Choose the memory duration

Because a C UDR executes in the memory space of the database server, its dynamic memory allocations can increase the memory usage of the database server. For this reason, it is important that a UDR release its dynamically allocated memory as soon as it no longer needs to access this memory.

To help ensure that unneeded memory is freed, the database server associates a memory duration with memory allocation made from its shared memory. The portion of shared memory that the database server provides for dynamic allocation by C UDRs is organized into several memory pools. Each memory pool is associated with a memory duration, which specifies the lifetime of the memory allocated from the pool. Keeping related memory allocations in one pool helps to reduce memory fragmentation.

The following figure shows a schematic representation of the shared memory of the database server, including the memory-duration memory pools.

Tip: For more information about the use and structure of database server memory pools, see your Informix® Administrator's Guide. For more information about how to monitor the amount of shared memory that exists in each of the memory pools, see Monitor shared memory.

When the database server calls a UDR, it creates a memory context. This memory context records all of the allocations that the UDR makes before the routine returns. The UDR might run for some time, calling other UDRs or DataBlade® API functions. The database server automatically reclaims shared memory based on its memory duration. When a particular memory duration expires, the database server marks the associated memory pool for deallocation.