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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 extensionsYou 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 GuideThe 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 routineThis 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 processorsTo 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 VPA well-behaved C UDR must preserve the availability of the CPU virtual processor (CPU VP).
Avoid blocking I/O callsTo preserve concurrency, a well-behaved C UDR must avoid system calls that perform blocking input and output operations (I/O).

Avoid blocking I/O calls

To preserve concurrency, a well-behaved C UDR must avoid system calls that perform blocking input and output operations (I/O).

The following operating-system calls are examples of calls that can block input and output operations:

accept() bind() fopen() getmsg()
msgget() open() pause() poll()
putmsg() read() select()
semop() wait() write()

When a C UDR executes any of these system calls, the CPU VP must wait for the I/O to complete. In the meantime, the CPU VP cannot process any other requests. The database server can appear to stall because the concurrency of the CPU VP is impaired.

If your C UDR needs to perform file I/O, do not use operating-system calls to perform this task. Instead, use the DataBlade® API file-access functions. These file-access functions allow the CPU VP to process the I/O asynchronously. Therefore, they do not block the CPU VP. For more information, see Access to operating-system files.

If your UDR must issue blocking I/O calls, assign the routine to execute in a user-defined VP class. When a UDR blocks a user-defined VP, only those UDRs that are assigned to that VP are affected. You might need to use a single instance of a user-defined VP, which would affect client response. Your UDR must also handle any problems that can occur if the thread yielded; for example, operating-system file descriptors do not migrate with a thread if it moves to a different VP.

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