning Other Values

Some functions return values other than the OCI error codes listed in Table 2-3. When using these function be sure to take into account that they return a value directly from the function call, rather than through an OUT parameter. More detailed information about each function and its return values is listed in the reference chapters. Some examples of these functions are:

• OCICollMax()
• OCIRawPtr()
• OCIRawSize()
• OCIRefHexSize()
• OCIRefIsEqual()
• OCIRefIsNull()
• OCIStringPtr()
• OCIStringSize()

Additional Coding Guidelines

This section explains some additional factors to keep in mind when coding applications using the Oracle Call Interface.

Parameter Types

OCI functions take a variety of different types of parameters, including integers, handles, and character strings. Special considerations must be taken into account for some types of parameters, as described in the following sections.

Address Parameters

Address parameters pass the address of the variable to Oracle. You should be careful when developing in C, which normally passes scalar parameters by value, to make sure that the parameter is an address. In all cases, you should pass your pointers carefully.

Integer Parameters

Binary integer parameters are numbers whose size is system dependent. Short binary integer parameters are smaller numbers whose size is also system dependent. See your Oracle system-specific documentation for the size of these integers on your system.

Character String Parameters

Character strings are a special type of address parameter. This section describes additional rules that apply to character string address parameters.

Each OCI routine that allows a character string to be passed as a parameter also has a string length parameter. The length parameter should be set to the length of the string.

Inserting Nulls into a Column

You can insert a null into a database column in several ways. One method is to use a literal NULL in the text of an INSERT or UPDATE statement. For example, the SQL statement

     INSERT INTO emp (ename, empno, deptno)
        VALUES (NULL, 8010, 20)

makes the ENAME column null