The HEAPZONES and HEAPCHK runtime options are useful for debugging overlay damage problems that occur in the user heap. Though similar in that both options can be used for debugging purposes, the runtime options activate very different behavior in the runtime when specified.
HEAPZONES is a lightweight mechanism that detects heap overlay damage only during the freeing of an element. It looks for damage in the heap check zone of the freed element only.
Selecting a non-quiet output option causes HEAPZONES to display information about the damaged heap element. When messaging is requested, the address of the damaged element along with information specific to the heap check zone are included in the message. Depending on the type of damage, the value of the heap check zone is displayed. The data area of the damaged location is displayed following any issued informational messages. This runtime option can also be used as a mechanism to tolerate heap overlay damage by simply requesting no output (QUIET).
Depending on the size of the heap check zone and the number of allocation requests, the user may notice a significant amount of extra storage being used by the application. Performance may be affected due to the overhead of examining each heap check zone.
HEAPCHK investigates the entire user heap for damage during heap related calls at a frequency based on the specified settings in the option. Because HEAPCHK will traverse the entire user heap, a slow down in application performance will occur. Information about HEAPCHK diagnostic output is discussed in Using Language Environment debugging facilities.
When deciding which runtime option is better suited to use with your application, consider the differences between HEAPZONES and HEAPCHK relating to performance, storage usage, and time of damage detection. Although both runtime options affect performance, an application that chooses HEAPCHK will perform slower than an application that chooses HEAPZONES. If storage usage is a concern, HEAPCHK will not consume extra amounts of storage in the manner that HEAPZONES will. Determining when heap damage has occurred may be simpler to accomplish if HEAPCHK is chosen because of the frequency and scope of its analysis.
For more information about the HEAPZONES and HEAPCHK runtime options, see z/OS Language Environment Programming Reference.
