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Extending Informix®
Beyond standard relational database objects, HCL Informix® can be extended to handle specialized data types, access methods, routines, and other objects. Informix includes many built-in extensions that are fully integrated in the database server. Informix also provides DataBlade® modules, which are packages of extended database objects for a particular purpose and that are installed separately from the database server. Alternatively, you can create your own user-defined objects for Informix.
Creating extensions
You can create user-defined data types, routines, access methods, and other database objects to suit your needs. You can use application programming interfaces to write user-defined routines and applications that access data in Informix® databases.
DataBlade® API Programmer's Guide
The Informix® DataBlade® API Programmer's Guide describes information about the DataBlade API, the C-language application programming interface (API) provided with HCL Informix.
Create user-defined routines
Write a user-defined routine
This section outlines some implementation issues for C user-defined routines (UDRs). This section also describes the specific features and tasks of a C UDR.
Virtual processors
To service multiple client-application SQL requests, the database server uses virtual processors (VPs). The database server breaks the SQL request into distinct tasks, based on the resource that the task requires. Different VP types, called virtual-processor classes (VP classes), service the different kinds of tasks.
Create a well-behaved routine
Avoid restricted system calls

Extending Informix®
Beyond standard relational database objects, HCL Informix® can be extended to handle specialized data types, access methods, routines, and other objects. Informix includes many built-in extensions that are fully integrated in the database server. Informix also provides DataBlade® modules, which are packages of extended database objects for a particular purpose and that are installed separately from the database server. Alternatively, you can create your own user-defined objects for Informix.
- Informix® extensions and DataBlade® modules
  These topics describe how to use built-in database extensions and separately installed DataBlade® modules.
- Creating extensions
  You can create user-defined data types, routines, access methods, and other database objects to suit your needs. You can use application programming interfaces to write user-defined routines and applications that access data in Informix® databases.
  - DataBlade® API Programmer's Guide
    The Informix® DataBlade® API Programmer's Guide describes information about the DataBlade API, the C-language application programming interface (API) provided with HCL Informix.
    - DataBlade® API overview
    - Data manipulation
    - Database access
    - Create user-defined routines
      - Develop a user-defined routine
        A C user-defined routine (UDR) is a UDR that is written in the C language and uses the server-side implementation of the DataBlade® API to communicate with the database server. C UDRs (functions and procedures) are implemented as C-language functions. DataBlade modules often include C UDRs that are made available for use by registering them in the database.
      - Write a user-defined routine
        This section outlines some implementation issues for C user-defined routines (UDRs). This section also describes the specific features and tasks of a C UDR.
        Code a C UDR
        When you code a C UDR, you define routine parameters, obtain argument values, define a return value, and code the routine body to write a C function.
        Virtual processors
        To service multiple client-application SQL requests, the database server uses virtual processors (VPs). The database server breaks the SQL request into distinct tasks, based on the resource that the task requires. Different VP types, called virtual-processor classes (VP classes), service the different kinds of tasks.
        Create a well-behaved routine
        Preserving availability of the CPU VP
        A well-behaved C UDR must preserve the availability of the CPU virtual processor (CPU VP).
        Write threadsafe code
        A well-behaved C UDR must be threadsafe.
        Avoid restricted system calls
        Unsafe operating-system calls
        An unsafe system call is one that blocks, causing the virtual processor to stall the CPU until the call returns, or one that allocates resources local to the virtual processor instead of in shared memory. A system call within a transaction is not terminated by a rollback, so a suspended transaction can wait indefinitely for the call to return.
        External-library routines
        It is recommended that a C UDR avoid the use of routines from existing external libraries. Some of these external routines might contain system calls that are restricted in your VP. If your C UDR must use an external routine, it might be ill behaved.
        Safe operating-system calls
        Choose the user-defined VP class
        Define a user-defined VP
        You define a new virtual-processor class in the onconfig file with the VPCLASS configuration parameter.
        Assign a C UDR to a user-defined VP class
        When you register an ill-behaved C UDR, you assign it to a class of user-defined virtual processors with the CLASS routine modifier of the CREATE FUNCTION or CREATE PROCEDURE statement.
        Manage virtual processors
        To manage virtual processors, you need to initialize VP classes, add and drop VPs, and monitor VPs.
        Control the VP environment
        The routine manager executes your C UDR in a virtual-processor (VP) environment.
        Perform input and output
        Because a C UDR executes in the context of the database server, it must not use the standard input/output (I/O) calls such as scanf() and printf(). The DataBlade® API provides support for I/O on a generic stream and I/O on an operating-system file from a UDR.
        Access the UDR execution environment
      - Manage memory
        A C user-defined routine (UDR) has access to shared memory for dynamic allocations and stack memory for routine arguments (including the MI_FPARAM structure), local stack variables, and return values.
      - Create special-purpose UDRs
        This section describes how to create special UDRs, such as iterator functions, user-defined aggregates, and optimization functions.
      - Extend data types
        This section describes the ways to extend data types with C user-defined routines (UDRs).
    - Appendix
      This section contains additional reference information.
  - DataBlade® API Function Reference
    The Informix® DataBlade® API Function Reference describes the DataBlade API functions and the subset of Informix ESQL/C functions that the DataBlade API supports.
  - DataBlade® Developers Kit
    The DataBlade® Developers Kit (DBDK) guides contain information about the tools you can use to develop and package DataBlade modules, which extend the functionality of HCL Informix® databases.
  - J/Foundation Developer's Guide
    The J/Foundation Developer's Guide describes how to write user-defined routines (UDRs) in the Java™ programming language for Informix®.
  - R-Tree Index User's Guide
    The Informix® R-Tree Index User's Guide describes the HCL Informix R-tree secondary access method and how to access and use its components.
  - User-Defined Routines and Data Types Developer's Guide
    The Informix® User-Defined Routines and Data Types Developer's Guide describes how to define new data types and enable user-defined routines (UDRs) to extend HCL Informix.
  - Virtual-Table Interface Guide
    The Informix® Virtual-Table Interface Programmer's Guide explains how to create a primary access method with the Virtual-Table Interface (VTI) so that users have a single SQL interface to HCL Informix tables and to data that does not conform to the storage scheme of HCL Informix.
  - Virtual-Index Interface Guide
    The Informix® Virtual-Index Interface Programmer's Guide explains how to create a secondary access method with the Virtual-Index Interface (VII) to extend the built-in indexing schemes of HCL Informix typically with a DataBlade® module.

Avoid restricted system calls

A well-behaved C UDR must avoid the use of restricted system calls, which can have the following adverse effects:

They might block I/O, which causes the operating system to suspend the process that calls them.
This suspension slows down both the C UDR that contains the calls and any other threads that share the same CPU virtual processor.
Many system calls allocate resources local to the process and are not re-entrant.

Informix® cannot provide a definitive list of unsafe system calls because system calls that are unsafe vary among versions of operating systems and different types of operating systems. Additionally, the implementation of the VPs is different between UNIX™ or Linux™ and Windows™:

On UNIX™ or Linux™, the VPs are implemented as separate processes.
On Windows™, each VP is a thread of a common process.

The difference in VP implementation means that some system calls are acceptable when the C UDR runs on Windows™ but not when this same UDR runs on UNIX™ or Linux™. There are also differences in how UNIX™ or Linux™ handles shared libraries and how Windows™ handles dynamic link libraries (DLLs) that can affect the platform on which operating-system calls are valid. Therefore, UDRs might not be portable from one operating system to another.