Memory Overlay Detection System (MODS)

Some of the most difficult types of problems to debug are what are generally called "memory overlays." Memory overlays include the following:

• Writing to memory that is owned by another program or routine
• Writing past the end (or before the beginning) of declared variables or arrays
• Writing past the end (or before the beginning) of dynamically allocated memory
• Writing to or reading from freed memory
• Freeing memory twice
• Calling memory allocation routines with incorrect parameters or under incorrect conditions.

In the kernel environment (including the kernel, kernel extensions, and device drivers), memory overlay problems have been especially difficult to debug because tools for finding them have not been available. Starting with AIX® 4.2.1, however, the Memory Overlay Detection System (MODS) helps detect memory overlay problems in the kernel, kernel extensions, and device drivers.